Patent Publication Number: US-11050256-B2

Title: Distributed energy resource topology and operation

Description:
RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 16/539,150, which was filed on Aug. 13, 2019 and issued as U.S. Pat. No. 10,530,157. The &#39;150 application is a continuation of U.S. application Ser. No. 16/260,411, which was filed on Jan. 29, 2019 and issued as U.S. patent 10, 418,815. The &#39;411 application is a continuation of U.S. application Ser. No. 15/272,575, which was filed on Sep. 22, 2016 and issued as U.S. Pat. No. 10,218,177. 
    
    
     BACKGROUND 
     Most electricity using facilities are connected to a regional electrical grid maintained and powered by an electric utility and draw alternating current (AC) power from that regional electrical grid. Increasing numbers of facilities are supplementing or replacing the AC power drawn from the regional electrical grid with distributed power generated on-site by renewable energy sources such as solar or wind that do not generate power at a constant rate throughout the day. Additionally, because load demand on the regional electric grid can vary with respect to time of day, date, or weather conditions but electrical generation must always meet load demand, electrical utilities must maintain the ability to ramp up power generation to meet load demand and avoid brownouts. Further, the increase of distributed energy resources on the grid can have a negative impact to grid stability of voltage and/or frequency. Further, in order to discourage users from drawing more power during periods of high load demand, electric utilities may increase prices on electricity during high load demand periods and decrease prices during low load demand periods. In order to make use of energy generated during one time of day in a second time of day, meet load demand with additional power, arbitrage energy across different rate periods, or improve grid stability as well as other reasons, interest in on-site energy storage with grid support capability is increasing. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The figures described below depict various aspects of the system and methods disclosed herein. It should be understood that each figure depicts an embodiment of a particular aspect of the disclosed system and methods, and that each of the figures is intended to accord with a possible embodiment thereof. Further, wherever possible, the following description refers to the reference numerals included in the following figures, in which features depicted in multiple figures are designated with consistent reference numerals. 
         FIG. 1  shows a block diagram of the components of a distributed energy resource (DER) in accordance with various disclosed embodiments; 
         FIG. 2  shows a block diagram of the components of a system including one or more DERs in accordance with various disclosed embodiments; 
         FIGS. 3A-D  is a flow chart illustrating an example DER operation method in accordance with various disclosed embodiments; 
         FIG. 4  is a flow chart illustrating an example DER installation method in accordance with various disclosed embodiments. 
     
    
    
     SUMMARY 
     Embodiments may include a distributed energy resource (DER) comprising: a plug configured to couple to an AC circuit and receive or deliver AC power from or to the AC circuit; a receptacle configured to receive a plug of a load device and transmit AC power to the load device; an energy storage circuit; one or more controllers; a program memory storing executable instructions that when executed by the one or more controllers cause the DER to: determine whether to set the DER to a charge state or a discharge state, determine an amperage difference between a first threshold value and the amperage of an AC power transmitted to the load device, if the amperage difference is above a charge threshold value and the DER is set to a charge state, charge the energy storage circuit with AC power received from the AC circuit, and if the DER is set to a discharge state, deliver AC power to the AC circuit by discharging the energy storage circuit. 
     Embodiments may also include a method for operating a distributed energy resource (DER) having a plug, a receptacle, and an energy storage circuit, the method comprising: receiving AC power from an AC circuit via the plug of the DER; transmitting AC power to a load device coupled to the DER via the receptacle of the DER; determining whether to set the DER to a charge state or a discharge state; determining an amperage difference between a first threshold value and the amperage of the AC power transmitted to the load device; if the amperage difference is above a charge threshold value and the DER is set to a charge state, charging the energy storage circuit with AC power received from the AC circuit; and if the DER is set to a discharge state, delivering AC power to the AC circuit by discharging the energy storage circuit. 
     Embodiments may further include a distributed energy resource (DER) comprising: a plug configured to couple to an AC circuit and receive AC power from the AC circuit; a receptacle configured to receive a connector to a load device and transmit AC power to the load device; a bidirectional DC/AC converter; one or more controllers; a program memory storing executable instructions that when executed by the one or more controllers cause the DER to: determine whether to set the DER to a reactive power source state; and if the DER is set to a reactive power source state, use the DC/AC converter to source or sink reactive power to the AC circuit. 
     DETAILED DESCRIPTION 
     The following detailed description is merely illustrative in nature and is not intended to limit the embodiments of the subject matter of the application or uses of such embodiments. As used herein, the word “exemplary” means “serving as an example, instance, or illustration.” Any implementation described herein as exemplary is not necessarily to be construed as preferred or advantageous over other implementations. Furthermore, there is no intention to be bound by any expressed or implied theory presented in the preceding technical field, background, brief summary or the following detailed description. 
     This specification includes references to “one embodiment” or “an embodiment.” The appearances of the phrases “in one embodiment” or “in an embodiment” do not necessarily refer to the same embodiment. Particular features, structures, or characteristics may be combined in any suitable manner consistent with this disclosure. 
     Terminology. The following paragraphs provide definitions and/or context for terms found in this disclosure (including the appended claims): 
     “Comprising.” This term is open-ended. As used in the appended claims, this term does not foreclose additional structure or steps. 
     “Configured To.” Various units or components may be described or claimed as “configured to” perform a task or tasks. In such contexts, “configured to” is used to connote structure by indicating that the units/components include structure that performs those task or tasks during operation. As such, the unit/component can be said to be configured to perform the task even when the specified unit/component is not currently operational (e.g., is not on/active). Reciting that a unit/circuit/component is “configured to” perform one or more tasks is expressly intended not to invoke 35 U.S.C. § 112, sixth paragraph, for that unit/component. 
     “First,” “Second,” etc. As used herein, these terms are used as labels for nouns that they precede, and do not imply any type of ordering (e.g., spatial, temporal, logical, etc.). For example, reference to a “first” plug does not necessarily imply that this plug is the first plug in a sequence; instead the term “first” is used to differentiate this plug from another plug (e.g., a “second” plug). 
     “Based On.” As used herein, this term is used to describe one or more factors that affect a determination. This term does not foreclose additional factors that may affect a determination. That is, a determination may be solely based on those factors or based, at least in part, on those factors. Consider the phrase “determine A based on B.” While B may be a factor that affects the determination of A, such a phrase does not foreclose the determination of A from also being based on C. In other instances, A may be determined based solely on B. 
     “Coupled”—The following description refers to elements or nodes or features being “coupled” together. As used herein, unless expressly stated otherwise, “coupled” means that one element/node/feature is directly or indirectly joined to (or directly or indirectly communicates with) another element/node/feature, and not necessarily mechanically. 
     “Inhibit”—As used herein, inhibit is used to describe a reducing or minimizing effect. When a component or feature is described as inhibiting an action, motion, or condition it may completely prevent the result or outcome or future state completely. Additionally, “inhibit” can also refer to a reduction or lessening of the outcome, performance, and/or effect which might otherwise occur. Accordingly, when a component, element, or feature is referred to as inhibiting a result or state, it need not completely prevent or eliminate the result or state. 
     In addition, certain terminology may also be used in the following description for the purpose of reference only, and thus are not intended to be limiting. For example, terms such as “upper”, “lower”, “above”, and “below” refer to directions in the drawings to which reference is made. Terms such as “front”, “back”, “rear”, “side”, “outboard”, and “inboard” describe the orientation and/or location of portions of the component within a consistent but arbitrary frame of reference which is made clear by reference to the text and the associated drawings describing the component under discussion. Such terminology may include the words specifically mentioned above, derivatives thereof, and words of similar import. 
     In the following description, numerous specific details are set forth, such as specific operations, in order to provide a thorough understanding of embodiments of the present disclosure. It will be apparent to one skilled in the art that embodiments of the present disclosure may be practiced without these specific details. In other instances, well-known techniques are not described in detail in order to not unnecessarily obscure embodiments of the present disclosure. 
     Referring now to  FIG. 1 , a block diagram showing various components of a distributed energy resource (DER)  100  is shown. The DER  100  may include energy storage  102 , a DC/AC converter  104 , a controller  106 , a program memory  107 , a communications module  108 , plug retention device  110 , a plug  112 , a cord  114 , a voltage sensor  116 , a disconnect  118 , an overcurrent protector  120 , a load sensor  122 , a load receptacle  124 , and a user interface  126 . While only one of each component of the DER  100  is shown in  FIG. 1 , it will be understood that the DER  100  may include more than one of any or all of the shown components (e.g., more than one controller  106 , more than one communications module  108 , more than one DC/AC converter  104 , more than one energy storage  102 , etc.). Further, it will be understood that the DER  100  may not include all of the components shown in  FIG. 1 . For example, the DER  100  may not include energy storage  102  or a user interface  126 . 
     Energy storage  102  may be an array of one or more rechargeable batteries (e.g., Li ion batteries) and necessary components to receive a DC voltage from the DC/AC converter  104  and charge the one or more rechargeable batteries if the DER  100  is in a charge stage or deliver DC voltage to the DC/AC converter  104  if the DER  100  is in a discharge stage or to store energy if the DER  100  is in an idle state. Alternatively or additionally, energy storage  102  may include other energy storage devices such as capacitors, inductors, or other devices to store and deliver electrical energy. Alternatively or additionally, energy storage  102  may include other forms of energy storage such as a flywheel or compressed air coupled with suitable circuits for conversion to or from DC electrical power. Energy storage  102  may be sized to store any of a number of different amounts of power (e.g., 1 kilowatt hour, 10 kilowatt hours, 100 kilowatt hours, etc.). As discussed herein, the DER  100  may use energy storage  102  for time of use arbitrage, duck curve mitigation, energy delivery when the utility needs to deliver more power to the grid storage of solar-generated power for use during times in which less solar power can be generated, uninterruptable power supply (UPS) functionality, etc. 
     As used herein, the term “time of use arbitrage” refers to using the DER  100  to store energy during a first period of time when AC power (e.g., from the utility grid) is priced at a first amount of money and discharging the stored energy during a second period of time when AC power is priced at a second amount of money, wherein the second amount of money is greater than the first amount of money. Additionally, as used herein, the term “duck curve” refers to the circumstance where distributed renewable power generation (e.g., distributed solar power generation) occurs at midday but utility peak loads occur later in the day, so the time of use load curves seen by utilities dip in the middle of the day for their load profiles. Accordingly, it may be advantageous for a utility to mitigate the duck curve by having distributed energy storage (e.g., the energy storage  102  of the DER  100 ) charge during the middle of the day and discharge later in the day to help level utility load curves. 
     The DC/AC converter  104  may be a bidirectional DC/AC inverter or any other device that is configured to convert AC power to DC power in one direction (e.g., when charging energy storage  102 ) and convert DC power to AC power in the other direction (e.g., when discharging energy storage  102 ). Collectively the DC/AC converter  104  and energy storage  102  may also be referred to herein as an “energy storage circuit.” The DC/AC converter  104  may be the interface between energy storage  102  and the AC circuit to which the DER  100  is coupled (e.g., an AC branch circuit  212  discussed herein). When energy storage  102  is connected, the DC/AC converter  104  may charge or discharge the battery based upon a signal from the controller  106  to source or sink real power to the AC circuit. With or without energy storage  102 , the DC/AC converter  104  may also be capable of providing reactive power to the AC circuit in support of e.g., CA Rule  21 , Hawaii Electric Rule  14 , UL 1741 SA or pending changes to IEEE 1547 requirements. As discussed herein, reactive power functions may provide voltage and/or frequency grid support functions and other supportive functions desired by electric utilities. The DC/AC converter  104  may provide information to the controller  106  about its status including operating real power, reactive power, voltage and/or current on the AC circuit side, and/or voltage and/or current on the energy storage  102  side of the DC/AC converter DC/AC. 
     The controller  106  may include one or more computer processors capable of executing instructions causing the DER  100  to implement the actions specified by the instructions. The controller  106  may include a program memory  107 . The program memory  107  may be configured to store computer-readable instructions that when executed by the controller  106  cause the DER  100  to implement the methods described herein. The controller  106  may also include a random access memory (RAM) (not pictured), an input/output (I/O) circuit (not pictured). The various components of the controller  106  may be interconnected via an address/data bus (not pictured). 
     The communications module  108  may include any of a number of wired (e.g., USB, Ethernet) or wireless (e.g., 802.11 WiFi, Zigbee radio) communications connections. The communications module  108  may be configured to transmit and/or receive communications from some or all of a customer user, (e.g., the owner of the home in which the DER  100  is installed communicating with the DER  100  with a local device  220  as shown in  FIG. 2 ), a utility user (e.g., a technician monitoring the status of the grid), or a program running on a remote server (e.g., a remote server  218  as shown in  FIG. 2 ). As discussed herein, the communications module  108  may receive commands to put the DER  100  in a charge state, a discharge state, an idle state, or a reactive power state. Alternatively or additionally, the communications module  108  may receive information (e.g., weather forecast information, chronological information) that the DER  100  (via the controller  106 ) may use to determine whether to put itself into a charge state, a discharge state, an idle state, or a reactive power state. Additionally, the communications module  108  may be used at the manufacturing site at which the DER  100  is built to preconfigure a unit for a schedule of operation of the DER  100 , may be used to determine time of day and date, may be used to provide firmware or software updates to the DER  100 , etc. 
     The plug  112  may be any of a number of known plugs used to couple to a receptacle coupled to a preexisting AC circuit. For example, the plug  112  may be a plug configured to be inserted into an electrical receptacle in a wall. Depending on the local standard, the plug  112  may have a plurality of prongs (e.g., 3 prongs, 4 prongs, etc.). In some embodiments, the plug  112  is a National Electric Manufacturers Association (NEMA) 10-30 electrical clothes dryer plug. In other embodiments, the plug  112  is a NEMA 14-30 electric clothes dryer plug. The plug  112  may also be a plug of the type used to power an electric dryer, an electric range and oven, an electric water heater, a refrigerator, or other specialized plug or a standard AC plug in the country in which the DER  100  will be installed. The plug  112  may be configured to receive or deliver AC power from or to the AC circuit to which it is coupled. 
     Because it may be required (e.g., by local electrical code) that the plug  112  not be inadvertently removed by a casual user in some embodiments, the DER  100  may include a plug retention device  110 . Such a plug retention device  110  may, for example, make the DER  100  more likely to be listed by UL or other safety organizations. The plug retention device  110  may be a mechanical securement that requires a tool to operate (e.g., key) such as a special cover plate for the receptacle to which the DER  100  is coupled. The special cover plate may mechanically couple to the plug  112  such that the plug  112  may only be decoupled from the receptacle after using the key (or other tool) to unlock the retention mechanism. The plug  112  may also include a lamp indicating that voltage is present as a convenience to a user so that they know when voltage is present. 
     The cord  114  may be a standard insulated power cord of any of a number of lengths (e.g., 6 feet, 12 feet, 100 feet, etc.). For example, the cord  114  may be 6 feet long, which will enable the DER  100  to be mounted to the wall near the receptacle to which the plug  112  is coupled (e.g., mounted on the wall in the laundry room if the DER  100  is coupled to an electric dryer receptacle). Alternatively, the cord  114  may be short enough such that the entire DER  100  may mount to the receptacle of the AC circuit. A longer cord  114  may enable the DER  100  to be located at a distance from the receptacle of the AC circuit (e.g., if the receptacle is a dryer receptacle in a laundry room, most of the DER  100  may be located in the garage with only the cord  114  and plug  112  disposed in the laundry room. 
     The DER  100  may include a voltage sensor  116  located on the AC circuit side of the disconnect  118 . The voltage sensor  116  may be used to measure grid voltage and/or grid frequency, and communicate such measurements to the controller  106 . Grid voltage and grid frequency sensing may be used to determine when grid support functions of the DER  100  may be needed. The voltage sensor  116  may also be used to allow for synchronization between the DC/AC converter  104  and the grid prior to re-closing the disconnect  118 . 
     The DER  100  may include a disconnect  118 . The disconnect  118  may be an AC contactor (or other controllable switch) that disconnects the AC circuit allowing the remainder of the DER  100  to provide power and energy from the energy storage  102  to the load receptacle  124  when operating in an uninterruptible power supply (UPS) mode of operation. The disconnect  118  may disconnect the AC circuit from the remainder of the DER  100  based on a command from the controller  106  and may connect the AC circuit to the remainder of the DER  100  based on a command from the controller  106 . 
     The DER  100  may include an overcurrent protector  120 . The overcurrent protector  120  may be any of a number of known devices (e.g., a circuit breaker, a fuse, a combination of the two, etc.) configured to disconnect the load device from AC power if the amperage of the AC power transmitted to the load device through the DER  100  is greater than or equal to a threshold value. Such a threshold value may be the maximum rated current of the AC circuit. For example, as discussed herein in connection to  FIG. 2 , if the AC circuit is a dedicated branch circuit  212  for an electric dryer, the maximum rated current may be 30 amps. In such an example, the overcurrent protector  120  may be a 30 amp circuit breaker that will open the circuit automatically if more than 30 amps flows through the 30 amp circuit breaker. Alternatively, overcurrent protection may be provided to the load receptacle through proper operation of the controller  106  using the signals from the load sensor  122  and reliance upon the facility over current protection already provided to the AC circuit (e.g., the overcurrent protection devices  211  discussed herein in connection to  FIG. 2 ). 
     The load sensor  122  may include any of a number of known devices capable of determining how much load current is being drawn by a load device  214  coupled to the load receptacle  124  (e.g., an electrical dryer). For example, the load sensor  122  may be a current transformer connected to two lines coupled to the load receptacle (e.g., a line one and a line two). The load sensor  122  may also include a voltage sensor to determine the voltage of the load receptacle  118  to determine a load voltage measurement. The load current measurement and load voltage measurement may be used (e.g., by the controller  106  or a controller included in the load sensor  122 ) to determine the real and reactive power drawn by the load device  214  as well as total rms current drawn by the load device  214 . The load sensor  122  may output any or all of the measurements it makes to the controller  106 . Alternatively, the controller  106  may use the voltage and or current signals of the load sensor  122  to determine power and other parameters drawn by the load device  214 . 
     The load receptacle  124  may be a receptacle configured to receive a plug of a load device  214  and transmit AC power to the load device  214 . The load receptacle  124  may be a receptacle of the same type as the receptacle of the AC circuit to which the plug  112  is coupled. For example, if the receptacle of the AC circuit is a NEMA 14-30 receptacle and the plug  112  is a NEMA 14-30 plug, the load receptacle  124  may be a NEMA 14-30 receptacle. Alternatively, the load receptacle  124  may be a different type of receptacle from the receptacle of the AC circuit (e.g., if the receptacle of the AC circuit is a NEMA 10-30 receptacle, the load receptacle  124  may be a NEMA 14-30 receptacle) if such a difference is permitted by local electrical code. 
     The DER  100  may also include one or more user interfaces  120 . The one or more user interfaces  120  may include output devices such as an LCD display, an OLED display, or an array of lights (e.g., LEDs) to display information visually. The one or more user interfaces  120  may also include input devices such as a keyboard, one or more buttons, one or more switches, one or more dials, and/or a touchscreen display (e.g., an LCD touchscreen) to receive local user input. The user interface  126  may also include a graphical user interface displayed over a network (e.g., a network  216 ) to receive input and display output to a user accessing the DER  100  via the network. Additionally or alternatively, the user interface  126  may include an application program interface (API) configured to receive commands from and display information to an application monitoring the energy use (and in some embodiments the distributed renewable power generation) of the facility at which the DER  100  is installed. Such an application may include the SunPower® Residential Monitoring System, for example. 
     Referring now to  FIG. 2 , a block diagram of a system  200  including one or more DERs  100  is shown. The system  200  may include a distributed renewable energy source  202 , a distributed generation meter  204 , a utility AC grid connection  206 , a utility meter  208 , an electric panel  210 , one or more AC branch circuits  212 , a network  216 , a remote server  218 , and a local device  220 . The majority of the components of the system  200  (i.e., all but the network  216  and remove server  218 ) may be located at the same facility (e.g., a house, commercial installation, etc.). It will be understood that while  FIG. 2  shows one of each of the distributed renewable energy source  202 , distributed generation meter  204 , utility AC grid connection  206 , utility meter  208 , electric panel  210 , network  216 , remote server  218 , and local device  220 , the system  200  may include one or more of each in various embodiments or may not include all of the components shown (e.g., a system  200  may not include a distributed renewable energy source  202  or a distributed generation meter  204 ). 
     The distributed renewable energy source  202  may be one or more renewable energy sources of AC power (e.g., a solar photovoltaic (PV) array, a solar thermal array, a wind turbine, a biofuel generator, etc.). It will be understood that the distributed renewable energy source  202  may not generate AC power at a consistent rate throughout the course of a day (e.g., a PV array may generate more AC power during midday than during the morning or evening). The distributed generation meter  204  may be coupled to the distributed renewable energy source  202  and utility meter  208  and may track the amount of AC power generated by the distributed renewable energy source  202  and fed into the AC circuit of the system  200 . 
     The utility AC grid connection  206  may be coupled to the AC grid and to the utility meter  208 . The utility AC grid connection  206  may be configured to draw AC power from the AC grid, and to also deliver power (e.g., AC power generated by the distributed renewable energy source  202 , AC power discharged from a DER  100 ) to the AC grid. The AC grid may be coupled to a regional electric grid operated by an electric utility. 
     The utility meter  208  may be coupled to the distributed generation meter  204 , utility AC grid connection  206 , and the electric panel  210 . As will be understood, the utility meter  208  may measure the amount of AC power drawn from the AC grid via the utility AC grid connection  206 , and in some embodiments may subtract the amount of power generated by the distributed renewable energy source  202  that is delivered to the AC grid. The utility meter  208  may deliver AC power (i.e., from the AC grid and/or from the distributed renewable energy source  202 ) to the electric panel  210  and may deliver AC power discharged from the one or more DERs  100  to the AC grid. The utility meter  208  may also track the amount of AC power delivered to the AC grid that has been discharged by the one or more DERs  100 . 
     The electric panel  210  may be coupled to the utility meter  208  and the one or more AC branch circuits  212 . The electric panel  210  may deliver AC power from the AC grid to the AC branch circuits  212  and may also deliver AC power discharged by the one or more DERs  100  to the AC grid via the utility meter  208 . The electric panel  210  may include one or more overcurrent protection devices  211  (e.g., circuit breakers, fuses, a combination of both, etc.). The electric panel  210  may be coupled to the one or more AC branch circuits  212  via the overcurrent protection devices  211  (e.g., an overcurrent protection device  211 A may be coupled to a first AC branch circuit  212 A). As discussed herein, each AC branch circuit  212  may be designed to conduct an AC current (at the voltage and frequency specified by local electrical grid specifications) below a maximum rated current (e.g.,  50 , amps, 30 amps, 20 amps, 10 amps). 
     In embodiments, the electric panel  210  and/or overcurrent protection device  211  may communicate with the DER  100  (e.g., over a network  216 ) and receive information regarding the amount of current flowing into the DER  100  via the plug  112  and out of the DER  100  via the load receptacle  124 . In such a case, the electric panel  210  and/or overcurrent protection device  211  may coordinate with the DER  100  to ensure that the current flowing through the AC branch circuit  212  does not exceed the maximum rated current. For example, if the maximum rated current of the first AC branch circuit  212 A is 30 amps and the DER  100  is discharging its energy storage  102  to deliver 5 amps to the load device  214 A and communicates information regarding this 5 amp delivery to the electric panel  210  and/or overcurrent protection device  211 , the electric panel  210  and/or overcurrent protection device  211  may ensure that no more than 25 amps is delivered from the AC grid to the AC branch circuit  212 A. In some embodiments, electric panel  210  and/or overcurrent protection device  211  have no way of knowing what is coupled to each AC branch circuit  212 , including whether a DER  100  is installed. In such embodiments, the overcurrent protection device  211  will only disconnect AC power from the AC branch circuit  212  if the maximum rated current is exceeded (e.g., 40 amps is flowing to an AC branch circuit  212  with a 30 amp maximum rated current). 
     The one or more AC branch circuits  212  are AC circuits connecting one or more loads  214  to AC power (at the voltage and frequency specified by local electrical grid specifications) and may be coupled to the electric panel  210  via AC power cables sufficient to carry that AC power. In  FIG. 2 , three AC branch circuits  212 A,  212 B, and  212 C are shown, although fewer or more branch circuits may be present in the system  200 . Each AC branch circuit  212  may include a DER  100  and a load device  214 . Each AC branch circuit  212  may be a dedicated branch circuit for a single load device  214 . For example, the first AC branch circuit  212 A may be a dedicated branch circuit for a load  212 A that is an electric dryer, the second AC branch circuit  212 B may be a dedicated branch circuit for a load  212 B that is an electric range, and the third AC branch circuit  212 B may be a dedicated branch circuit for a load  212 C that is an electric water heater. Of course, it will be understood that each branch circuit  212  may include more than one load  214 . Each AC branch circuit  212  may have a maximum rated current (e.g.,  50 , amps, 30 amps, 20 amps, 10 amps, etc.) and AC wiring sufficient to carry the maximum rated current. As discussed herein, if a current higher than the maximum rated current flows from the electric panel  210  to the AC branch circuit  212 , the overcurrent protection device  211  coupled to the AC branch circuit  212  may disconnect (e.g., by a circuit breaker tripping, a fuse opening) AC power from the AC branch circuit  212  to which it is coupled. 
     The network  216  may be any of a number of wired (e.g., USB, Ethernet) and/or wireless (e.g., 802.11 WiFi, Zigbee) communication networks coupled to various devices in the system  200 . In some embodiments, the network  216  includes the Internet. While the network  216  is shown coupled to the DERs  100 , remote server  218 , and local device  220 , in some or all of the other components in the system  200  may be communicatively coupled to the network  216 . 
     The remote server  218  may be one or more computing devices located in a separate geographic location than the rest of the system  200  and communicatively coupled to the DER  100  via the network  216 . As discussed herein, the remote server  218  may be used to operate an interface (e.g., a GUI, an API, etc.) receiving information from and sending information to the one or more DERs  100  (e.g., to display data gathered by a DER  100 , to put the DER  100  in one or more of a charge state, discharge, state, idle state, or reactive power state). The remote server  218  may also receive a command (e.g., from a local user, a remote utility technician, or other software) to put a DER  100  in one or more of a charge state, discharge state, idle state, or reactive power state and relay such a command to the appropriate DER  100  over the network  216 . 
     The local device  220  may be any of a number of devices configured to communicate with a DER  100  over the network  216 . For example, the local device  220  may be a computer communicatively coupled to a home WiFi network  216  to which the one or more DERs  100  are communicatively coupled. A local device  220  may be one or more of a laptop, tablet computer, smartphone, wearable computer, etc. 
     Referring now to  FIGS. 3A-3D , a flowchart illustrating a DER operating method  300  is shown. The DER operating method  300  may be implemented all or in part by the DER  100 . Additionally, the DER operating method  300  may be partially implemented using the remote server  218  and/or local device  220 . At block  302 , the DER  100  receives AC power from the AC circuit (e.g., via the plug  112  coupled to the receptacle of the AC circuit). At block  302 , the DER  100  transmits AC power to the load device  214  as discussed herein (e.g., via the load receptacle  124  coupled to a plug of the load device  214 ). 
     At block  306 , the DER  100  determines (e.g., with the load sensor  122  and/or controller  106 ) the amperage difference between the amperage of the maximum rated current of the AC circuit and the amperage of the AC power transmitted to the load device  214 . This amperage difference may be based upon rms measurements of each current individually or the rms of the difference of the currents. The DER  100  may receive a value for the maximum rated current from a user (e.g., an installer who checks the maximum rated current of the AC branch circuit  212  to which the DER  100  is installed and inputs it to the DER  100  via the user interface  126  and/or via the communications module  108 ), from a remote server  218 , and/or a local device  220 . Alternatively, the DER  100  may be preprogrammed with the maximum rated current of the AC circuit. Accordingly, an installer may have to select the appropriate DER  100  for the AC circuit. The maximum rated current may depend on the type of plug  112  and load receptacle  124 . For example, if the plug  112  and load receptacle  124  are NEMA 10-30 or NEMA 14-30 type plugs and receptacles, respectively, the maximum rated current is 30 amps. The DER  100  may determine the amperage of the AC power transmitted to the load device  214  using the load sensor  122 . The DER  100  may store the amperage difference in memory (e.g., RAM of the controller  106 ). 
     At block  308 , a determination is made whether to put the DER  100  in a charge state, a discharge state, or idle state. This determination may be made by the DER  100  and/or may be made by a remote server  218 , local device  220 , and/or a user. The determination of whether to put the DER  100  in a charge state, a discharge state, or idle state may be based in part on a measurement made by the DER  100  of the difference between the maximum capacity of energy storage  102  and the amount of energy currently stored in energy storage  102  (also referred to herein as the charge level of energy storage  102 ). If energy storage  102  is empty, it may be advantageous to charge energy storage  102 . If energy storage  102  is not fully charged, it may be advantageous to discharge or charge energy storage  102 . If energy storage  102  is fully charged, it may be advantageous to discharge energy storage  102 . Charging or discharging energy storage  102  may not be merely an on/off function but may have variable amounts of charging or discharging power levels as determined by the controller  106  or network  216 . As discussed herein, the DER  100  may be used for time of use arbitrage, duck curve mitigation, energy delivery when the utility needs to deliver more power to the grid, storing energy generated by a distributed renewable energy source  202  for use when the distributed renewable energy source  202  is generating less energy, sourcing or sinking reactive power for grid support, or for UPS functionality (collectively “DER use cases”). Accordingly, the determination is made whether to put the DER  100  in a charge state, a discharge state, a reactive power state, or idle state depending on the charge level of energy storage  102  and the DER use cases. 
     If the DER  100  is performing time of use arbitrage, the DER  100  may receive (e.g., via the communications module  108 , via the user interface  126 ) information relating to the price the utility providing power to the system  200  is charging during different time periods (e.g., a first time period, a second time period). For example, the utility may charge $ 0 . 11  per kilowatt hour during a first time period between noon and 4:00 PM and charge $0.15 per kilowatt hour during a second time period between 4:00 PM and 8:00 PM. In such a case, the DER  100  may store energy during the first time period and provide energy during the second time period. Accordingly, if the DER  100  is performing time of use arbitrage, energy storage is not fully charged, and the time is between noon and 4:00 PM, the DER  100  may be put in a charge state. Further, if the DER  100  is performing time of use arbitrage, energy storage is not empty, and the time is between 4:00 PM and 8:00 PM, the DER may be put in a discharge state. 
     Similarly, if the DER  100  is performing duck curve mitigation, whether the DER  100  is put in a charge state, a discharge state, or an idle state may be based on the amount of energy currently stored in energy storage  102 , the time of day, and configurations from the local utility or operator. As discussed above, to mitigate the duck curve, the DER  100  may store energy at midday and discharge stored energy in the evening when less energy is being generated by PV arrays. The utility may determine (based on generating capacity, current and past loads, the weather forecast, etc.) when it requires (or anticipates it will require) additional energy to be delivered to the AC grid from one or more DERs  100  in the utility&#39;s service area. If the utility requires (or anticipates it will require) additional energy to be delivered to the grid, the utility may determine (e.g., via the communications module  108  and network  216 ) the amount of energy currently stored in energy storage  102  in the DER  100  and command the DER  100  to enter a discharge state or alternatively command the DER  100  to not enter a charge state (thus adding more load to the AC grid). In order to have one or more DERs  100  with energy stored in energy storage  102  available for discharge, the utility may command the DERs  100  to enter a charge state during a period of time when the utility does not require additional energy to be delivered to the grid (e.g., during midday when PV arrays are delivering AC power to the AC grid). 
     Additionally or alternatively, the DER  100  may be communicatively coupled to the distributed renewable energy source  102  (e.g., via the network  216 ) and may receive a measurement or estimate of how much AC power the distributed renewable energy source  102  is generating. If the distributed renewable energy source  102  is generating AC power above a threshold level (e.g., 10% of the maximum power output of the distributed renewable energy source  102 , 20% of the maximum power output of the distributed renewable energy source  102 , etc.) and energy storage  102  is not fully charged, the DER  100  may enter a charge state. If the distributed renewable energy source  102  is not generating AC power above the threshold level and energy storage  102  is not empty, the DER  100  may enter a discharge state. 
     Additionally or alternatively, the DER  100  may be used for UPS functions. The DER  100  may determine that it is receiving AC power from the AC circuit and transmitting AC power to the load device  214  during a first time period and then detect that the DER  100  has stopped receiving AC power from the AC circuit during a second time period. In order to allow continuous operation of the load device  214 , if energy storage  102  is not empty, the DER  100  may enter a discharge state and open the disconnect  118  so that power generated by the DC/AC converter  104  does not back feed into the AC circuit. When the AC circuit has been restored to nominal operation as sensed through voltage sensing  111 , the controller  106  may synchronize operation of the DC/AC converter  104  to the AC circuit and then re-close the disconnect  118 . If the DER  100  is providing UPS functionality, it may be advantageous for the DER  100  to enter a charge state whenever energy storage  102  is not fully charged (and the DER  100  has not received a command to not enter a charge state). 
     If the DER  100  is set to a charge state, the method  300  continues on  FIG. 3B . If the DER  100  is set to a discharge state, the method  300  continues on  FIG. 3C . If the DER  100  is set to an idle state, the method  300  continues on  FIG. 3D . 
     Referring now to  FIG. 3B , a flowchart illustrating the actions performed by a DER  100  in a charge state is shown. At block  310 , the DER  100  determines whether the amperage difference (i.e., the maximum rated current—amperage of AC power transmitted to the load device  214 ) is less than a charge threshold value. The charge threshold value may be the minimum current required to charge energy storage  102 . Alternatively, the charge threshold value may be as low as 0 amps. If the amperage difference is less than the charge threshold value, the DER  100  may charge energy storage  102  at block  312 . In order to prevent the overcurrent protector  211  of the electric panel  210  from disconnecting power to the AC circuit, it is important that the DER  100  not draw more current than the amperage difference to charge energy storage  102  and may adjust the level of charge current to prevent exceeding the amperage difference. Accordingly, the DER  100  may only draw an amperage less than the amperage difference. If the amperage difference is greater than the charge threshold value, then the DER  100  does not charge energy storage  102  at block  314 . After block  312  or  314 , the method  300  may continue on to  FIG. 3D  discussed herein. 
     Referring now to  FIG. 3C , a flowchart illustrating the actions performed by a DER  100  in a discharge state is shown. At block  316 , the DER  100  determines whether the amperage difference is greater than 0 amps. If the amperage difference is greater than 0 amps, the DER  100  may discharge energy storage  102  and deliver AC power to the AC circuit at amperage less than the maximum rated current of the AC circuit at block  318 . As will be understood, if the amperage difference is 0 amps (or less), then the load device  214  is drawing the maximum rated current for the AC circuit (e.g., an AC branch circuit  212 ). If the DER  100  were to discharge AC power when the load device  214  is drawing the maximum rated current for the AC circuit, then a current higher than the maximum rated current may flow on the circuit between the DER  100  and the load device  214 , which may result in damage to the AC circuit, the DER  100 , the load device  214 , or the facility or the overcurrent protector  120  disconnecting the load device  214  from AC power. If the amperage difference is less than or equal to 0 amps, the DER  100  may not discharge energy from energy storage  102  at block  320 . After block  318  or  320 , the method  300  may continue on to  FIG. 3D  discussed herein. 
     Referring now to  FIG. 3D , a flow chart illustrating the action performed by a DER  100  providing grid support functions is shown. At block  322 , the DER  100  may measure (e.g., with the voltage sensor  116  and/or load sensor  122 ) the voltage of the AC circuit, the frequency of the AC circuit, and the reactive power drawn by the load device  214  (collectively “reactive power measurements”). At block  324 , the DER  100  determines whether to enter a reactive power state. As discussed above, the local utility may use one or more DERs  100  to provide voltage support and other functions to improve the quality of the AC grid based on one or more standards and the reactive power measurements. For example, a DER  100  may be used to provide reactive power in support of e.g., CA Rule  21 , Hawaii Electric Rule  14 , UL 1741 SA or pending changes to IEEE 1547 requirements. The DER  100  may enter a reactive power state based on a command received from the utility (e.g., from a remote server  218  via the network  216 ) or the DER  100  may determine itself whether to enter a reactive power state based on the reactive power measurements and one or more standards. Having determined to enter a reactive power state, the DER  100  may use energy storage  102  to source or sink reactive power based on the reactive power measurements and one or more standards at block  326 . It will be understood that the DER  100  may only source or sink reactive power if doing so does not result in drawing more current from the AC circuit than the max rated current of the AC circuit. After providing reactive power (or not) the method  300  may loop back to the start and repeat on  FIG. 3A . Reactive power operation may occur independent of the real power control mode of charging state, discharging state or idle state. 
     Referring now to  FIG. 4 , a flowchart illustrating a DER installation method  400  is shown. The method  400  may be performed to safely add a DER  100  to an AC circuit (e.g., an AC branch circuit  212  of a facility). At block  402 , the installer may deactivate the AC circuit. At block  404 , the installer may insert the plug  112  of the DER  100  into the receptacle coupled to the AC circuit. If the DER  100  includes a plug retention device  110 , the plug retention device  110  may be installed to secure the plug  112 . At block  404 , the installer may insert the plug of the load device  214  into the load receptacle  124 . At block  408 , the installer may activate the AC circuit, providing AC power to the DER  100 . At block  410 , the installer (or another user) may configure the DER  100  (e.g., inputting the maximum rated current of the AC circuit, setting up communication with the network  216 , etc.). At block  412 , the installer (or other user) may activate the load device  214  (e.g., by turning on an electric dryer). 
     Although specific embodiments have been described above, these embodiments are not intended to limit the scope of the present disclosure, even where only a single embodiment is described with respect to a particular feature. Examples of features provided in the disclosure are intended to be illustrative rather than restrictive unless stated otherwise. The above description is intended to cover such alternatives, modifications, and equivalents as would be apparent to a person skilled in the art having the benefit of this disclosure. 
     The scope of the present disclosure includes any feature or combination of features disclosed herein (either explicitly or implicitly), or any generalization thereof, whether or not it mitigates any or all of the problems addressed herein. Accordingly, new claims may be formulated during prosecution of this application (or an application claiming priority thereto) to any such combination of features. In particular, with reference to the appended claims, features from dependent claims may be combined with those of the independent claims and features from respective independent claims may be combined in any appropriate manner and not merely in the specific combinations enumerated in the appended claims.