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Contract No. DE-AC36-08GO28308

DC-connected Solar Plus 

Storage Modeling and Analysis 

for Behind-The-Meter Systems 

in the System Advisor Model

Preprint 

Nicholas A. DiOrio, Janine M. Freeman, 

and Nate Blair

National Renewable Energy Laboratory

Presented at the 45th IEEE Photovoltaics Specialist Conferences

Waikoloa, HI

June 10 – 15, 2018

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Suggested Citation 

DiOrio, Nicholas, Janine Freeman, and Nate Blair. 2018. “DC-connected 

Solar Plus Storage Modeling and Analysis for Behind-The-Meter Systems in 

the System Advisor Model.” Golden, CO: National Renewable Energy 

Laboratory. NREL/CP-6A20-71636.

https://www.nrel.gov/docs/fy18osti/71636.pdf.

Conference Paper

NREL/CP-6A20-71636

July 2018
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provided by U.S. Department of Energy Office of Energy Efficiency and Renewable Energy Solar Energy 

Technologies Office. The views expressed in the article do not necessarily represent the views of the DOE or the 

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or reproduce the published form of this work, or allow others to do so, for U.S. Government purposes.

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DC-connected Solar Plus Storage Modeling and Analysis for

Behind-The-Meter Systems in the System Advisor Model

Nicholas A. DiOrio, Janine M. Freeman, and Nate Blair

National Renewable Energy Laboratory, Golden, CO 80401, USA

Abstract—A detailed model for PV plus DC-connected batteries was developed. This model was compared to an existing ACconnected battery model in the System Advisor Model (SAM)

tool using a hypothetical Honolulu residence with a PV plus

storage system. The stand-alone PV system was shown to provide

the most value; however due to the increased difficulty of PV

systems being approved for grid interconnection in Hawaii, it

is valuable to install an AC or DC connected battery to enable

full PV self-consumption under a less restrictive interconnection

option. In that case both battery systems enabled full PV selfconsumption at the residence, requiring no power to be exported

to the grid. The DC-connected battery system captured energy

from the PV array which would have also been clipped due to

inverter limitations. For a PV system with DC/AC ratio of 1.23,

the battery configurations produced net-present values within

1% of each other, but when the DC/AC ratio increased to 1.85,

the DC-connected system increased the net-present value by

13%.

Index Terms—solar plus storage, topology, pv, batteries, acconnected, dc-connected, SAM,

I. INTRODUCTION

As costs for photovoltaic (PV) and energy storage have

continued to decrease, interest in additional opportunities for

coupling these technologies has increased. As PV penetration

climbs in certain regions, issues of grid integration have

become a primary concern, further raising the stakes in the

consideration of how to design combined PV plus energy

storage systems. For example, in Hawaii, high PV penetration

led Hawaii Electric Industries to close Hawaiis Net Energy

Metering (NEM) program in 2015 and replace it with a

customer self-supply option (CSS), where customers with

energy storage can receive expedited approval for PV systems

in high penetration areas, or customer grid-supply option

(CGS), where PV customers are compensated for exports with

a feed-in-tariff [1].

Consequently, behind-the-meter energy storage is of increasing interest to enable PV-self consumption and reduce

exports to the electric grid in high-penetration regions with

reduced or no compensation for excess generation, in addition

to providing conventional backup power, energy arbitrage, or

demand charge reduction services.

PV plus storage systems can be configured in several ways.

A PV system with an AC-connected battery assumes that

the storage system is packaged as a DC battery with battery

management system (BMS) and its own inverter/charger. The

AC-battery can then be charged from AC power from the

PV inverter or grid and discharge to meet AC loads. A

PV system with a DC-connected battery assumes that the

battery is connected behind a shared bidirectional inverter

and can be charged from regulated DC PV power. It may

also be considered that AC grid power can be sent through

the bidirectional inverter to charge the battery.

Other researchers have presented methods to optimize

power flow for grid connected PV systems with DC-connected

batteries. In [2], a detailed model is presented for the battery

DC/DC converter. In [3], the authors propose optimal power

management with dynamic programming, modeling converter

efficiency with a quadratic interpolation of an experimental

curve. In [4], the authors implement a linear programming

routine assuming a lossless inverter While each approach represents valuable progress in modeling PV plus DC-connected

battery systems, the models are largely written for a research

audience and aren’t easily accessible to homeowners or installers interested in answering how much value such a system

would provide to their site compared to another system.

Previous work has suggested that DC-coupling of PV plus

storage and allowing the storage to only charge from PV is the

most profitable battery design for large-scale systems due to

reduced balance-of-systems costs, reduced power electronics

costs, and the ability to fully capture the ITC [5]; however,

relatively little work has been done considering the tradeoffs at the residential scale. Considering multiple system

topologies presents a specific set of challenges, including

answering:

• Which solar plus storage system topology maximizes

system value in a PV self-consumption scenario?

• What is the ideal DC/AC ratio?

• Is it valuable for energy storage to be set up on the DCside to capture otherwise clipped PV power and store it

for later use?

We have integrated models of AC and DC-connected batteries into the detailed PV model in NREL’s System Advisor

Model (SAM) [6] to answer these questions 1. A model for a

battery energy storage system (BESS) connected on the DCside of a PV array will be presented with an example case

study that illustrate how to approach and answer the posed

questions.

II. MODEL

A DC-connected battery model was developed to consider

the case where a PV array can feed power to a battery

1Models are open-source at github.com/NREL/ssc, will be released in

official SAM version at sam.nrel.gov in summer 2018

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications. 1
Fig. 1. PV plus DC-connected battery storage system diagram.

Fig. 2. PV plus AC-connected battery storage system diagram.

through DC/DC power converters and share a common bidirectional inverter to send power to the electric load or charge

the battery from the grid. The DC/DC power converters

add additional system losses which must be considered. An

existing AC-connected battery model [7] will be used as a

point of comparison. Within the AC-connected battery model,

all power electronics conversions to and from the battery are

modeled as single-point losses. The AC-connected battery

has similar power flow calculations as the DC-connected

model without the complexity of the shared inverter. The DCconnected battery model can be summarized by a description

of Figure 1, which illustrates the assumed system design. The

assumed AC-connected battery system design is shown in

Figure 2.

There are several automated dispatch options available

within SAM for both behind-the-meter and front-of-meter

analysis as described in [8], which initialize the battery power

target for a specific application. This work will describe the

underlying behavior of a manual dispatch strategy where the

user programs their preferred battery operation. Within the

manual dispatch, a user assigns profiles to hours throughout

the day and year which specify whether the battery can charge

from PV, can charge from the electric grid, or can discharge

to meet the electric load.

At every time-step, the photovoltaic model computes the

net DC power and voltage of the PV array. The PV power

then can flow through an optional DC power optimizer,

which is modeled as a single point loss. The resulting DC

power and AC electric load are sent to the battery charge

controller, which determines how to operate the battery given

the programmed dispatch and system constraints. The charge

controller is an iterative controller which initializes the battery

power, checks constraints, and updates the power until all

constraints are met. This iterative approach is required due

to the non-linear response of the shared bi-linear inverter and

battery voltage.

A. Dispatch initialization

The initial battery power is calculated by considering if the

battery can discharge in the current time step, has a state-ofcharge (SOC) higher than the minimum SOC, and has less

PV power production than is required to meet the electric

load. In this case, the battery is initialized to the maximum

discharge power.

If these conditions are not satisfied and the battery has

greater PV production than required to meet the electric load

and can charge from PV, it is initialized with the excess PV

power. If the battery can charge from the grid, the battery

is initialized with the maximum charge power. The model

considers battery power discharge to be positive, and battery

charge power to be negative. The model also restricts the

programmed power over the time step to satisfy SOC and

current throughput limits.

B. Dispatch iteration

At this point, the dispatch iteration begins by requesting a

constant current to or from the battery using the previously

initialized battery power and the battery voltage from the last

time step. The battery capacity, voltage, temperature, and lifetime properties are updated, and the native DC battery power

before the battery management system (BMS) is calculated.

From this power, various power flow quantities throughout

the system can be calculated. These calculations depend on

whether the battery is discharging or charging.

1) Power Flow: Battery Discharging: The first quantity

of interest is the DC battery power on the other side of the

BMS at common interconnection point before the inverter.

The BMS is assumed to raise the battery voltage to interface

with the PV power and feed into the inverter.The effective

discharge power after the BMS is calculated as in (1). The

battery power combines with DC PV generation in (2).

Pbatt dc = Pbatt pre bms dc ∗ ηBMS (1)

Pdc = Ppv dc + Pbatt dc (2)

In (1), ηBMS is the assumed single-point efficiency of

the battery management system to either lower the charging

input voltage down to the battery bank voltage or raise the

battery discharge voltage to the PV voltage. The combined

DC power is sent through the shared inverter using either the

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications. 2
Sandia Inverter Model [9] or a Part Load Curve, which assigns

inverter efficiency as a function of output power percentage.

The outputs of the inverter model include the combined AC

power, weighted inverter efficiency ηinv, clipping losses due

to AC power limits, and other inverter losses. From these

outputs, the DC power can be decomposed into AC power

produced by the PV array in (3), and AC power discharged

from the battery (4). Calculated power quantities are assumes

to be AC unless otherwise indicated.

Ppv = Ppv dcηinv (3)

Pbatt = Pbatt dcηinv (4)

Remaining AC power quantities of interest include how

much of the PV and battery went to meet the electric load

or to the grid. While the battery is discharging, the model

assumes that PV power always serves the load first and that

excess PV generation is sent to the grid. Any unmet electric

load is served by the battery according to (5). Excess battery

power is assumed to go to the grid (6), and the grid is assumed

to make up any necessary deficit in serving the load (7).

Pbatt to load = min(Pbatt, Pload − Ppv to load) (5)

Pbatt to grid = Pbatt − Pbatt to load (6)

Pgrid to load = Pload − Ppv to load − Pbatt to load (7)

2) Power Flow: Battery Charging: If the battery is charging the effective power is calculated as in (8), which assumes

that in order to achieve the native charging power calculated

at the battery, additional power had to be sent through the

BMS, which has a single-point efficiency ηBMS

Pbatt dc = Pbatt pre bms/ηBMS (8)

There is additional complexity in calculating the charging,

since some of the PV may be used for charging and some

may go to serve the electric load. In a DC-connected battery,

PV power can charge the battery without being converted to

AC. If the battery is allowed to charge from PV, it is assumed

that all of the battery charging was from PV, while excess PV

goes through the inverter (9). If there was insufficient PV to

charge the battery, then the rest of the battery charging is

assumed to come from the grid (10). The net amount of DC

power through the inverter is calculated assuming one-way

power flow (11).

Ppv inv dc = Ppv dc − Ppv to batt (9)

Pgrid to batt dc = abs(Pbatt dc) − Ppv to batt (10)

Pdc = Ppv inv − Pgrid to batt dc (11)

The DC power is sent through the specified inverter model,

returning the AC power and weighted inverter efficiency. The

AC power components are again calculated by decomposing

the net AC power.

Ppv = Ppv inv dcηinv (12)

Pgrid to batt = Pgrid to batt dc/ηinv (13)

From here, the calculations are identical to the discharging

case, with the exception that Pbatt to load and Pbatt to grid

are zero.

3) Check Constraints: Once the power flow components

have been calculated, the model checks whether any constraints have been violated. The constraints present in the

model include:

• Minimum and maximum state-of-charge

• Maximum charging and discharge current

• Maximum charging and discharge power

• Disallow battery charging from grid unless explicitly

allowed

• Disallow PV to export to grid if the battery is charging,

is allowed to charge from PV and has additional capacity

to charge

• Force PV to meet electric load before charging the

battery

• Disallow battery export to grid while behind the meter

(assumes that self-consumption is primary goal)

If all constraints are satisfied, the model continues to the

next time step. If a constraint has been violated, the battery

is reset to its state at the beginning of the time step and

dispatched with the updated current level that satisfied the

constraint. As one constraint is satisfied, another may be

violated, so the iteration will continue until all constraints are

satisfied up to a limited number of iterations. As the battery

current is modified, the battery voltage and power will change,

and the efficiency of the shared inverter will change, resulting

in changes to the power flow calculations.

The battery state, including voltage, state-of-charge, temperature, and maximum capacity is tracked at every time-step,

imposing degradation based on user-specified cycle-life and

calendar-life effects.

III. CASE STUDY

To illustrate the novel DC model behavior, a case study was

developed for comparing three scenarios: PV Only, PV plus

DC-connected battery, and PV plus AC-connected battery.

The scenarios reveal trade-offs that must be considered when

adding energy storage to a PV system and present configuration options for navigating a complicated grid interconnection

environment.

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications. 3
A. System Assumptions

A residential scenario in Honolulu was considered where a

home has a peak-annual load of 3.4 kW with a proposed

4.7 kW PV array, DC/AC ratio of 1.23, and a 5kW, 24

kWh lithium-ion battery system to completely capture any

excess PV generation. The PV system was laid out with seven

335 Wdc modules per string and two strings of modules, all

feeding into one 3.8 kWac inverter. Under the scenario, the

behavior of a DC-connected battery system was compared

to an AC-connected system to understand differences in

operation and potential benefits for each configuration. The

system parameters are shown in Table 1.

B. Cost and Financing Assumptions

The system costs were assumed to be the same for each

configuration, taken from the 2017 NREL Annual Technology

Baseline (ATB) [10]. Storage costs are from residential GTM

behind-the-meter cost trends [11]. Financial assumptions for

the analysis were taken to be the SAM default residential

settings adjusted for Hawaii specific state-taxes and incentives. The battery is not allowed to charge from the grid in

either configuration, so that the full federal investment tax

credit (ITC) can be captured [12]. System costs and financial

assumptions are shown in Table 1.

C. Electricity Rate and Metering Assumptions

The base electricity for each scenario was assumed to be

the Hawaiian Electric Company Inc Schedule R Residential

Service, Single Phase rate as specified in the OpenEI Utility

Rate Database [13]. The electricity rate bills the first 350

kWh of the month at $0.257/kWh, amounts between 350-1200

kWh at $0.268/kWh, and anything over 1200 kWh/month

at $0.287/kWh. The rate also assumes a fixed charge of

$10.27/month.

All PV-only scenarios assume that the customer qualifies

for the CGS option and can export excess PV energy at the

feed-in tariff rate. This rate was taken to be the reported

$0.1507/kWh on Oaha [1]. CGS customers must pay a

$25/month minimum charge.

All PV-plus-storage scenarios assumed the customer qualifies for the CSS option and cannot export excess PV energy.

It is also assumed that CSS customers must pay a $25/month

minimum charge.

D. Results

Figure 3 shows the PV generation and load profile for a

sample day where PV generation exceeds the inverter capacity

in the middle of the day. Some minor clipping of PV power

due to the inverter AC power limit occurs at noon. The electric

load rises in the early morning, levels off in the middle of

the day and increases in the early to mid evening hours.

Figure 4 shows the same day with the DC-connected

system. System operation with a PV plus DC-connected

battery results in the PV array meeting the full electric load

in the middle part of the day and charging the battery with

Fig. 3. The site electric load and PV production on March 24

Fig. 4. Operation with a DC-connected battery on March 24

excess generation. The DC-connected battery system can fully

capture all excess PV generation for the day.

System operation with a PV plus AC-connected battery

system is similar in most regards, but due to the configuration

requiring PV power to be inverted before going to the battery,

any excess PV generation is clipped. This is illustrated in

Figure 5.

Throughout the year, these clipping opportunities accumulate but are minimal, at 0.07% of the annual energy

produced by the PV system. This result illustrates that for

this site with a PV system with a DC/AC ratio of 1.23, the

magnitude of clipping is very small. Throughout the year, PV

power exceeds the inverter input power limits 49 times and

is clipped. The average power clipped during each event is

0.118 kW, demonstrating that the PV array rarely exceeds the

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications. 4
TABLE 1

SYSTEM DESIGN COSTS, FINANCIAL ASSUMPTIONS

Variable Value Variable Value

PV Capacity 4.7 kWdc PV system $2.92/Wdc

Inverter Capacity 3.93 kWdc PV O&M $24/kW-yr

Inverter Efficiency Modeled Battery System $2000/kW

Battery Power 5 kWdc Analysis Period 25 years

Battery Capacity 24 kWhdc Inflation Rate 2.5%/year

Battery DC to DC Efficiency 98% Nominal Discount Rate 8.14%/year

Battery AC to DC Efficiency 96% Federal Tax Rate 30%/year

Battery DC to AC Efficiency 96% State Excise Tax 4%/year

Battery Replacement Capacity 50% State Tax Credit 35% up to $5000

Fig. 5. Operation with a AC-connected battery on March 24

inverter capacity and does not exceed it by a large degree.

Table 2 shows the lifetime economics of these systems

which have the same size and assumed cost of components,

varying only in system topology. The AC-connected battery

operates more efficiently than the DC-connected system,

resulting in slightly higher value in this case.

E. Sensitivity Analysis: Increase DC/AC ratio

The value of a DC-connected battery improves as the

DC/AC ratio of the PV system increases. Possible reasons for

boosting the DC/AC ratio include trying to levelize the system

AC output throughout the day, keeping plant AC output

consistent over the module lifetime, and taking advantage of

time-of-use utility rates with a DC-connected battery charged

from excess PV generation.

If an additional string of seven PV modules were added to

the inverter, the PV capacity would increase to 7.04 kWdc,

and DC/AC ratio would increase to 1.85. In this scenario,

1220 kWh/year (12%) of PV power would be clipped. In

this case, the DC-connected battery has increased value as

illustrated in Table 3.

IV. DISCUSSION

The most beneficial scenario is if a PV-only system can

be installed under Hawaiian Electrics CGS program using

the most recently available feed-in-tariff [1]. Adding an AC

or DC connected battery decreases the annual utility bill, but

reduces the lifetime value due to increased capital costs from a

battery purchase in Year 0 and during later years depending on

battery operation. In the PV plus battery cases, no PV power

is exported to the grid, making the customer eligible for the

CSS program, potentially making it easier to get approved

while still enjoying most of the benefit of installing a standalone PV system.

No benefit is attained by installing a battery on the DCside rather than the AC-side for a PV system with a DC/AC

ratio of 1.23. In the DC-system energy which would have

otherwise been lost due to clipping can be captured, but the

lost energy is not large enough to justify installing the DC

system over AC system.

When the PV system DC/AC ratio is increased to 1.85,

the DC-connected battery system shows increased value over

an AC-connected battery due to increased opportunity for

charging from PV power that would have otherwise been

clipped, illustrating that the DC-connected battery topology

holds promise for sites where it makes sense to install PV

systems with high DC/AC ratios, including locations beyond

Hawaii which were not studied here.

V. CONCLUSION AND FUTURE WORK

A detailed model for PV plus DC-connected batteries was

developed in SAM. This model was compared to an existing

AC-connected battery model using a hypothetical Honolulu

residence with a PV plus storage system. The stand-alone

PV system was shown to provide the most value if the

customer is approved for the Customer-Grid Supply option.

If a customer is not approved, it is valuable to install an

AC or DC connected battery to enable increased PV selfconsumption under the Customer Self-Supply option with

no compensation for exporting PV power to the grid. The

DC-connected battery system captured energy from the PV

array which would have also been clipped due to inverter

limitations. For a case with a PV DC/AC ratio of 1.23, the

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications. 5
TABLE 2

CASE STUDY RESULTS: 1.23 DC/AC RATIO

Variable No PV PV Only PV plus DC battery PV plus AC battery

Year 1 Utility Bill $3,973 $1,926 $1,985 $1,984

Year 1 Energy Clipped 0 kWh 5.66 kWh 0 kWh 5.66 kWh

Year 1 Battery Energy Charged 0 kWh 0 kWh 2,256 kWh 2,179 kWh

Year 1 Battery Energy Discharged 0 kWh 0 kWh 2,027 kWh 1,969 kWh

Net Present Value $0 $19,948 $12,186 $12,402

TABLE 3

CASE STUDY RESULTS: 1.85 DC/AC RATIO

Variable No PV PV Only PV plus DC battery PV plus AC battery

Year 1 Utility Bill $3,973 $1,240 $1,045 $1,345

Year 1 Energy Clipped 0 kWh 1220 kWh 4.21 kWh 1220 kWh

Year 1 Battery Energy Charged 0 kWh 0 kWh 5,435 kWh 4,152 kWh

Year 1 Battery Energy Discharged 0 kWh 0 kWh 4,886 kWh 3,756 kWh

Net Present Value $0 $24,315 $16,247 $14,384

different battery configurations produced net-present values

within 1% of each other, indicating that system topology was

not important. For a case with a PV DC/AC ratio of 1.85, the

DC-connected battery system produced a net-present value

13% higher than the AC-connected battery system due to

increased opportunity to capture PV power which would have

been clipped.

Future work will consider system topology, value streams,

and control for large front-of-the-meter PV plus battery

systems who have an incentive to install high DC to AC

ratios. It would also be of interest to implement the model

in a mixed-integer linear program and determine the optimal

operation for a tightly coupled system. Finally, the converters

which are currently modeled as single-point efficiencies may

be expanded to non-linear response models.

ACKNOWLEDGMENT

This work was authored by Alliance for Sustainable Energy, LLC, the manager and operator of the National Renewable Energy Laboratory for the U.S. Department of Energy

(DOE) under Contract No. DE-AC36-08GO28308. Funding

provided by U.S. Department of Energy Office of Energy

Efficiency and Renewable Energy Solar Energy Technologies

Office. The views expressed in the article do not necessarily

represent the views of the DOE or the U.S. Government. The

U.S. Government retains and the publisher, by accepting the

article for publication, acknowledges that the U.S. Government retains a nonexclusive, paid-up, irrevocable, worldwide

license to publish or reproduce the published form of this

work, or allow others to do so, for U.S. Government purposes.

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This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications. 6