Is Power from a PV System Used by the Correct Apartment? - Page 1
Forum section: electronics / general

author: Michaelpier
author_rank: Contributor
timestamp: September 03, 2024, 08:39:42 pm
content: Hi everyone,In a building with 8 apartments, one of the apartments (specifically apartment number 7) recently installed a PV system on the roof. The PV system is connected straight to the mains after the main switch in the meter room, where all the other apartments' main switches are also located.I’m curious about how the power generated by this PV system is managed. How can we ensure that the electricity produced by the PV system is first used by apartment 7, and if apartment 7 doesn’t need the power, then it will be sent to other apartments or back to the grid? How does the inverter determine apartment 7 receives the power?Are there specific technical setups or principles that ensure this? How does the connection work in such multi-apartment buildings to manage this scenario effectively?

author: ahbushnell
author_rank: Frequent Contributor
timestamp: September 03, 2024, 11:18:02 pm
content: How is the system metered? Is there a meter for each apartment? Which side of the meter is the PV connection?

author: Michaelpier
author_rank: Contributor
timestamp: September 04, 2024, 11:54:44 am
content: Each apartment has its own meter and main switch. For Apartment 7, the path to the inverter is as follows: from the grid to the fuse, then to the meter, and to the main switch. From the main switch, electricity is distributed to both the apt and the inverter.The PV is a net metering system.

author: Zero999
author_rank: Super Contributor
timestamp: September 04, 2024, 01:07:05 pm
content: If the apartment's PV system is connected to a grid tie inverter, then if it's not used locally, the power will be distributed to the other apartments or buildings on the grid.Electricity follows Ohm's law, so the place where it's being generated, the voltage is higher and most of it will go the part of the circuit with the lowest resistance i.e. highest load. Think of it like water flowing down a hill. Most of it will flow though the deepest channels which have the least resistance.The inverter doesn't need to know about the other loads on the grid. It just feeds a constant current, up until the point when the grid voltage is too high or the frequency drifts significantly

author: Michaelpier
author_rank: Contributor
timestamp: September 04, 2024, 05:55:24 pm
content: QuoteWhy not the inverter send the power to the neighborhood apartment first, and let number 7 draw electricity directly from the grid? Whats the technical/theory ?

author: Someone
author_rank: Super Contributor
timestamp: September 04, 2024, 11:52:39 pm
content: Quote from: Michaelpier on September 04, 2024, 11:54:44 amWithout a country to associate this with "net metering" means almost nothing:https://en.wikipedia.org/wiki/Net_meteringIs there only a single meter on apartment 7 ? And both the solar and all loads in the apartment are on the same side of that meter? If so then that is correct and normal, it is like a canal of water. Excess that apartment 7 is not using will flow back out and feed everyone else, but that will be counted by the meter accurately.

author: Siwastaja
author_rank: Super Contributor
timestamp: September 05, 2024, 06:03:55 am
content: Zero999 is right that electricity is "automagically" used more locally than remotely, but there is no way to tag the electrons with names (besides, in AC circuit the same carriers move back and forth so you really can't tag them even in your imagination). But what Zero999 is missing is billing, and most of us are interested in our billing, not just general good.I always struggle to understand textual descriptions. Therefore a wiring schematic showing exactly the locations of energy meters and the inverter would be helpful.Plus the explanation what is exactly meant by net metering. It varies by place and time. For example, here we are in transition from hourly sum of buy + sell over all 3 phases into 15-minute sum. And of course, how that metering affects the billing, not only how the exported production ismeasuredbut also how it is billed.

author: Michaelpier
author_rank: Contributor
timestamp: September 05, 2024, 10:24:57 am
content: Net metering works like when the solar panels generate electricity, it powers your home first. Any extra electricity that isn’t used right away is sent to the national grid, and you get credit for it. You can then use those credits when the sun isn’t shining, like at night.

author: Siwastaja
author_rank: Super Contributor
timestamp: September 05, 2024, 10:59:47 am
content: Well, in this configuration Apartment7 gets all the benefits in billing - they get the self-use subtracted from their cost, they also get paid for the electricity sold out, or get the export credits or penalties or however the local system works.It is as if Apartment7 owned the whole PV system, and others had nothing.You need some kind of monetary compensation system in place, it would need to combine consumption data from all of the apartment meters plus production data out of the inverter, then you could share the monetary value of production between the apartments. This means both extra hardware and software. Or you can just make estimates and agree on fair compensation.

author: Zero999
author_rank: Super Contributor
timestamp: September 05, 2024, 12:07:56 pm
content: Quote from: Michaelpier on September 04, 2024, 05:55:24 pmI already told you the theory. It's Ohm's law. The electrical power is simply consumed by the load drawing the most curremt, i.e. the one having the lowest resistance. If the apartment with the inverter isn't using much power, but but someone in the neighbouring building is, say a shop, then said shop will get the power. If the meter in the apartment with the inverter is set up properly, then account will be creddited with the electricity generated and the shop will pay for it.

author: Siwastaja
author_rank: Super Contributor
timestamp: September 05, 2024, 04:34:31 pm
content: It's like water pipes. Say you have a ground water source, a well, with a pump in it. You then have T pieces after the pump going to different usage locations. Water gets where a tap is opened and water needed by simple laws of physics. You can't ask the water pump to "send" water to any specific location. You sure can add water meter(s) wherever you want and they will measure accurately how much going through that very meter, but nothing else.

author: Michaelpier
author_rank: Contributor
timestamp: September 05, 2024, 04:56:37 pm
content: Quote from: Zero999 on September 05, 2024, 12:07:56 pmIn my example, apartment 7 has a minor load of 300W, making it a "larger" resistance, while apartment 8, running four air conditioners and an oven (a total of 10kW), represents a much smaller resistance. Are you suggesting that apartment 8 will draw power from the inverter first, while apartment 7 will source its power from the grid, assuming the inverter can't cover both loads?

author: Ian.M
author_rank: Super Contributor
timestamp: September 05, 2024, 05:03:40 pm
content: Quote from: Siwastaja on September 05, 2024, 10:59:47 amQuote from: Michaelpier on September 03, 2024, 08:39:42 pmIf Apartment 7 did pay for the installation and the section of roof its on is owned and maintained by Apartment 7, there's nothing to see here and the other apartments should have no expectation of any benefit from the system. OTOH if the roof area in question is a common part, terms and remuneration for leasing the necessary roof space should have been agreed with Apartment 7 before installation, bearing in mind that the presence of solar panels and the roof penetrations required to mount them do increase the risk of roof leaks and increase the cost of future roof maintenance. Whether or not a lease agreement can be negotiated after installation is a matter for the building management committee as advised by its lawyer.Everything 'upstream' of the utility company meters is 'the grid'. The fact that some of the grid electricity is being generated by apartment 7 is irrelevant to the other apartments. It is unlikely that any utility company will allow feed-in credit from one customer to be shared with others.

author: Michaelpier
author_rank: Contributor
timestamp: September 05, 2024, 05:23:32 pm
content: Quote from: Ian.M on September 05, 2024, 05:03:40 pmI'm really interested in understanding how an inverter works and how electricity gets distributed, rather than focusing on the legal aspects.

author: Siwastaja
author_rank: Super Contributor
timestamp: September 05, 2024, 06:49:54 pm
content: Quote from: Michaelpier on September 05, 2024, 05:23:32 pmReally, inverter just pushes energy and it goes where consumption is. Inverter is in parallel with all the house loads, after the meter. Loads consume what they consume; if inverter produces more, rest flows through the meter into grid.E.g. inverter 1000W, loads 1000W, 0W goes through meter. Inverter 1000W, loads 500W, 500W goes through meter into grid (aka export aka sell). Inverter 1000W, loads 1500W, 500W goes through meter from grid (aka import aka buy).Other tenants play no role in any of this. They just buy from grid, just like the house next door, and the house 100 kilometers away. We can build theories how closer-by customers somehow get larger share but it is hard to calculate and irrelevant in any case because buildings in many kilometers of radius could be sharing pretty significant percentages. This is just how grid works, all the electricity is "common" to everyone, there is no way to label from which power plant is which electron coming from. Therefore meters are installed where billing matters.Even if not interested about legal aspects, are you interested in money at all, or just Common Good? Because now if your schematic is accurate, Apartment7 completely owns the whole PV system and all gains from it, and other tenants are in similar position than e.g. me here, not getting anything out of it. So hopefully Apartment7 completely paid for the system as well. If not, you need to come up with some kind of compensation system. In most countries there ultimately is some sort of freedom of contract so depending on how difficult your neighbors are you might be able to come up with some simple compensation system and still everybody being happy. There probably are also governmental/legal ways to deal with this situation because it's a common one.

author: Zero999
author_rank: Super Contributor
timestamp: September 05, 2024, 07:12:23 pm
content: Quote from: Michaelpier on September 05, 2024, 04:56:37 pmYes, that's exactly what will happen. The inverter in apartment 7 will increase the voltage slightly, causing the electrical power to flow into apartment 8 and contribute power to the air conditioners. This is why it's important for apartment 7 to have their meter set up properly, so they get paid for generating the power.

author: Michaelpier
author_rank: Contributor
timestamp: September 05, 2024, 07:58:59 pm
content: Quote from: Zero999 on September 05, 2024, 07:12:23 pmThank you for your explanation. However, from my understanding, it seems unclear because I thought power or current in a contactor can't flow in both directions at the same time. Could you please clarify this for me?

author: Michaelpier
author_rank: Contributor
timestamp: September 05, 2024, 08:14:21 pm
content: Quote from: Siwastaja on September 05, 2024, 06:49:54 pmYes, let’s say the PV system is only for Apt 7. Your water analogy made a lot of sense to me. From what I understand, Apt 7 gets first use of the power, no matter what’s happening with other apartments or the grid. The reason is simple: power can’t flow in two directions at once. So, Apt 7 uses the power it needs from PV, and when that’s not enough, it pulls the rest from the grid. If there’s extra power, it can be sent back to the grid. What are your thoughts on this?

author: Someone
author_rank: Super Contributor
timestamp: September 05, 2024, 09:53:33 pm
content: Quote from: Michaelpier on September 05, 2024, 07:58:59 pmIts like water, we've said this a few times now. If the inverter is making power (pouring water into the canal/pipe) and people are drawing power (taps letting water out) then the flow through the #7 meter can only be in one directionapartment 7 is using more power than generating, power coming inapartment 7 is using less power than generating, power going out

author: Someone
author_rank: Super Contributor
timestamp: September 05, 2024, 09:59:05 pm
content: Quote from: Zero999 on September 05, 2024, 07:12:23 pmI'm not sure that will happen from a metering point of view, as:"while apartment 7 will source its power from the grid, assuming the inverter can't cover both loads?"The #7 meter will not be counting incoming power when inverter generation > #7 consumption, no matter what the other loads.Then it gets wildly complex with how the outflowing energy through the meter is shared, theoretically every single user of power gets some (usually tiny) fraction of that solar power delivered back into the grid.

author: TimFox
author_rank: Super Contributor
timestamp: September 05, 2024, 09:59:24 pm
content: Direction of net power flow on a wire carrying AC is determined by the phase relationship between the current along the wire (in a defined direction) and the voltage on the wire.If DC, then the same thing except it is the polarity of the current along the wire and the voltage on the wire.Note that the current (scalar) is the integral of the current density passing through an area that the wire penetrates, and the current density is a vector quantity.

author: Siwastaja
author_rank: Super Contributor
timestamp: September 06, 2024, 07:51:19 am
content: In AC system, current always flows alternatively, in both directions. Therefore talking about current is meanignless, and we should say "power flow". While the direction of current alternates back and forth 50/60 times a second, voltage alternates in sync and because power = voltage * current, direction of power does not alternate. E.g. +1A * +230V = +230W, and 10 milliseconds later -1A * -230V = still +230W. Direction of power is simply from balance of consumption and load as explained above.If it is a 3-phase system, it's possible some of the phases export and some import at the very same time. And some types of loads can change pretty abruptly, too. So it is possible to import/export "at the same time" or "nearly at the same time". Whatever "net meter" exactly means depends, but I think atvery leastit would calculate net sum over all three phases, so that this doesn't matter. Usually "net metering" also implies summing production and consumption oversome time periodso that duringthat period, there is no simultaneous buy/sell (even if therephysicallyis).For example, you could sell every other second and buy every other by turning your electric stove on and off all the time. Here we are transitioning into 15-minute net interval. That would mean summing all those numbers together so that during 15 minutes, you only sold or bought, not both.If Apartment7 bought the whole PV system and got permission to use common roof etc. or whatever (you get the point), then the role of other apartments is no different at all compared to the next house, and the next, and the next. They self-use what they can, and rest is sold to grid for common use by everybody on that grid (possibly spanning many countries!), and your other apartments are as much consumers as everybody else.

author: Zero999
author_rank: Super Contributor
timestamp: September 06, 2024, 10:24:43 am
content: Quote from: Siwastaja on September 06, 2024, 07:51:19 amQuote from: TimFox on September 05, 2024, 09:59:24 pmQuote from: Someone on September 05, 2024, 09:59:05 pmIt's true, power can't go down a cable in boh directions simulaniously, but it can travel in both directions, just not at the same time.And please don't complicate things. The original poster is only just starting to grasp Ohm's law and that power flows from the part of a circuit with a higher potential, compared to the load.

author: Michaelpier
author_rank: Contributor
timestamp: September 06, 2024, 03:00:23 pm
content: Quote from: Zero999 on September 06, 2024, 10:24:43 amYou're suggesting that apartment 8 will draw power from the inverter first, while apartment 7 will rely on the grid if the inverter can't support both loads. How would this setup work? According to the diagram below, power would need to flow in both directions for this to happen, which, as we've agreed, is not possible.Can you elaborate please? Forgive me but it seems like there's a contradiction in what you're saying.

author: Siwastaja
author_rank: Super Contributor
timestamp: September 06, 2024, 04:14:31 pm
content: Does this help? (Your picture was lacking number for PV production, so I made it 1000W for the example. If it was, say, just 200W, then the direction of Apartment 7's arrow would change.)

author: Michaelpier
author_rank: Contributor
timestamp: September 06, 2024, 04:57:15 pm
content: Yes, it seems that's the logical conclusion. Power cannot flow in both directions through a cable at the same time. As a result, Apartment 7 should be supplied first from the PV system, with the remaining 700W flowing to the other apartments(in our example to Apt8)

author: Zero999
author_rank: Super Contributor
timestamp: September 06, 2024, 05:08:38 pm
content: Quote from: Michaelpier on September 06, 2024, 03:00:23 pmImagine a large factory with a compressed air system. The compressed air is used to pump up tyres, blow dust away, power hand tools i.e. screwdrivers, drills etc. The main compressor has a control system which maintains a roughly constant pressure, irrespective of how many things are connected to it. If someone were to connect their own compressor to one of the air outlets, then the pressure around it will increase slightly and power tools within the immediate vicinity. This main compressor's control system will reduce the power output of the compressor, as the pressure has increased slightly.The same thing happens with an PV inverter connected to the mains. The voltage near the inverter will increase, causing the main generator to reduce its power output and the inverter will power the loads in the immediate vicinity.

author: Michaelpier
author_rank: Contributor
timestamp: September 06, 2024, 05:28:42 pm
content: This example attached suggests that the inverter first sends power to its own line, regardless of the loads or resistance. Any excess power then flows to the next load on the easiest path. It seems like the grid acts as a 'wall of resistance,' so the inverter's power prefers to flow to the loads opposite the grid. I dont know if that make any sense

author: Michaelpier
author_rank: Contributor
timestamp: September 06, 2024, 05:33:56 pm
content: Quote from: Zero999 on September 06, 2024, 05:08:38 pmWe agree on this.

author: Andy Chee
author_rank: Super Contributor
timestamp: September 06, 2024, 05:57:18 pm
content: Quote from: Michaelpier on September 06, 2024, 05:28:42 pmIf it makes it easier for you to think, the inverter is "smarter" than the grid and can adjust its own "resistance".When the inverter adjusts itself to have lower resistance than the grid, then the power will flow from the inverter to load, instead of the grid to load.This is somewhat close to how a grid inverter operates in reality. The terminology "resistance" is incorrect.

author: Siwastaja
author_rank: Super Contributor
timestamp: September 06, 2024, 06:10:32 pm
content: Quote from: Andy Chee on September 06, 2024, 05:57:18 pmI'm sorry but I find this confusing, I don't understand the point at all. You are using word "resistance" as a metaphor, but to achieve what?Especially the smartness comment is very misleading because it has absolutely nothing to do with direction of power flows, no smartness needed for that, same works for a dumb rotating generator.

author: Andy Chee
author_rank: Super Contributor
timestamp: September 06, 2024, 06:24:29 pm
content: a) I prefer not to use the word "resistance" becuse it is definitely the incorrect physics adjective, however the OP used that name to label a concept in their thinking. I simply reused the same label in order to reference the same concept in the OP's mind.b) same thing with "smart". I completely understand that a dumb generator can shift phase in order to achieve the required power flows. However "phase" is a concept that is clearly beyond the OP's grasp of understanding!The point is to bridge the gaps in the OP's technical understanding and technical vocabulary.

author: Someone
author_rank: Super Contributor
timestamp: September 06, 2024, 09:19:29 pm
content: Quote from: Michaelpier on September 06, 2024, 05:28:42 pmThat is true as an approximation/analogy which is accurate enough for your original question and for all practical purposes related to the billing/metering. But it is not technically true so if you start making a detailed model then that concept of walls/exact quantities falls apart.