Solar Questions

[Charlie W]

Before I get into the guts of it, I want to start with praise for this site. When I was looking into panels 4 or so years ago, this place was invaluable. Truth without the hype, and I can't tell you how much I appreciated it. I humbly ask for some patience, and promise that I will listen.

My going-in belief (which might be wrong, and if it is, I want to know) is that the typical panel array is grid-tied, and send all of the juice back to the utility. The result is a credit against use, typically at retail rates, i.e., you trade a kWh for a kWh hour, and sometimes the meter runs backward. But when it comes to drawing power, you do it from the utility rather than the panels themselves. I believe this because of the stories I've read about California panel users being surprised that their panels didn't supply electricity during power failures there.

I know, or think I know, that panels are DC, and the inverter converts it to AC. I believe that you can go straight from the inverter to the house, but I believe (see above) that most panel systems don't do that but send everything to the utility. So the first question is: Is that true, and if so, roughly what share of panel systems send everything to the utility vs. those that can send it to the house.

Second question is what happens if someone has a backup generator for power failures. (I know that Calif is banning generators -- the idiots -- but we aren't in California.) Generators are very popular where we live, and not restricted. If the generator comes on, does the connection from the panels need to be shut off? Is there a safety issue? Could appliances be fried in that situation?

I am asking this on behalf of a neighbor who has panels but doesn't know much of anything about his system, which came with the house that he bought. Rather than wing it, I am coming here to ask people who know more than I do.

Thanks in advance, and I hope that this isn't too dang long. By the way, if anyone has any use for it, I have a pretty deep understanding of telecommunications technology, and am happy to share that knowledge, patiently, with anyone who has questions, if there's any demand for it here.

[SolTex]

Charlie W,

I am a fairly new PV enthusiast. Our system was turned on the end of Mar 2022. I have been reading and learning as much as possible, but I'm far from being a PV expert. We have a Enphase based 20 kW (nominal) ground mounted PV system with two 10 kWh battery modules. Our system is wired for full house backup, which means everything in our house is powered when the grid goes down.

From what I have read, most basic grid-tied PV systems, during normal operation, first attempt to supply the consumption requirements of the dwelling, and then if there is any excess production send that to the grid. The priority is directly supplying consumption. If a PV system does not produce enough power to fully cover the consumption, then grid power will be imported to make up the difference. There is a LOT of variation in how the excess production being exported to the grid is metered and credited to the account. Some old school mechanical electric meters might actually "run backwards" but in most cases electric providers now provide electronic meters that record the power imported from the grid on one channel, and record the power exported to the grid on a second channel. At the end of the month the owner is billed for the actual amount of power imported. The owner MAY also get some kind of credit for the amount of power exported, but as I said this varies greatly. The contract between the system owner and the utility will spell out the details of how exported power is metered and credited.

A basic no-frills grid-tied PV system must be connected to the operational grid to operate. If the grid goes down, the PV system goes down. Hence the stories you hear... You CAN have a grid-tied PV system that will continue to operate normally even when the grid goes down. This type of PV system usually includes some backup batteries, although there are special inverters that will allow limited backup capability even without any batteries in the system. (Google Enphase Sunlight Backup for more info)

There are ways to integrate a backup generator into a PV system. Each inverter manufacturer will have special inverter models that allow generator hookup. This is something that must be carefully planned during the design phase to ensure everything will "play nice" together.

Hope these quick answers are helpful. I think you are in the right place to ask questions. There is a tremendous store of PV knowledge represented here on this Forum. And best of all, it is free!

[Mike90250]

Welcome. I'm glad you've found this site to be useful in the past. Most of us are pretty straight shooters, and may plug the highlights of our own gear.

to your questions

Grid Tie Back Feed. You almost got it right. There are time of use costs, and many different plans, but they all generally revolve around selling you power from the grid if you need it, at prevailing rates, ( say $0.30 KWh ) Any surplus you generate in the daylight hours, the "Grid" buys for $0.03 KWh. Ouch !! What you generate, you use on site, without going through the meter. If you have a Zero Export agreement, your inverter sends nothing to the grid, and some power is unused.

Backup generators.
They will need a transfer switch and a sub panel for your critical loads ( Light's, kitchen, well pump/sump pump, garage door opener, minimal entertainment systems, minimal HVAC systems.) You will be paying for the fuel, so selecting your loads is important. And very few folks need more than a 40Kw generator. Fuel can be gasoline, propane/nat gas or diesel, whatever you think will be least expensive and most available in your neighborhood long term. Buying too large of a generator just results is burning more fuel.

There are also Hybrid Inverters that require a battery, but can utilize your PV array in case of a grid failure, saving you some fuel in the sunny part of the day. Just costs more $$ to be more flexible !!

[Charlie W]

Mike, I am familiar with the transfer switch and subpanel requirements for a generator, so I don't need help there. I guess what I'm trying to figure out is whether the generator flipping on during an outage will interfere with the panels, or vice versa.

Let's imagine an outage at 1 p.m. on a summer day. The panels are producing. The generator comes on. If it's not hybrid, then the panels simply stop producing and the power from the generator supplies the house and my neighbor doesn't have to worry about any surges as a consequence of having both panels and a generator, correct? If it's hybrid, then the panels send juice to the house. Would they go into the same sub-panel that the generator uses? Are there two sub-panels? Is there any conflict between the juice from the panels and the juice from the generator?

Does that question make sense? I hope so!

[SolTex]

Charlie,

I am only familiar with Enphase products, so I will supply a link below to a technical brief that answers your question. The brief covers some of the details involved in the integration of a generator into a PV system. I hope I am not violating any forum rules by posting this link. I am in NO way pimping for Enphase or trying to sell their products. There are several other major players in the PV market that offer comparable (but different) solutions to the same question. At least you will see that there IS at least one way to allow both a generator and a PV system to simultaneously supply a home with power during a grid outage.

https://enphase.com/sites/default/files/2021-10/Enphase-Generator-Support-Technical-Brief.pdf

[Charlie W]

Thank you, SolTex. I have asked my neighbor for more details about his solar system. My guess (which might be wrong) is that he does not have a hybrid system. I hope he doesn't, because I think it's a waste of money in our area. Outages here don't happen in the clear blue sky. They happen during storms and heavy snow that bring trees into power lines. If we have a mid-day outage, it's because of work on the distribution network. Our electricity costs 9.62 cents/kWh, which is why I didn't install panels. My neighbor wouldn't have done it either, but they came with the house. Not only that, but we don't have time-of-day pricing. It wouldn't make any sense where we are.

So I'm trying to know whether adding a generator will screw things up. From what I've managed to glean so far, the answer is that if it's a non-hybrid grid-tied system, he's got no worries. If I find out that the prior owner installed a hybrid system ("Just put in the best and greatest, screw the cost"), then my next question will probably be how to bypass it entirely. That Enphase document seems to imply that some more equipment than just the hybrid inverter might be required. At the very least, some batteries. And to what end? At the current inflated cost of propane, electricity would cost about 70 cents/kWh. (Thanks so much, Joe.) The 60-cent differential would require a lot of hours of operation to beat a generator. It would work only for pretty short outages.

I'd already figured it out (sans the numerical semi-precision) before coming here. I even question whether it makes any sense in time-of-use jurisdictions. Where we are, solar panels strike me as virtue-signaling devices, vanity for rich liberals. Hey, that's okay. I'm a rich winger, and my 1-ton pickup is my vanity statement. Each to their own. The solar weenies around here probablly laugh at me from their Prius or Tesla. At least we're laughing, right? Really, I just want my neighbor to know that if he adds a generator, he's not going to fry the family cat. LOL

In any case, SolTex, thank you very much for your reply and for the Enphase document. Regardless of my sparks off the wheel, I really appreciate it.

[Mike90250]

in a non hybrid system, the Grid is wired to the house main panel. When the Grid faults, the panels shut off, no relays / switches needed
The generator is wired between the main panel and it's critical load sub panel. A transfer switch ( manual or automatic ) is installed between the
critical load sub panel and the main panel. Solar never sees the generator . When grid returns, non critical loads power up and you then disconnect and shut down the generator

[peakbagger]

Have the neighbor spend a few bucks and buy Solar Power your Home for Dummies. Its a wise investment and he can lend it to you when he is done reading it.

It is likely that your neighbor has run of the mill grid tie system. probably microinverters under the panels and gateway to talk to the "mothership" to report output and possibly SREC sales. The other option is optimizers under the panels and string inverter. They will stop working with generator in the loop wihtout a lot of extra hardware.

To use direct DC from the panels to cover loads is not an option with a microinverters or optimizers. If the panels are on rooftop they need to meet Rapid Shutdown requirements and unless someone does a home brew non code hack its not worth the hassle and liability to try to use them opther than grid tied . Note that a gird tie system can work in an off grid scenario via AC coupling where a hybrid inverter with battery creates a local microgrid that can also include a generator. Look around at You Tube for video on Sunny Island inverters. They create a clean local power supply that tricks the other grid tied inverters on the microgrid to keep running and also deals with reducing their output if there is excess power in the system with no where to go.

I took advantage of large bankruptcy auction to get a whole lot of hardware for about 20% of cost so I have a preconfigured Sunny Island system sitting in the front yard that will happily AC couple my other grid tied system and also has a Kubota Diesel for real long term grid events. Its vast overkill for my house but one of these years it will get towed over to a house site for me to build a new home and then its pays for itself instead of running generators for a couple of months.

[bcroe]

Connecting panel DC from a grid tie system directly to a big load, is a lot
more complex than might be first thought. You end up building up an
alternate off grid system. Bruce Roe

[Charlie W]

in a non hybrid system, the Grid is wired to the house main panel. When the Grid faults, the panels shut off, no relays / switches needed
The generator is wired between the main panel and it's critical load sub panel. A transfer switch ( manual or automatic ) is installed between the
critical load sub panel and the main panel. Solar never sees the generator . When grid returns, non critical loads power up and you then disconnect and shut down the generator

That's exactly what I was looking for. Thank you very much. It's also what I thought before I posted, but I wanted to be certain so I told my neighbor that I knew of a no-b.s. solar website full of people who know what they're talking about and don't tell lies, and I'd double check to be sure. Thanks again. And I'm fine with solar, by the way. I can easily imagine having panels if our rates were higher. But where we are, I see no reason to have panels when a) the bottom line is that the juice would be more expensive, and b) there's a hassle factor relative to the utility. But still, even if it's a vanity purchase, well, I have a huge $63,000 pickup truck that was expensive to drive even when diesel was $1.97 rather than $6.40. I don't need a truck that big, but it's redneck cool. LOL