Solar panels and generator connection

Questions on how we spend our money and our time - consumer goods and services, home and vehicle, leisure and recreational activities
Post Reply
Topic Author
Starfish
Posts: 2996
Joined: Wed Aug 15, 2018 6:33 pm

Solar panels and generator connection

Post by Starfish »

This might seem a strange question for a financial forum but I am often amazed by the wealth of information here, so here it goes :D

I own a small commercial building where I installed solar panels. Due to frequent power outages in the area later I also installed a Generac Guardian 22kW whole house gas generator.

In my naivety I assumed the electrician who installed the generator knew how to deal with the solar panels and I did not pay any attention during the installation process, but at the end, after asking him several questions I am not so convinced anymore.
He installed the Generac box (which I assume is the switch) upstream from the solar panel box (which I assume is also a switch + inverters). My theory is that when there is no power the first switch (Generac) decouples the grid, but the solar panels continue to feed in power because they see the generator output. If the power consumption in the building is higher than what the panels produce there is no problem, but what happens when the power consumption in the building is lower? Will the power from panels be back fed into the generator, damaging either the generator or the inverters?
Is this the right way to install the generator?

Thank you.
Iorek
Posts: 1569
Joined: Fri Mar 08, 2013 8:38 am

Re: Solar panels and generator connection

Post by Iorek »

I am by no means an expert but I believe the electrical code generally requires solar panels to be tied in to the grid and not to your house.

My guess is that if the grid is down your panels are not supplying electricity to anywhere and your generator will kick in.
Topic Author
Starfish
Posts: 2996
Joined: Wed Aug 15, 2018 6:33 pm

Re: Solar panels and generator connection

Post by Starfish »

Iorek wrote: Tue Sep 28, 2021 10:08 pm I am by no means an expert but I believe the electrical code generally requires solar panels to be tied in to the grid and not to your house.

My guess is that if the grid is down your panels are not supplying electricity to anywhere and your generator will kick in.
Not necessarily, if you have batteries.
If the generator kicks in, the panels see the generator and assume the grid is up, so they do provide electricity.
User avatar
willthrill81
Posts: 32250
Joined: Thu Jan 26, 2017 2:17 pm
Location: USA
Contact:

Re: Solar panels and generator connection

Post by willthrill81 »

I think that your concern is valid. In my view, a normally closed contactor, a special kind of switch, would be needed so that when the generator is providing power, it opens (aka 'breaks') the circuit connecting the solar panels to the line (I installed a normally closed contactor in my RV literally just yesterday, so this has great top of mind awareness for me!). That wouldn't be pricey, but without it, it does seem that you could have both the generator and the solar panels providing power on the same line, which could be problematic even if that power isn't being consumed.

Have you asked the electrician who installed the generator about this issue?
The Sensible Steward
smitcat
Posts: 13308
Joined: Mon Nov 07, 2016 9:51 am

Re: Solar panels and generator connection

Post by smitcat »

Here is some information....

"The bottom line is you cannot allow solar panels and a generator to work in parallel. They must be electrically isolated at all times. If solar inverters “see” voltage from a generator, they will attempt to sync with the generator and backfeed power to it. Any time solar production exceeds loads in the building, solar inverters attempt to send power to the utility grid. As a huge “battery” of sorts, the grid can handle this small amount of backfeed. Typical residential standby generators cannot."

And the link for more details...
https://floridasolardesigngroup.com/ins ... ar-panels/
SmallSaver
Posts: 641
Joined: Mon Mar 26, 2012 11:34 am

Re: Solar panels and generator connection

Post by SmallSaver »

Assuming your solar contractor is good this would probably be a question for them. They're generally more up to speed on the details than a general electrician.
Topic Author
Starfish
Posts: 2996
Joined: Wed Aug 15, 2018 6:33 pm

Re: Solar panels and generator connection

Post by Starfish »

smitcat wrote: Wed Sep 29, 2021 9:23 am Here is some information....

"The bottom line is you cannot allow solar panels and a generator to work in parallel. They must be electrically isolated at all times. If solar inverters “see” voltage from a generator, they will attempt to sync with the generator and backfeed power to it. Any time solar production exceeds loads in the building, solar inverters attempt to send power to the utility grid. As a huge “battery” of sorts, the grid can handle this small amount of backfeed. Typical residential standby generators cannot."

And the link for more details...
https://floridasolardesigngroup.com/ins ... ar-panels/
Thank you. This is what I was looking for. Now I have to figure out what can be done.
suemarkp
Posts: 1380
Joined: Sun Nov 12, 2017 7:18 pm
Location: Somewhere in WA State

Re: Solar panels and generator connection

Post by suemarkp »

It can work, but as others said, you need to have a battery in the system to take the excess power. The inverters and generator also need to be compatible, and that can be a problem. I think the latest inverters are getting better at dealing with generator based systems, but if you've had solar for a while you probably don't have this type.

If you want a system that works on grid, and completely off grid with a generator, it is coming and may be here now with some installers with specific generators. If you want that type of system, I'd wait a few more years.

Finally, be sure to have the electrician test the system on generator on a sunny day to make sure you don't have any problems. If your inverters can't deal with the generator, you want the electrician to be the one who burns something up (most likely the generator or its voltage regulator).
Mark | Somewhere in WA State
skis4hire
Posts: 101
Joined: Thu Oct 11, 2018 12:54 am

Re: Solar panels and generator connection

Post by skis4hire »

This gets complicated pretty quickly.
You didn't specify, but I assume your solar panels are hooked up to a grid-tie inverter with no batteries.

In that case you certainly don't want the solar and generator on at the same time as linked above.

The inverter typically senses the grid by checking the frequency of the line voltage.
You may be able to configure the generator and inverter such that the generator runs at a slightly different frequency that would generally not be noticeable but that the inverter would recognize as not being grid power and thus shut down. That would require a deep dive into the inverter manual or calling the solar installer. A general electrician is unlikely to solve that for you.

The alternate is to rewire the solar array upstream of the generator transfer switch.

There may be other ways to upgrade the inverter to work with the generator, likely in combo with a battery bank, but this would be much more expensive than the other two options.
Point
Posts: 660
Joined: Mon Jul 10, 2017 9:33 pm

Re: Solar panels and generator connection

Post by Point »

I have a 22kw Generac with an ATS, and also solar panels with a SolarEdge inverter. When the grid goes down the solar shuts off as I don’t yet have the 16kwh battery installed (on order). The Generac comes on and the ATS switches from grid to Generac. The solar does not come on as the generac power looks different than from the grid (sine wave is different).

When the 16kwh battery arrives and is installed it will be configured differently. Ultimately, the design will change to allow the Generac to come online when the battery is low, and the Generac will charge the battery, then shut off and the solar and battery will take over.
mervinj7
Posts: 2496
Joined: Thu Mar 27, 2014 3:10 pm

Re: Solar panels and generator connection

Post by mervinj7 »

Starfish wrote: Wed Sep 29, 2021 12:10 pm
smitcat wrote: Wed Sep 29, 2021 9:23 am Here is some information....

"The bottom line is you cannot allow solar panels and a generator to work in parallel. They must be electrically isolated at all times. If solar inverters “see” voltage from a generator, they will attempt to sync with the generator and backfeed power to it. Any time solar production exceeds loads in the building, solar inverters attempt to send power to the utility grid. As a huge “battery” of sorts, the grid can handle this small amount of backfeed. Typical residential standby generators cannot."

And the link for more details...
https://floridasolardesigngroup.com/ins ... ar-panels/
Thank you. This is what I was looking for. Now I have to figure out what can be done.
Maybe talk to an experienced electrician? It's seems like a common enough situation.
User avatar
willthrill81
Posts: 32250
Joined: Thu Jan 26, 2017 2:17 pm
Location: USA
Contact:

Re: Solar panels and generator connection

Post by willthrill81 »

skis4hire wrote: Wed Sep 29, 2021 2:02 pm This gets complicated pretty quickly.
You didn't specify, but I assume your solar panels are hooked up to a grid-tie inverter with no batteries.

In that case you certainly don't want the solar and generator on at the same time as linked above.

The inverter typically senses the grid by checking the frequency of the line voltage.
You may be able to configure the generator and inverter such that the generator runs at a slightly different frequency that would generally not be noticeable but that the inverter would recognize as not being grid power and thus shut down. That would require a deep dive into the inverter manual or calling the solar installer. A general electrician is unlikely to solve that for you.

The alternate is to rewire the solar array upstream of the generator transfer switch.

There may be other ways to upgrade the inverter to work with the generator, likely in combo with a battery bank, but this would be much more expensive than the other two options.
As I noted above, an option would be to install a normally closed contactor between the inverter and the line, and the contactor would be controlled by the generator. The contactor will normally leave the solar panels connected, but when the generator turns on, it will open the contactor and disconnect the inverter from the line. This shouldn't be too expensive.
The Sensible Steward
Topic Author
Starfish
Posts: 2996
Joined: Wed Aug 15, 2018 6:33 pm

Re: Solar panels and generator connection

Post by Starfish »

willthrill81 wrote: Tue Sep 28, 2021 11:48 pm I think that your concern is valid. In my view, a normally closed contactor, a special kind of switch, would be needed so that when the generator is providing power, it opens (aka 'breaks') the circuit connecting the solar panels to the line (I installed a normally closed contactor in my RV literally just yesterday, so this has great top of mind awareness for me!). That wouldn't be pricey, but without it, it does seem that you could have both the generator and the solar panels providing power on the same line, which could be problematic even if that power isn't being consumed.

Have you asked the electrician who installed the generator about this issue?
This seems to be a simple enough solution, but I am not sure how easy is to implement in my case, as the inverters and grid switch seem to be integrated in the same box.
I did ask the electrician way too late, at the end of the installation. His answer was along the lines "That's a good question"/head scratch.
Topic Author
Starfish
Posts: 2996
Joined: Wed Aug 15, 2018 6:33 pm

Re: Solar panels and generator connection

Post by Starfish »

Point wrote: Wed Sep 29, 2021 6:41 pm I have a 22kw Generac with an ATS, and also solar panels with a SolarEdge inverter. When the grid goes down the solar shuts off as I don’t yet have the 16kwh battery installed (on order). The Generac comes on and the ATS switches from grid to Generac. The solar does not come on as the generac power looks different than from the grid (sine wave is different).

When the 16kwh battery arrives and is installed it will be configured differently. Ultimately, the design will change to allow the Generac to come online when the battery is low, and the Generac will charge the battery, then shut off and the solar and battery will take over.
What is the advantage of both battery and generator?
I like the battery option (a lot more elegant) but it is pricey for a decent capacity. My application is a dental office running compressors, heating/AC etc in a region where 3 days power shutoff are not uncommon, so I would need a lot of batteries. A combination of both sounds like an even pricier option.

skis4hire wrote: Wed Sep 29, 2021 2:02 pm This gets complicated pretty quickly.
You didn't specify, but I assume your solar panels are hooked up to a grid-tie inverter with no batteries.

In that case you certainly don't want the solar and generator on at the same time as linked above.

The inverter typically senses the grid by checking the frequency of the line voltage.
You may be able to configure the generator and inverter such that the generator runs at a slightly different frequency that would generally not be noticeable but that the inverter would recognize as not being grid power and thus shut down. That would require a deep dive into the inverter manual or calling the solar installer. A general electrician is unlikely to solve that for you.

The alternate is to rewire the solar array upstream of the generator transfer switch.

There may be other ways to upgrade the inverter to work with the generator, likely in combo with a battery bank, but this would be much more expensive than the other two options.
'

Very interesting! Thank you.
I had no idea that the inverters are so intelligent to evaluate frequency or sine purity.
User avatar
willthrill81
Posts: 32250
Joined: Thu Jan 26, 2017 2:17 pm
Location: USA
Contact:

Re: Solar panels and generator connection

Post by willthrill81 »

Starfish wrote: Thu Sep 30, 2021 1:16 pm
willthrill81 wrote: Tue Sep 28, 2021 11:48 pm I think that your concern is valid. In my view, a normally closed contactor, a special kind of switch, would be needed so that when the generator is providing power, it opens (aka 'breaks') the circuit connecting the solar panels to the line (I installed a normally closed contactor in my RV literally just yesterday, so this has great top of mind awareness for me!). That wouldn't be pricey, but without it, it does seem that you could have both the generator and the solar panels providing power on the same line, which could be problematic even if that power isn't being consumed.

Have you asked the electrician who installed the generator about this issue?
This seems to be a simple enough solution, but I am not sure how easy is to implement in my case, as the inverters and grid switch seem to be integrated in the same box.
I did ask the electrician way too late, at the end of the installation. His answer was along the lines "That's a good question"/head scratch.
It wouldn't be difficult to implement. There has to be a line running from the solar panels to the inverter or from the inverter to the main line, and a contactor could be placed at either point. It should be a simple wiring job.
The Sensible Steward
Point
Posts: 660
Joined: Mon Jul 10, 2017 9:33 pm

Re: Solar panels and generator connection

Post by Point »

Our house is in a rural area and 4,500' elevation. The summers are hot, the winters are cold. Our solar array this summer was producing ~55Kwh on average daily. We consume more than that to cool one zone of the house and have other usages. So, in an outage, with smoke (we are inundated by the fires in the west), the production is down. We're not interested in curtailing our use due to lack of sun or battery. Besides, we had the generator before the solar (and soon also the battery). Last year we had 11 extended outages due to PSPS - winds. The infrastructure is being updated so with luck outages will be 1/2 that rate this year. The battery is a whim, and we'll see how it does. Worst case I think we'll be able to rate arbitrage to recover the costs as, yes, they are expensive, even after tax credits. Regardless, before we moved here I told my wife power would not be an issue. It's not :D
Starfish wrote: Thu Sep 30, 2021 1:19 pm
Point wrote: Wed Sep 29, 2021 6:41 pm I have a 22kw Generac with an ATS, and also solar panels with a SolarEdge inverter. When the grid goes down the solar shuts off as I don’t yet have the 16kwh battery installed (on order). The Generac comes on and the ATS switches from grid to Generac. The solar does not come on as the generac power looks different than from the grid (sine wave is different).

When the 16kwh battery arrives and is installed it will be configured differently. Ultimately, the design will change to allow the Generac to come online when the battery is low, and the Generac will charge the battery, then shut off and the solar and battery will take over.
What is the advantage of both battery and generator?
I like the battery option (a lot more elegant) but it is pricey for a decent capacity. My application is a dental office running compressors, heating/AC etc in a region where 3 days power shutoff are not uncommon, so I would need a lot of batteries. A combination of both sounds like an even pricier option.

skis4hire wrote: Wed Sep 29, 2021 2:02 pm This gets complicated pretty quickly.
You didn't specify, but I assume your solar panels are hooked up to a grid-tie inverter with no batteries.

In that case you certainly don't want the solar and generator on at the same time as linked above.

The inverter typically senses the grid by checking the frequency of the line voltage.
You may be able to configure the generator and inverter such that the generator runs at a slightly different frequency that would generally not be noticeable but that the inverter would recognize as not being grid power and thus shut down. That would require a deep dive into the inverter manual or calling the solar installer. A general electrician is unlikely to solve that for you.

The alternate is to rewire the solar array upstream of the generator transfer switch.

There may be other ways to upgrade the inverter to work with the generator, likely in combo with a battery bank, but this would be much more expensive than the other two options.
'

Very interesting! Thank you.
I had no idea that the inverters are so intelligent to evaluate frequency or sine purity.
Post Reply