Transfer switches for power outages

[Alchete]

I haven't found too much discussion about this topic and am wondering what folks are doing about having a grid-tied panel setup usable during a power outage?

It sounds like a conventional transfer switch won't work, as most inverters require the grid element to be active or else they won't convert any power (for safety)...

Is battery storage the only solution, or are people able to use some sort of transfer switch, or does everyone just go happily dark when the neighborhood goes out?

[thastinger]

There are hybrid systems available these days but they are the most expensive and the batteries would need to be replaced every 5-7 years regardless of whether you use them or not. Since the batteries are so expensive, they are usually designed to only run critical loads for short periods of time (less than a day) before you would drain them and need a generator. If all you care about is a fridge and a couple of LED lights, that is doable, central AC is not unless you're willing to spend barrels of cash.

I'm no expert in this area, I'm an off-grid guy, but if you search, there may be some info as I recall some discussion recently on the subject. Probably best to discuss with a pro. I'm sure there are plenty of ways to illegally "trick" system components to work and you could trip the main house breaker or even have a lockout in the panel but you won't find anyone here willing to discuss any of that, nor should they probably.

[Alchete]

Thanks thastinger. Yes, as an EE I was already imagining ways one could trick the system. )

I'm actually a bit surprised that this is not more of a common concern or that there don't appear to be more baked-in solutions -- assuming my perception is correct(?). If I'm going to spend $30K+ to generate my own electricity, I'm not going to sit in the dark with everyone else after a big storm or watch from the shadows when the zombies start walking down the road. And, buying a diesel generator seems a bit ridiculous. Yes, battery storage would be a solution, but is it the main solution?

Cheers.

[solarix]

This whole area is kind of a bug-a-boo in the solar arena. A lot of people buy a grid-tied solar system not realizing it can't operate without a utility grid to synchronize to. "What! - you mean I just paid $25k for this solar stuff and if the grid goes down, I'm gonna be OTL just like my neighbors?

There is no easy/inexpensive way to convert an on-grid system to an off-grid system. One of the things that makes an on-grid system efficient and cost effective is it runs the array at high DC voltages whereas the off-grid systems are limited by a <50V battery bank.

Here are the options:
1) Use the one grid-tied inverter that does have some off-grid capability - the SMA SunnyBoy TL inverters with the Secure Power Outlet feature that can produce 1500 watts when the sun is up. This is a built in feature that is no extra cost except the outlet itself.
2) Add on the SMA SunnyIsland Inverter system that will create its own "grid island" and keep a grid-tied inverter going during an outage. Works with any inverter not just SMA, (although works best with their own). They are quite expensive and of course needs batteries as well as the 120/240 transformer if you only use one of them.
3) Put in a Hybrid type inverter that works in both on-grid and off-grid regimes, although with some compromises. These are definitely not cheap as well and are more difficult to get permitted.
4) Use one of the new, way expensive, charge controllers that can tie to a 600V DC array and down convert it efficiently to 48Vdc to charge batteries during and outage. Probably wouldn't want to leave it connected all the time so also need a high voltage DC transfer switch to handle that.

People that have tried to use generators and such to "fool" a GT inverter into operating report that it doesn't work as they are not stable enough and of course can't absorb power from the inverter.

[sensij]

Thanks thastinger. Yes, as an EE I was already imagining ways one could trick the system. )

I'm actually a bit surprised that this is not more of a common concern or that there don't appear to be more baked-in solutions -- assuming my perception is correct(?). If I'm going to spend $30K+ to generate my own electricity, I'm not going to sit in the dark with everyone else after a big storm or watch from the shadows when the zombies start walking down the road. And, buying a diesel generator seems a bit ridiculous. Yes, battery storage would be a solution, but is it the main solution?

Cheers.

Unless you have batteries (or some kind of energy storage), yes you will sit in the dark with everyone else, even if you could trick your inverter into working. The array only makes power when the sun is out, when it is light, not dark. I don't know what other "main solution" could exist.

Trying to run anything directly off an array without some kind of energy storage means every cloud that goes by will knock out your power. Most post-apocalyptic scenarios aren't all that sunny, ya know?

[bcroe]

Thanks thastinger. Yes, as an EE I was already imagining ways one could trick the system. )

I'm actually a bit surprised that this is not more of a common concern or that there don't appear to be more baked-in solutions -- assuming my perception is correct(?). If I'm going to spend $30K+ to generate my own electricity, I'm not going to sit in the dark with everyone else after a big storm or watch from the shadows when the zombies start walking down the road. And, buying a diesel generator seems a bit ridiculous. Yes, battery storage would be a solution, but is it the main solution?

Every so often someone suggests it, despite the obvious requirement for serious sun. The grid tie
inverter would have to be converted from a current source, to a voltage source. The talkers never
actually accomplish it, and I wouldn't waste my time trying. 25 years ago I bought an 8 hp gasoline
generator, which works rain or shine, minimum cost, no maintenance. Only used it 4 times; probably
less than 10 hours running time. Keeps the furnace and the water systems alive, shut the rest down.

If you want to go on as though the power never went off, get a permanently installed gen that runs on
natural gas or your propane tank. Solar could NEVER do that. Bruce Roe

[paul65k]

Every so often someone suggests it, despite the obvious requirement for serious sun. The grid tie inverter would have to be converted from a current source, to a voltage source. The talkers never actually accomplish it, and I wouldn't waste my time trying. 25 years ago I bought an 8 hp gasoline generator, which works rain or shine, minimum cost, no maintenance. Only used it 4 times; probably less than 10 hours running time. Keeps the furnace and the water systems alive, shut the rest down. If you want to go on as though the power never went off, get a permanently installed gen that runs on natural gas or your propane tank. Solar could NEVER do that. Bruce Roe

Agreed......I installed a simple manual transfer switch to isolate the grid and power the home directly with a 9KW portable generator which can run on either propane or gasoline. When I manually flip the transfer switch I also turn off the breakers for the solar system and can power the entire house including a high efficiency 3 ton AC unit, 2 refers and a deep freeze along with the lights and TV although I do not use the electric range, microwave or Washer and Dryer we can keep cool in the 100+ degree summer here in the desert and my 20GA Propane tank will run the system at 50% load for 24+ hours, then I could run for 8 hours more on the 5GA tanks I keep around for the BBQ.....which substitutes as our cooking center during an outage........all that being said it's too bad that there isn't a good way to utilize the 4.4Kw Grid-tied system during the daylight hours without the costly Battery set-up for the 3-4 power outages we have during a 12 month period.......sounds like a business opportunity for someone

[thastinger]

Agreed......I installed a simple manual transfer switch to isolate the grid and power the home directly with a 9KW portable generator which can run on either propane or gasoline. When I manually flip the transfer switch I also turn off the breakers for the solar system and can power the entire house including a high efficiency 3 ton AC unit, 2 refers and a deep freeze along with the lights and TV although I do not use the electric range, microwave or Washer and Dryer we can keep cool in the 100+ degree summer here in the desert and my 20GA Propane tank will run the system at 50% load for 24+ hours, then I could run for 8 hours more on the 5GA tanks I keep around for the BBQ.....which substitutes as our cooking center during an outage........all that being said it's too bad that there isn't a good way to utilize the 4.4Kw Grid-tied system during the daylight hours without the costly Battery set-up for the 3-4 power outages we have during a 12 month period.......sounds like a business opportunity for someone

I also have a 9K for backup power which is tri-fuel capable. I'm fortunate enough to have a natural gas supply at home and that is the primary fuel source but I did buy a 100LB propane tank and test ran it from that for a while because if mine is under any significant load it ices up the 20lb bottles in just a few minutes.

[Sunking]

If I'm going to spend $30K+ to generate my own electricity, I'm not going to sit in the dark with everyone else after a big storm or watch from the shadows when the zombies start walking down the road. And, buying a diesel generator seems a bit ridiculous. Yes, battery storage would be a solution, but is it the main solution?

A hybrid battery system requires a generator.

[paul65k]

I also have a 9K for backup power which is tri-fuel capable. I'm fortunate enough to have a natural gas supply at home and that is the primary fuel source but I did buy a 100LB propane tank and test ran it from that for a while because if mine is under any significant load it ices up the 20lb bottles in just a few minutes.

Candidly I haven't had the icing up issue here in Arizona.......even in the winter (if you can call it that) we're in the 50's or 60's and the humidity is around 10% ........Interesting though I'll keep an eye on that next time I use a 20lb bottle.....thanks!

[lkruper]

Every so often someone suggests it, despite the obvious requirement for serious sun. The grid tie
inverter would have to be converted from a current source, to a voltage source. The talkers never
actually accomplish it, and I wouldn't waste my time trying. 25 years ago I bought an 8 hp gasoline
generator, which works rain or shine, minimum cost, no maintenance. Only used it 4 times; probably
less than 10 hours running time. Keeps the furnace and the water systems alive, shut the rest down.

If you want to go on as though the power never went off, get a permanently installed gen that runs on
natural gas or your propane tank. Solar could NEVER do that. Bruce Roe

What if there was a way to switch the panel output to a separate Charge Controller that feeds a battery bank which powers a separate inverter connected to a manual transfer switch?

Some more details to minimize cost. The battery bank is sized conservatively to be able to power just things needed in survival mode and kept healthy with a small maintainer. If the power goes out, the solar panel is switch to the charge controller to power the battery bank. During the day the surplus of power goes straight to the inverter and larger appliances can be run. At night when the sun goes down, the batteries take over, and large appliances are not run until the sun comes up again.

This allows the solar panels to be used to power the house even when the grid goes down, but it does require additional purchased of charge controller, inverter and batteries.

[sensij]

What if there was a way to switch the panel output to a separate Charge Controller that feeds a battery bank which powers a separate inverter connected to a manual transfer switch?

Yes, this is fine, although I'm not sure that it is more cost effective than just buying a single hybrid inverter. You would need make sure you have an interlocked transfer switch that prevents grid back-feed when operating in off-grid mode, and also some way to maintain the batteries when operating in grid-tie mode.

[bcroe]

What if there was a way to switch the panel output to a separate Charge Controller that feeds a battery bank which powers a separate inverter connected to a manual transfer switch?

Some more details to minimize cost. The battery bank is sized conservatively to be able to power just things needed in survival mode and kept healthy with a small maintainer. If the power goes out, the solar panel is switch to the charge controller to power the battery bank. During the day the surplus of power goes straight to the inverter and larger appliances can be run. At night when the sun goes down, the batteries take over, and large appliances are not run until the sun comes up again.

This allows the solar panels to be used to power the house even when the grid goes down, but it does require additional purchased of charge controller, inverter and batteries.

The original question was about tricking a grid tie inverter to supply an AC island. Now you are proposing a
pretty conventional charge controller/battery/inverter system, done all the time. Comes with the usual
expense and maintenance issues. I see the biggest problem is grid tie systems typically run at much higher
DC voltage than battery systems, but that could be worked out.

Energy management of large loads, to keep the battery small, is in theory possible. In practice very difficult,
since the loads don't know what the sun is doing. I just start the generator when its critical to run some
combination of the well, the furnace, and the microwave. Otherwise, flashlights. good luck, Bruce Roe

[jflorey2]

What if there was a way to switch the panel output to a separate Charge Controller that feeds a battery bank which powers a separate inverter connected to a manual transfer switch?

Most grid-tie systems are >250 volt system voltages which means you would need a $1000+ MPPT charge controller to make this work. No cheap HV MPPT controllers yet.

If the power goes out, the solar panel is switch to the charge controller to power the battery bank. During the day the surplus of power goes straight to the inverter and larger appliances can be run. At night when the sun goes down, the batteries take over, and large appliances are not run until the sun comes up again.

If you want to do this, consider getting an SMA GT inverter with the "secure power" feature. This can give you up to 1500 watts at 120 volts even when the grid is out. If you want backup power at night, get a $100 UPS from Best Buy and plug it into the "secure power" outlet.

[lkruper]

Most grid-tie systems are >250 volt system voltages which means you would need a $1000+ MPPT charge controller to make this work. No cheap HV MPPT controllers yet.

If you want to do this, consider getting an SMA GT inverter with the "secure power" feature. This can give you up to 1500 watts at 120 volts even when the grid is out. If you want backup power at night, get a $100 UPS from Best Buy and plug it into the "secure power" outlet.

Yes, I see that Xantrex and Morningstar have $1000+ MPPT controllers. That's probably a $400+ premium but you do get more amps. what about switching just one string of the solar array to a smaller controller? Would that be practical?