Usage during outage and generator connection?

[desmo907]

I am new to solar (and have general knowledge of electricity) and was considering solar panels for my home.
I may have other questions but for now I wondered about a friend in this situation.
He has solar panels for electrical production. It is tied to the grid and in summer he makes more then he needs (it goes back out to the grid) so works off the "bank" at other times.
Last year we had a power outage for ~7 days and he said they he could not run anything off the solar power he was producing. This didn't make sense to us. (He did ask but didn't receive a satisfactory answer from installer.)

Why couldn't one still run devices in their home off the electricity the panels produce?
I understand you may not be able to send the excess back out to the grid since power lines were down, etc. Does one need a particular type of device (or installation procedure) to enable this?

Also, thinking about my possible installation, I may want to add a portable, gas generator (for backup at night during power outages) and before I considered adding solar I was researching either a transfer switch or interlock (on the board). If I have solar power as well is it still possible to interface a gas generator and/or do I need to tell this to the installers when they install everything (would they have to do something extra, use certain devices, etc.)?

Thx!

[billvon]

Why couldn't one still run devices in their home off the electricity the panels produce?

Because his inverter was not designed to stand alone. It needed utility power to synchronize to and to feed back power to.

I understand you may not be able to send the excess back out to the grid since power lines were down, etc. Does one need a particular type of device (or installation procedure) to enable this?

You mean operation without utility power? Yes, you need three things:

1) A standalone inverter
2) A battery bank
3) A charge controller

You cannot get by without a battery bank.

If you want to feed power back when the utility is available you'll need a very specific type of inverter, a grid tie/standalone inverter. An example of this sort of inverter is the GTFX3048 from Outback. It has the circuitry needed to allow both operation from (solar charged) batteries and to and from the grid.

Also, thinking about my possible installation, I may want to add a portable, gas generator (for backup at night during power outages) and before I considered adding solar I was researching either a transfer switch or interlock (on the board). If I have solar power as well is it still possible to interface a gas generator and/or do I need to tell this to the installers when they install everything (would they have to do something extra, use certain devices, etc.)?

Yes, that's possible. Generally you'll need to add a transfer switch, and that will likely be easiest to do during installation.

[desmo907]

Thanks for the reply.

It sounds like, when off the grid (due to power outage) I would operate off the batteries (and the panels charge them).
I am curious as to what happens when they reach full charge?

I guess I need to perform more research if I want this option, as I would need the 3 components you mentioned. I suspect that could be quite expensive. I am looking at a system to produce 5.28kW. I guess I could just rely on the gas generator as well and hope for a short power outage too

[billvon]

It sounds like, when off the grid (due to power outage) I would operate off the batteries (and the panels charge them).
I am curious as to what happens when they reach full charge?

The batteries take less and less power from the panels until there's no power flowing, even in sunlight. The panels are effectively disconnected at that point.

I guess I could just rely on the gas generator as well and hope for a short power outage too

Depends on your area. Here in San Diego we almost never get power outages; I've been inconvenienced by power outages once in all the time I've been here (20 years.) In an area that saw a lot of storm related outages it might make sense to do the battery backed thing.

[Sunking]

Last thing you want is batteries for backup, you cannot afford it. Use a either a LP or diesel generator. never use gasoline.

[biggreen]

Can you use a small battery bank for a large load if you only use it during high energy producing times? For example, In an emergency could you run a 10 amp.X 120V well pump with a 3000w array with 2 100AH batteries?

[billvon]

Can you use a small battery bank for a large load if you only use it during high energy producing times? For example, In an emergency could you run a 10 amp.X 120V well pump with a 3000w array with 2 100AH batteries?

Yes, but you need to be able to handle the power demands. For example, a well pump could easily draw 5-10x the steady state current during startup (or 6 to 12kW in your example) and both the inverter and the battery bank has to handle that. Most inverters have surge ratings, and thus you might be able to get a 3kW inverter that will surge to 6kW for a second or so. 6kW at 12 volts would be 500+ amps. A T-105 will handle up to about 250 amps, so you'd need 4 T-105's to supply 500A at 12V. So that would be 4 6V 220ah batteries, but would be marginal.

At that point it might make a lot of sense to switch to a smaller well pump and use an accumulator to maintain water flow during periods of transient high demand.

Here's a few articles on this issue:

http://homepower.com/article/?file=HP98_pg140_QandA_4

http://www.wholesalesolar.com/Information-SolarFolder/Invt-sizingforwellpump.html

[desmo907]

.....
Yes, that's possible. Generally you'll need to add a transfer switch, and that will likely be easiest to do during installation.

Is this something I should have done at the same time as the solar install?
Do they need to be aware I will add a generator later and do something special?
Also instead of a transfer switch I was considering just adding an interlock to my panel, my understanding is that it cuts off the incoming (grid) circuit and opens the circuit to the generator. I wonder if this would work with the solar in place as well.
(I am not well versed how all this wiring works.)

Thx

[biggreen]

thank you for the answer Billvon.

[Designer_Mike]

You are FAR better off just leaving your panels idle during the power outage and using the generator. The economics aren't even close. You would probably spend more in batteries than a decent 8Kw generator would cost you and the generator doesn't lose capacity after a couple years.

To have enough usable capacity you would need a complete stand alone system and still need a backup generator.

[mikegoldnj]

We had our 16 kW natural gas generator installed about five years ago. Had a grid tied solar system installed last summer.

When utility power is out (we've had a few 3 or 4 day outages in the last year) the solar is automatically shut down and our generator kicks in. No problems running most everything in the house.

[Sunking]

No problems running most everything in the house.

That is how it is suppose to work.

[biggreen]

We had our 16 kW natural gas generator installed about five years ago. Had a grid tied solar system installed last summer.

That would be sweet. Approx. what does a 16 kw natural gas generator cost and how much fuel does it use per day?

[Sunking]

That would be sweet. Approx. what does a 16 kw natural gas generator cost and how much fuel does it use per day?

I can help you out with that a Kohler Dual Fuel (NG pr LPG) 17 KVA runs about $4100. Fuel cost is hard to answer because it does depend on how much you load it down. Fo ra 17 KVA LPG loaded at 50% you are looking at about .8 gal/hour. If you have NG I would recommend you go that route.

[biggreen]

Thanks. So at 50% load about 19 gal/day - that's pretty thirsty.