Running my A/C on a generator...

[Toller]

I am looking into a NG standby generator; and am seeing if I can run my A/C off it. Not a big deal, but if it is simply done... I would be willing turn everything off for a few hours to run the A/C by itself to cool the house off a bit; and then turn the A/C off and everything else back on.

It is a Trane X14 2.5 ton. The generator installer "thinks" it will work with a 10kw, but that is not very encouraging.
I emailed Trane. They say I should talk to my local dealer or generator installer.
I emailed my dealer. He has no idea.
I emailed Generac. They say I have to rely on the generator installer.

The answer to a question on the Generac website says a 3 ton may run on a 8kw depending on what else is going on. That is encouraging, but the fact it is buried in the Q&A suggests it may not be authoritative.

Any ideas?

[kangaroogod]

10kw with a load-shed panel

[heaterman]

The short answer is yes, 10kw would supply adequate power. Is your generator installer a licensed electrician? If so, he should have all of the answers you need. If not, maybe look for a qualified installer.

[Sleuth]

.

Great question and you've received two great answers (post #2 & #3).

From experience 8kw is marginal and 10kw is good to go in most cases.
..

[timebuilder]

...and yes, use an electrician.

A proper setup uses a transfer switch and a second panel which contains the loads you intend to run, properly sized for the correct generator.

If all is correct, no problem at all.

[motoguy128]

The incremental cost going up 1 size in the generators isn't that significant and will give you some wiggle room in case you want to run a few more loads and you have multiple appliances start at the same time.

[TwincamDave]

what ever size they give you, make sure the generator has a "line conditioner" to clean up the dirty sine waves so it doesn't mess up any and all electronic circuit boards you might have in what ever you are running.

[timebuilder]

I don't believe the Generac product has a power quality issue.

[Toller]

A website selling Generac said the 8kw will start a 3 ton A/C.
I asked where they got that information and they said from Generac. As long as the LRA on my A/C was under 50, I was good.
I looked and found mine says 107!
Presumably I need a 20kw to start it?!

If so, I guess I won't have A/C during a power outage...

[kangaroogod]

10kw generator with a load shed panel. Make sure start components are added to your air conditioner. I would be highly surprised if you had an issue.

[bmac]

Generac has detailed guidelines on this. Your contractor needs to size the generator for everything you want to run and then check to make sure the generator he chooses can handle the surge of the A/C. Our Honeywell (Generac) 10KW has a surge capability of 63 LRA. 15KW is 125RLA. According to the sizing guide, they would NOT recommend a 10KW. Also, be sure to check the capacity of your gas meter, our 10KW needs 156,000 btu's, 15KW is 231,800 btu's. So add up the total btu's that your house needs and make sure the gas meter can handle that plus the generator btu's. I've seen some electricians ignore this and then have problems later. You shouldn't have to get opinions, the manufacturer spells out the sizing guidelines, you just need someone that understands them to help you.

[Toller]

I couldn't find the guidelines. I don't even see a 15kw, but the 17kw is $2,000 more; so I will pass on that.

According to their website the 8kw uses 139cf/hour. A cf of NG is 1029 btu, so the 8kw uses 140,000btu. Does that seem right to you?

My furnace is 80,000btu and my water heater is 40,000. The utility says my supply is 250,000btu. So I am a bit over the maximum. Is that likely to matter? As a practical matter, I can't imagine I will ever use over 3.5kw and at half capacity the generator only uses 80,000btu, so I am okay there.

I have run everything for years off a 1.6kw generator; so even adding microwave oven and a few lights will still be under half capacity; right?

[motoguy128]

That adds up to 260. I unless you gas pressure is on the low side, I doubt you'll have an issue. Install the generator near the gas meter if possible and run it's branch to be the first piece of equipment off the meter. Be sure the piping is sized adequately.


It's funny, if that unit really uses 80k BTU @ 4kw output, that's only $0.17/kwhr at $0.85/therm. Not too bad.

[ga-hvac-tech]

A loose rule of thumb... and we should NOT follow rules of thumb:

It takes around 3 times the watts to START an elec motor as it does to RUN the same motor. Whatever your run watts are (volts x amps)... then 3 times with some wiggle room probably will work.
I agree with Motoguy... a larger unit is a better choice. Remember that generally speaking: A generator will produce close to the same watts on LP gas as gasoline... yet around 20-25% LESS on NG.

Have a LICENSED electrician handle this for you.

[genduct]

How about a hard start kit to reduce LRA on start=up?