# whole home generator size for central a/c?



## mogie (Sep 18, 2017)

i have a 5 ton a/c 16seer and am looking at buying a briggs and straton 20kw whole home generator. do you think this will be enough to turn the a/c on? i actually have 2 central a/c units, the other is 4 ton. i figure if it can kick on the 5 ton, i can alternate them and cool the house. on top of the a/c i'd like to power the fridge and tv if possible. thanks


----------



## exmar (Jun 29, 2015)

20KW should be adequate if you're willing to alternate them, don't think it'll handle both. That is an excellent question for the Contractor you're going to have install it. You should do a load calc and see what you actually need in addition to frig and tv's. Lights are nice, hot water, range, etc. Factor in how often and duration of outages you experience. e.g. My brother bought a genset and figured out the minimum wattage required during an outage, worked fine for short outages. We had a bad ice storm and an 8 day outage, he has two daughters in high school, cold showers, no hot oil hair treatments, no blow dryers, putting on makeup by flashlight. At one point he asked if he could hide in my root cellar. He now has a whole house unit.


----------



## Osviur (Sep 7, 2015)

Hi mogie,

A 5 Ton A/C compressor takes about 145 amps for starting (locked rotor amps or LRA), this value is for a 240 V unit. The aprox. load in KW (at a power factor = 0.8) will be 240 V X 145 A X 0.8 =27,840 W or 27.84 KW.

The nearest upper value for this load is 30 KW. This is the minimum generator size for your A/C unit ONLY. 

Regards


----------



## exmar (Jun 29, 2015)

Can't disagree with Osviur. Theoretically, he's right. Going to a 30Kw unit would be correct, however, you're going to spend a lot of money on steady state running using no where near 30KW, not to mention added cost to upsize to the larger unit. Personally, I'm cheap. Here's an article that goes into it much better than I could. Pay particular attention to "hard start kits" which can save you a lot of money compared to upsizing your genset. JMHO. Again, I'd be chatting with your contractor as presumably he'll be the one doing the installation, startup and testing. 


Sizing a Generator to Start Air Conditioner and Motors


----------



## Osviur (Sep 7, 2015)

Hi Exmar,

Thanks for your valuable opinion. You are right, because my calculation didn't consider the starting lower PF and the voltage dip. Personally I consider that a 30% dip is too much even for a residential application.

It is important to take in consideration that an A/C unit is not a permanent load, it is continuously cycling, on and off and in any moment it can start when the fridge and other loads are connected. There must be a safe margin for the genset to overcome this momentary overload. The soft start or hard start kits are a very good idea.

Best regards


----------



## Predator (Sep 19, 2017)

Another option is to use two smaller generators....one to start and run the Condensing unit outside....and one to run the Evaporating unit inside.

Here's a really good blog on the subject.....enjoy!

Sizing a Generator to Start Air Conditioner and Motors


----------



## LarryHomeowner (May 20, 2018)

I took the cheap way out 

I bought a 9500/12000 watt portable, had a 50 amp transfer switch put in, and put everything but 240v on the transfer. I get everything except A/C, the stove and the oven. Oh, and the Dryer.

I even have heat.

As for the A/C I just spent about $800 on a couple of 14000 btu portable units that the genny can easily handle and they work awesome for spot cooling where needed.

I also REALLY wanted true whole-house but when the numbers were run there was just no way to afford it.


I got 95% of what I wanted for less than 2 grand


----------

