Whole House Generator Sizing

[roni]

This recent prolonged power outage has pushed me forward on a decision that we'd been considering for a while.

 

We were looking at the Generac Centurion 20000 Watts (LP)/18000 Watts (NG) Standby Generator with Automatic Transfer Switch. I thought this would be enough for the whole house but am a little concerned since I've seen a few posts about not being able to run the AC on the generator due to the high peak loads.

 

Would love any info from those of you who have whole house units. Obviously, if the generator can run the tank and the heater/furnace, that's really all I care about.

[LCDRDATA]

We've also thought about going that route but haven't decided yet. Regardless, your best bet on that question is probably to check with the manufacturer of your specific HVAC unit (if it isn't on the unit itself or in a manual) to find out what the peak draw is. Beyond that you could certainly look at your electric bill to see what your average consumption is. Also, don't forget to take into consideration an estimate of how much you can cut back when you actually need to put it in service. Those figures should go a long way toward letting you size your requirements.

[Flip]

I have a 20kw generator on LP with auto switch that works great - 36 hours continous for this outage. I had this install one year after have my house built in 2008. After living in South Florida for six years and living through some brutal hurricanes and week-long power outages, I would never own a home without a standby generator. The 20kw is probably the one you need for your size house, and it depends on your fuel source. I have a 500 gal LP tank in the ground. The generator runs more than half of my house, to include all major pumps and applicances. I am also on a well, so this investment was a no-brainer for me. Figure $10-12K installed. LP costs run about $100 per day under half load, assuming LP cost of $2 per gal. PM me for additional details.

[Coral Hind]

Roni, Do you have gas or electric hot water/range/heat? How many and what size A/C unit?

[roni]

That helps a lot. We've got natural gas so the propane tank being buried won't be necessary (from what I've heard, usually a major expense). We have 2 AC units, 1 upstairs and 1 downstairs. My feeling is that with the sump in the basement, as long as the downstairs AC could be on the unit, we'd probably be fine. From what I've read, the AC is usually one of the heaviest loads on the generator.

 

Lowe's is running a 10% discount on the Generac whole house right now. It's not a lot off but in general, the whole house generators don't seem to go down in price very much.

 

They'll hopefully be coming by in the next couple days so I'll report back what they quote in terms of sizing.

[Steve175]

Flip: do you run your AC on your generator currently?

 

This thread (and the heat) have motivated me to ask my electrician to come by while our generator is still running to see what current we are drawing and therefore see whether one of our two central AC units can be added (later) to the system. I'll report back here once I have the data. When I had it installed, I assumed that we would most be at risk of power loss during the fall and winter when AC is less of an issue. I haven't lived here long: how often should I expect these types of storms during peak summer?

[Coral Hind]

Roni, If you have a normal 2 1/2 ton unit then you should be able to run both off the 20kw unit, just stagger the starts if you can so the LRA doesn't peak the gen set. If you are going to pick just one A/C unit I would go with the upstairs. The cool air will find its way to the basement if you leave doors open and it is much better to have A/C for the family sleeping areas, makes for a happier wife. Even with one A/C unit running you can run the second unit's fan to help circulate the air in the house.

[roni]

Flip: do you run your AC on your generator currently?This thread (and the heat) have motivated me to ask my electrician to come by while our generator is still running to see what current we are drawing and therefore see whether one of our two central AC units can be added (later) to the system. I'll report back here once I have the data. When I had it installed, I assumed that we would most be at risk of power loss during the fall and winter when AC is less of an issue. I haven't lived here long: how often should I expect these types of storms during peak summer?

 

Not sure about in Maryland but our power has been pretty stable. Per our neighbor, only one other major outage and it was 8 years ago. Are you back up with power?

 

 

Roni, If you have a normal 2 1/2 ton unit then you should be able to run both off the 20kw unit, just stagger the starts if you can so the LRA doesn't peak the gen set. If you are going to pick just one A/C unit I would go with the upstairs. The cool air will find its way to the basement if you leave doors open and it is much better to have A/C for the family sleeping areas, makes for a happier wife. Even with one A/C unit running you can run the second unit's fan to help circulate the air in the house.

 

That's a great idea. I guess the largest load on the generator is that initial start so that might actually do it. I'll see what the installer says. Thankfully my wife is a trooper. She's much rather the fish and pets be fine and we have to sleep downstairs then her own comfort

[Manny]

What about those power switches to handle the startup load? Seems like an expensive way to deal with only a few seconds worth of load. Can one plug things in manually and in sequence?

 

Maybe we can do a group buy/installations

[Laurie]

We have a 25KVW generator hardwired to the house. Installed a transfer switch in the power panel. It is set up to run essentials. 1st and foremost is the fish tank and then water pump(gotta have a shower), computer, AC, TV, frig and some lights. Even my neighbors plug into it when the power is out for long times. The generator can run for 4 days non stop before we have to refuel. It is a godsend when mother nature is fooling with us.

[Flip]

Flip: do you run your AC on your generator currently?

 

This thread (and the heat) have motivated me to ask my electrician to come by while our generator is still running to see what current we are drawing and therefore see whether one of our two central AC units can be added (later) to the system. I'll report back here once I have the data. When I had it installed, I assumed that we would most be at risk of power loss during the fall and winter when AC is less of an issue. I haven't lived here long: how often should I expect these types of storms during peak summer?

 

 

I have two AC units (2-3 ton) that run on my Generac 20kw, plus all my sump pumps, major appliances and some lighting, an I still have some room. As previously mentioned, you may want to stagger your AC unit start-ups due to power surge. I believe the Generac or other brand names have calculators on their sites to give you an idea of what you can run on a certain size generator. Pricing jumps once you go north of 22kw as the engines become liquid cooled, as opposed to air cooled. I purchased mine through HD and it was installed by CDS. I am very happy with the investment.

 

Roni - You can always stop by and check out my standby system. I am a few miles away.