Sizing a standby generator without the sales chart

The driveway pitch often starts with the size of the house: 2,000 square feet, so you need 22 kW. Square footage does not run when the power dies. The fridge, the furnace blower, the well, and maybe one air conditioner do.

A generator is sized in kilowatts (1 kW is 1,000 watts) to the circuits you insist on keeping alive, plus the surge when motors start, plus a little headroom so the engine isn’t pinned at maximum. If you skip that list, you buy the chart, not the outage.

A standby generator sits on a pad, usually on natural gas or propane, and an automatic transfer switch starts it when the utility drops. The battery vs generator guide is which machine. Once you’ve picked it, this is how big the gas one should be.

Unit plus labor

HomeGuide puts a typical whole-house standby at $6,000 to $11,000 installed (unit plus labor), or $9,500 to $24,000 when the run is long or the unit is over 20 kW.

The generator itself is often $3,000 to $6,000 for a common air-cooled size, and labor $3,000 to $5,000, climbing to $5,000 to $12,000+ for bigger or farther jobs.

Their size table (unit only, not installed) is a starting map, not a written add-up of what you will run. 13 to 16 kW is $3,000 to $4,500, described as a full home under about 1,500 square feet.

17 to 20 kW is $4,000 to $6,000 for 1,500 to 3,000 square feet. 22 to 25 kW is $4,500 to $12,000 for 3,000 to 5,000 square feet.

Generac unit-only averages on that same page run from about $3,800 at 16 kW to $5,000 at 22 kW, before pad, gas, transfer switch, and permits.

HomeGuide: whole house generator cost

Those bands overlap on purpose. A 22 kW quote that includes a 200-amp automatic transfer switch, a long gas run, and a concrete pad is a different job than a 14 kW essentials package with a smaller switch. Align the circuit list before you haggle the kilowatts.

Why the sales chart is a weak size

Two houses with the same floor area can have a gas furnace and a gas water heater, or an electric furnace, a heat pump, a well, and a Level 2 charger. The second house will eat a 22 kW unit and ask for more.

The chart also ignores starting watts. A 3-ton air conditioner might run around 3,500 watts and spike several times that for a second or two when the compressor starts. If two motors start at once, you size for the ugly second, not the quiet average.

Natural gas also derates some units compared with the propane sticker. If the brochure says 22 kW on propane and something lower on natural gas, the bid should use the fuel you actually have.

Load-shed modules (HomeGuide lists smart load managers around $130 to $250) let a smaller generator run more of the house by turning the range or the dryer off when the well starts. That’s often cheaper than jumping a size class.

Add running watts, then the greedy starts

Walk the critical-load list first: fridge, freezer, furnace or boiler controls and blower, sump, well, a lighting circuit per floor you use, bathroom, garage door if that’s the exit, modem and router. Leave the dryer, range, water heater, and whole-house AC off the first pass unless you are buying whole-home on purpose.

For each motor, take running watts from the metal tag (or volts times amps) and starting watts (often several times running for pumps and compressors). Add all the running watts you expect at the same time.

Then add the single largest start on top of that, not every start at once, unless two big motors can actually start together. Add 20% or so of headroom so the generator isn’t at 100% every time the well kicks.

A worked example, using order-of-magnitude nameplate ranges, not your house. Fridge: a few hundred running, a couple thousand at start. Furnace blower: hundreds to about 1 kW running. Sump: hundreds running, nasty start. Lights and internet: tens to a few hundred. No AC, no well, no electric heat.

That stack often lands in the low thousands of running watts. A 10 to 14 kW standby with an essentials subpanel covers it with room, which is why the 22 kW chart is frequently selling comfort, not physics.

Add a 3-ton central AC and a well, and you are in a different class because of start current. Soft starts on the AC and a delay so the well and the compressor don’t start together can pull you back a size.

Ask for that in writing if the bid is using it to justify 14 kW instead of 22 kW.

Essentials or whole house

Essentials means a small subpanel or a 6- to 10-circuit switch. The rest of the house stays dark. You can run that on a smaller generator, and you live in the kitchen and one bathroom until the utility is back. That’s a real product, and it’s cheaper.

Whole house means a transfer switch at service size (often 200 amp) that can feed everything, usually with load-shed so the range, dryer, and HVAC don’t all start together. That’s an 18 to 22 kW air-cooled job for a lot of gas-heat houses, and larger if you have two ACs or electric heat.

The automatic vs manual transfer switch guide is the hardware. Read the amp number on the quote.

A 14 kW “whole home” bid that quietly sheds the AC is an essentials job with better marketing. Make them list what stays on.

Which circuits, with watts?

Which circuits stay on, labeled the way they appear in the breaker box, with running watts and starting watts?

How many kilowatts on the fuel you actually have (natural gas vs propane), not only the bigger sticker?

Does the switch feed the whole breaker box, or a small extra box? How many amps?

If the generator can’t run everything at once, which circuits get turned off, and in what order?

Where’s the pad, how long is the gas pipe (HomeGuide puts that around $12 to $25+ per foot), how far from windows, who pulls the permit?

If the heat pump or AC is on the list, is there a soft start, a delay, or a bigger engine?

If one bid is 14 kW at $9,000 and another is 22 kW at $14,000, check whether the smaller unit includes the AC and the well. If it doesn’t, the extra $5,000 may be the compressor, not a brand.

HomeGuide’s unit and install bands

These use HomeGuide’s unit and install bands, not a quote for your panel.

About $8,000 to $11,000

14 kW air-cooled, essential-circuit subpanel, fridge, furnace, sump, lights, internet. No central AC. This is a complete job if that list is honest. It is unfinished if the well is on the house and missing from the sheet.

About $11,000 to $15,000

22 kW, 200-amp automatic transfer, load-shed for range and dryer, one AC with a start delay. HomeGuide’s typical installed band lives here. If this bid has no circuit list, you bought a chart.

$18,000 and up

Liquid-cooled or two ACs, long gas run, service upgrade. That’s a different house than the 14 kW job. Don’t average them.

Sized to square footage

They sized to square footage and the well trips the breaker every time it starts. Natural gas output is lower than the propane brochure and nobody derated. Load-shed was never programmed, so the first outage is a dark house with a screaming overload.

The furnace never got moved onto the essential panel. The unit sits too close to a window, which is a carbon monoxide and neighbor problem.

Never backfeed a dryer outlet. Inlet or hardwired standby, transfer switch or listed interlock, permit.

What did you miss last outage? That is the first-pass list. Decide whether AC in summer (or heat pump in winter) is on that list. That one line often jumps a size class.

Get two bids that use the same circuit list and the same fuel, then pick the job that covers the list, not the chart that matches your floor area.