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. This page is how big the gas one should be once you’ve picked it.

What does the machine actually cost?

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 load calc:

• 13 to 16 kW: $3,000 to $4,500, described as a full home under about 1,500 square feet
• 17 to 20 kW: $4,000 to $6,000, 1,500 to 3,000 square feet
• 22 to 25 kW: $4,500 to $12,000, 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 is the sales chart 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 nameplate. If the brochure says 22 kW on LP and something lower on NG, 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 is often cheaper than jumping a size class.

How do I add up the watts?

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, write running watts (the nameplate, 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.

Whole house or essentials?

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 is a real product, and it is 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 is a 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.

What should the quote include?

1. Circuit list, labeled as they appear in the panel, with running and starting watts or a load calc sheet you can keep.
2. Generator kW on your fuel (natural gas vs propane), not only the bigger nameplate.
3. Transfer switch type and amps, and whether it is whole panel or a subpanel.
4. Load-shed: which circuits get dropped, and in what order.
5. Pad, gas piping (HomeGuide puts gas line in a $12 to $25+ per foot band on that cost page), exhaust clearances, permit.
6. Elevation or long gas runs if they apply.
7. What happens if the heat pump or AC is on the list: start kit, delay, or a bigger engine.

If Bid A is 14 kW at $9,000 and Bid B is 22 kW at $14,000, check whether A includes the AC and the well. If it doesn’t, the extra $5,000 may be the compressor, not a brand.

What do three real-looking sizes mean?

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

Bid A, 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.

Bid B, 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 Bid B has no circuit list, you bought a chart.

Bid C, $18,000 and up. Liquid-cooled or two ACs, long gas run, service upgrade. That is a different house than Bid A. Don’t average them.

What goes wrong after you sign?

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.

How do I decide in one afternoon?

1. Write down what you missed last outage. That is the first-pass list.
2. Decide whether AC in summer (or heat pump in winter) is on that list. That one line often jumps a size class.
3. Get two bids that use the same circuit list and the same fuel.
4. Read kW on that fuel, transfer-switch amps, and load-shed. Then pick the job that covers the list, not the chart that matches your floor area.

You can always add load-shed and live with a dark laundry room. It is harder to live with a 22 kW engine you bought because the square-footage row said so.