Whole-house generator calculator
A 2,000 sq ft gas-heated house with a 3 ton AC and an electric water heater needs about 13.8 kW, which buys as a 14 kW standby. Your house is not that house, so set the real numbers below.
How the estimate works
General lighting and receptacle load follows the NEC convention: 3 watts per square foot, with the first 3,000 counted in full and the remainder at 35%, because nobody has every light and outlet loaded at once. On top of that base we add the refrigerator, your AC at roughly 1,200W per ton of cooling, the heat source, and each 240V appliance you flip on, at its demand figure rather than its nameplate maximum where the code allows it (the range counts as 8,000W, not the 12,000 on its sticker).
Air-cooled standby units sell in 10, 14, 18, 22, and 26 kW steps, so the answer rounds up to one of those. Past 26 kW you are pricing liquid-cooled equipment and the conversation changes; at that point load management stops being an option and becomes the sensible plan.
The honest caveat, once: this is budgeting math. The installer's Article 220 calculation on your actual nameplates is the one the permit office sees, and a good installer will also talk you into shedding loads instead of buying capacity. Let them.
Where common houses land
| House | Estimated demand | Standby class |
|---|---|---|
| 1,500 sq ft, gas heat + water, 2.5 ton AC | 8.1 kW | 10 kW |
| 2,000 sq ft, gas heat, electric water heater, 3 ton AC | 13.8 kW | 14 kW |
| 2,500 sq ft, heat pump, electric water + dryer, 3.5 ton | 22.5 kW | 26 kW |
| 3,000 sq ft, all-electric with range and level 2 EV | 46.2 kW | 30 kW+ liquid-cooled |
Questions people ask
What size generator runs a whole house?
Most gas-heated homes in the 1,500 to 2,500 sq ft range land on a 14 to 18 kW air-cooled standby once central AC is in the load. All-electric houses (resistance heat, electric water heater, range, dryer) push into 22 to 26 kW, and adding level 2 EV charging can shove the answer past air-cooled sizes entirely. The calculator above shows where your combination lands.
Do I really need to cover everything at once?
No, and this is where real installs save money. A load-management module sheds the water heater or AC for a few minutes when the panel gets busy, which routinely drops the sale one size, from a 22 kW to an 18, or an 18 to a 14. Ask the installer to quote both ways. Our number assumes no management, so treat it as the ceiling.
Is this an official load calculation?
It is the rule-of-thumb version. A permit application needs an NEC Article 220 calculation done by the installing electrician, and theirs counts nameplates, not chart averages. Expect their number to differ from ours by a kW or two in either direction; ours is for budgeting before they show up.
Portable or standby for home backup?
If the must-run list is fridge, furnace, sump, lights, and outlets, a 5,000 to 7,500W portable on an interlock covers it for a tenth of the installed price of a standby. Standby earns its cost when you need central AC, medical equipment, or backup that works while nobody is home. The guide on portable vs standby walks the tradeoff.