Generator Size Calculator | Home Generator HQ

Tell us what has to keep running during an outage, and this calculator does the same math as our sizing guides: it adds up running watts, adds the largest motor starting surge on top, applies headroom, and matches the result to generator classes and models from our dataset. Square footage alone doesn't draw power - appliances do - so the size field below only suggests a typical AC tonnage; the loads you tick are what actually get counted.

Your home and climate

Home size (sq ft) used only to suggest an AC size below Central air conditioner counted at about 1,040 running W per ton, double that at compressor start Climate adjusts the assumptions openly - the results state exactly what changed Heat pump aux heat strips (backup heat for a heat pump) the single biggest sizing load in the house when present Electric furnace (full electric heat) a standalone electric furnace is pure resistance heat: counted at full rated draw, with no starting surge

Heat with a full electric furnace? Use the electric furnace selector. Heat with a heat pump? The aux strips selector covers the same kind of load - pick one or the other for your heat source, not both.

EV charger (Level 2)

What must keep running?

Lights, outlets, TV, internet, phones (whole-house allowance) (800 W running / 800 W starting) Refrigerator (175 W running / 900 W starting) Freezer (chest or upright) (150 W running / 800 W starting) Furnace blower / air handler (gas or oil heat) (700 W running / 1,400 W starting) Sump pump (1/3 hp) (800 W running / 1,600 W starting) Well pump (1 hp) (2,000 W running / 4,000 W starting) Electric water heater (4,500 W running / 4,500 W starting) Electric range / oven (typical cooking load) (5,000 W running / 5,000 W starting) Electric dryer (5,000 W running / 5,500 W starting) Microwave (1,100 W running / 1,100 W starting) Hot tub / spa (240V, heater + pumps) (7,000 W running / 7,500 W starting) Medical equipment (e.g. oxygen concentrator) (300 W running / 300 W starting)

How this calculator works

Every watt figure is a typical value for that appliance type, labeled as such - a refrigerator at 175 running watts, a 1/3 hp sump pump at 800, and so on. Motors (compressors, pumps, blowers) draw far more for a fraction of a second when they start; those starting watts are higher than running watts, and the calculator adds the largest single surge on top of the running total, because motors rarely all start in the same instant. That allowance is an allowance, not a measurement - your appliances' nameplates (and, for a standby purchase, an installer's load calculation) are the exact sources. The checklist counts one of each load on purpose: a second fridge adds about 175 running watts next to the multi-kilowatt loads - air conditioning, electric heat, an EV charger - and those big loads are what decide the generator's size, so the form stays short and fast.

Heating and cooling are worked as two separate cases - the shared loads figured once with the air conditioner, once with the electric heat - and the larger case sets the size, because a house never heats and cools at the same time. The furnace blower runs in both seasons, so it is counted in both cases; the air conditioner and the electric heat source are never added together.

The standard 20% headroom keeps the generator at 50-80% of capacity, where it runs efficiently and lasts. The climate selector changes the assumptions in the open: hot climates get a stated +10% rule-of-thumb on top, because the AC runs near-continuously and dominates; cold climates get no multiplier, but the calculator warns - loudly - if electric resistance heat isn't counted, because 5-15 kW of heat-pump aux strips - or a 10-25 kW full electric furnace - dwarfs everything else on the list. Prefer a worked example for a specific house size? See the sizing guides for 1,500, 2,000, and 3,000 sq ft homes, or the whole-house cost guide for what the recommended class costs installed.

How this site works: We compare published specs, prices, and warranty terms - we don't sell gear and we don't fake hands-on tests. If you buy through links on this page, we may earn a commission at no cost to you. Prices shown are manufacturers' published prices, verified 2026-11-04 - they are not Amazon prices, and Amazon's price may differ. Prices are re-checked weekly against manufacturer listings.