Essential circuits
Fridge, lights, furnace fan, sump pump, internet — plus one small AC with load management.
Most of the house
Central air (one zone) + all essentials. The sweet spot for 1,500–2,500 sq ft homes.
True whole-house
Multiple AC zones, well pump, electric range, EV charger. 2,500+ sq ft without compromise.
Don't guess: the kW size calculator adds up your essential circuits in two minutes.
How the math works
Every load has two numbers: running watts (what it draws continuously) and starting watts (the brief surge when a motor starts). Your generator must cover all running watts at once, plus the single largest starting surge. Air conditioning dominates both — a 3-ton unit needs roughly 3,500 W running but 12,000 W to start.
The calculator adds a base 3,000 W for lights, fridge, freezer, internet and outlets, sums your checked loads, adds the biggest motor surge, then applies your safety margin and rounds up to the next standard generator size (14, 18, 20, 22, 24 or 26 kW).
One fuel caveat: several models derate on natural gas(a 20 kW LP unit may be 18 kW on NG). If you're on natural gas, size to the NG rating, not the headline number. Oursizing guide walks through the whole process, and theinstallation cost estimatorprices the project once you know your size.