Generator Sizing Calculator
A wattage tally, not a guess: check what has to run, add the biggest motor surge on top, leave 20% headroom, and land on a standard size class.
Check what has to run — typical planning watts shown; your nameplates win (add them as custom loads below).
Planning watts from typical manufacturer-worksheet figures — your appliances’ nameplates beat every preset here. Transfer switches, permits, and installation are a licensed electrician’s scope; this page sizes watts only.
How the math works
Total running watts = the sum of everything checked (plus your custom loads). Motors do not start at their running watts, so the starting need = running total + the single largest (starting − running) surge among the loads — only one motor is assumed to kick on at the worst moment. Recommended size = starting need × 1.20 headroom, rounded up to the next standard class.
The preset watts are typical planning figures of the kind printed on manufacturer sizing worksheets. Nameplates beat presets every time: a fridge’s sticker and an AC’s LRA rating are the real numbers, and the custom fields exist for them. Transfer equipment, permits, and installation are an electrician’s scope — this page sizes watts, nothing else.
A worked example
Fridge (725 W running / 2,175 starting), furnace blower (875 / 2,350), and lights (100 / 100): 1,700 W running, the furnace’s 1,475 W surge on top = 3,175 W starting need, × 1.20 = 3,810 W — a 4,500 W class machine. Check the sump pump too and the tally moves to 5,070 W — a 5,500 W class.
Questions from the field
What size generator do I need for a fridge and furnace? About 4,500 W class on typical nameplates: 1,700 W running plus the furnace blower’s starting surge, with 20% headroom. Your nameplates can move it a class either way.
Why does starting wattage matter so much? Motors draw 2–3× their running watts for the second they start. A generator sized to the running total alone stalls or trips the moment the biggest motor kicks — the surge line is the one that decides the size.
Should I size for the whole house? Only if everything truly has to run at once. Most backup plans power a critical-load list — fridge, heat, lights, medical, communications — and skip the range, dryer, and AC. The checklist is the plan.
Does this pick a fuel or a model? No. It lands on a wattage class. Fuel, noise, inverter vs conventional, and transfer equipment are shopping and electrician decisions downstream of the number.
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