Free • No signup • Shows the math
Generator Size Calculator
Pick what you want to power. You get the running watts, the one surge that actually decides the size, and the honest answer on how big the machine needs to be.
—total running watts
—peak (largest single surge)
—recommended size
How the math works (so you can check us)
- Add the running watts of everything you'll power at once.
- Find the largest motor-driven item's starting watts — the surge. Add that once.
- Multiply by 1.25 for headroom. A generator that runs flat-out for hours dies young.
- Round up to the nearest standard size: 2, 3, 3.5, 5, 6.5, 7.5, 9, 10, 12, 15 kW.
Needed = (Σ running watts + largest single surge) × 1.25
Wattage figures are typical nameplate values compiled from DOE/ENERGYSTAR appliance data and manufacturer spec sheets (Generac, Honda, Jackery). Check your own nameplates — yours may differ. Surge estimates: 3× running watts for pumps and compressors unless noted.
Winter-storm reality check
Hit the "Winter storm essentials" preset and you'll land around 3,600 running watts / 4,100 peak → a 5.1 kW need → buy a 6 kW class unit. That's the same answer generator manufacturers publish for the same scenario. Same math, no upsell.
Mistakes this calculator prevents
- Summing every surge at once. Motors almost never start at the same moment. You only pay for one surge, not five.
- Forgetting the furnace fan. In a winter outage your gas furnace still needs electricity to blow the heat around (~600–1,000 W running, ~2,000+ surge). Size without it and you'll have a furnace that runs but a house that stays cold.
- Running flat-out with no headroom. The 25% margin isn't padding — it's what keeps the engine alive past season two.
- Buying out of fear. A 15 kW standby for storm essentials burns more fuel sitting at low load and costs 3–4× what the right-sized inverter does.