Choose your essential equipment to estimate the running and starting watts you need and which generator class fits.
Generator sizing calculator
Tick the things you need during an outage. We add up running watts, add the biggest motor start, and suggest a generator size range and type.
Assumptions and limits
- Watts: Honda wattage guide, Champion wattage chart (upper values) and ENERGY STAR product data. Nameplate method: watts = amps × volts (DOE Energy Saver). Your equipment labels beat any chart.
- Starting surge: we add the single biggest starting jump (starting minus running watts) to the total running watts, assuming motors start one at a time (Honda wattage guide, Generac sizing guide: starting is often 2–3× running). Where a chart gives no starting figure we assume 2× running.
- Headroom: we suggest running the generator at no more than about 80% of its running rating, so the upper end of the range = running watts ÷ 0.8.
- Types (examples from maker specs): portable inverter about 1,800–3,500 W running / 2,200–4,500 W starting, 120 V; large portable about 7,500–9,200 W running with a 240 V L14-30 outlet; home standby for whole-house loads (Generac suggests 18–22 kW for many 2,000 sq ft homes, Generac sizing guide).
- 240 V: well pumps (often), central AC, electric water heaters, dryers and ranges need 240 V. A submersible well pump needs a generator rated well above its running watts (Franklin Electric AIM manual: 1 HP → 4 kW minimum, 50% more for 2-wire motors).
- Safety (CPSC): run portable generators outdoors only, at least 20 feet from the house, exhaust pointed away from windows, doors and vents; never in a garage, basement or shed. Put battery CO alarms on every level. Never plug a generator into a wall outlet: a licensed electrician must install a transfer switch or interlock.
- This is an estimate, not a design. A licensed installer must confirm sizing, wiring and permits.
Learn more in our Generators guide, or see all calculators.