What size generator do I need?
Tick everything you need running at the same time. The figure that decides your purchase is not the sum of the running wattages — it is your running total plus the largest single startup surge, because a motor draws two to three times its rated power for the instant it spins up.
Why not add every surge
Because motors never all start at the same instant. Summing every startup figure is the most common sizing error there is, and it sells people a generator roughly twice the size they need — often a thousand dollars past the right one.
Common questions
- Should I size against running watts or starting watts?
- Starting watts. Anything with a motor — a refrigerator, a well pump, an air conditioner — briefly draws two to three times its running wattage when the compressor kicks in. A generator sized only to the running total will trip or stall the moment that surge arrives.
- Why not just add up every appliance's starting watts?
- Because motors do not all start at the same instant. Adding every surge together typically doubles the generator you think you need. The correct figure is your total running load plus the single largest startup surge, which is what this calculator uses.
- How much headroom should I leave?
- About 20%. Running a generator pinned at full load is loud, burns more fuel per watt, and shortens engine life. It also leaves nothing spare when you inevitably plug in one more thing.
- Can I run my central air conditioner?
- A three-ton unit can surge past 10,000W, which pushes you into the largest portables. A soft-start kit cuts that surge substantially and costs far less than the extra generator capacity would.
These figures are typical values for sizing with headroom, not a substitute for electrical design. Check the nameplate on your own appliances — a deep well pump or an older compressor can draw considerably more than the typical figure. If you are wiring a generator into a transfer switch, that work belongs to a licensed electrician.