Generator sizing has a precise professional method, and a retail-floor alternative that sells generators twice the necessary size. Here's the method — running watts plus one surge — and how to apply it before the next storm season.
The two numbers on the box
Every generator carries running watts (continuous output) and starting watts (brief surge headroom). The asymmetry exists because motors compressors and pumps draw 2–3× their running load for a second at startup: an 800W fridge asks for ~1,300W for a blink; a 1,000W well pump asks 3,000.
The sizing method
- List what must run in an outage — not everything, the essentials. Use the checklist calculator: fridge, furnace fan, lights, phones, maybe the sump or well pump.
- Sum the running watts. That's your continuous floor.
- Add the single largest surge — in practice motors start one at a time, not in chorus.
- Add ~20% headroom for the appliance you forgot and the generator that won't love running at 100% forever.
Essentials-only lists typically land at 2,500–4,000W; adding pumps and window AC pushes 5,000–7,500W; whole-house-with-central-air is 10,000W+ permanent-standby territory.
The questions that change the answer
- Do you have a well pump? It's the surge king of ordinary homes and singlehandedly bumps many buyers a class.
- Gas heat or electric? A gas furnace needs only its blower (~800W). Electric heat is off the portable-generator menu entirely.
- Inverter or open-frame? Inverters cost more per watt but run quiet, sip fuel at partial load, and produce clean power for electronics — for fridge-and-Wi-Fi duty, the inverter class is the modern default.
The safety chapter, briefly but seriously
Connecting a generator to house wiring requires a transfer switch installed by an electrician — backfeeding an outage grid can injure line workers blocks away. Portable units run outdoors, 20+ feet from any window, never in a garage or on a porch; carbon monoxide is the storm's real second wave. And fuel ages: rotate stored gasoline every 6–12 months with stabilizer, or store propane. The best generator is the one that starts — on the third year of ownership, in the rain, by whoever is home.