Running watts vs starting watts — the sizing key
Motors (fridges, pumps, AC compressors) draw 2–3× their running watts for a second at startup. A generator must carry your total running load plus the largest single starting surge — not all surges at once, because appliances rarely start simultaneously (and you can stagger them). This calculator adds 20% headroom on top, the buffer most manufacturers recommend for continuous operation and altitude/temperature derating.
Typical generator classes
| Size | What it realistically runs | Type |
|---|---|---|
| 2,000 W | Fridge + lights + phone/laptop | Inverter, quiet |
| 3,500–4,500 W | + sump pump, furnace blower, TV | Portable |
| 6,500–8,000 W | + well pump, microwave, window AC | Portable, 240 V |
| 10–13 kW | Most of the house except central AC/electric heat | Large portable / small standby |
| 14–24 kW | Whole home incl. 3–5 ton AC | Standby, auto-transfer |
Practical notes before you buy
- 240 V loads (well pump, central AC, range) need a 240 V generator and a transfer switch or interlock — never backfeed through a dryer plug.
- Central AC is usually the deal-breaker: its surge pushes you into standby-generator territory. A soft-start kit can cut AC surge by 60–70% and let an 8 kW portable start a 3-ton unit.
- Fuel: at half load, a 7 kW portable burns roughly 0.6–0.9 gal of gasoline per hour — plan storage accordingly.
Frequently asked questions
What size generator do I need to run a house?
Essentials (fridge, lights, furnace blower, sump pump, TV) fit in 4,500–6,500 W. Add a well pump and window AC: 8,000 W. Whole-home with central air: 14–24 kW standby.
Will a 10,000 watt generator run central air?
Usually yes for a 3-ton unit (3,500 W running / ~8,700 W surge) with little else running, and comfortably with a soft-start kit installed. For 4–5 ton systems move to 13 kW+.
What's the difference between running and starting watts?
Running watts is continuous draw; starting watts is the 1–2 second surge when a motor spins up — typically 2–3× running. Size for running total + the single largest surge.
Can I just add up all starting watts?
No — that massively oversizes. Appliances don't all start at the same instant, and you can stagger startup. Use total running + largest single surge + ~20% headroom.