Generator Size Calculator — What Size Generator Do I Need?
Enter your essentials or whole-house loads to get running watts, starting surge and a generator size. No sign-up, no data stored.
Generator size
How generator size is determined
- Running watts = the sum of everything you plan to run at once.
- Starting surge = motors draw 2–4× their running watts for a few seconds. The generator's surge rating must cover the largest single motor start (AC or well pump).
- Peak = running watts + the largest surge (not every motor at once).
- Safety = add 20% so the generator isn't running at 100% continuously.
A 5,500-watt generator rated 8,000 surge covers essentials with a 1,100-watt well pump; the same generator trips if you also start a 3,500-watt AC compressor. The biggest motor start decides the size.
Typical appliance wattage reference
Running watts and approximate starting surge for common loads.
| Appliance | Running W | Starting surge |
|---|---|---|
| Refrigerator | 600–800 | 1,800–2,400 |
| Sump pump (1/2 hp) | 800–1,100 | 1,900–2,300 |
| Well pump (1 hp) | 1,000–1,500 | 3,000+ |
| Furnace blower | 800–1,200 | 2,000–3,000 |
| Central AC (3-ton) | 3,000–3,500 | 7,000+ |
| Electric water heater | 4,000–4,500 | — |
Last updated: October 2026 · Ratings vary by brand and model — use the label on each appliance for exact numbers. Never run a generator inside; keep it 20+ ft from doors and windows.
Generator FAQ
Essentials: fridge, lights, furnace, sump pump, phone chargers and a few outlets — roughly 4,000–4,500 running watts with headroom. Not central AC or an electric water heater.
Essentials only: 7,500 watts. Whole house with central AC: 15,000–20,000 watts and a transfer switch sized to your panel.
Yes — backfeeding without one can kill a lineman and is illegal in most areas. A manual interlock or transfer switch disconnects you from the grid safely. Never plug a generator into a wall outlet.
AC compressors surge 2–3× their running watts for a second or two. If your generator's surge rating is below that start, the breaker trips. That's the single most common undersizing failure.
No — a generator run at 10% load wastes fuel, carbon-loads the engine and can under-voltage your electronics. Size for your real peak plus 20%, not the biggest you can afford.
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