Sizing a generator comes down to one number: the total watts you need at the same time, plus surge headroom. Get it right and everything runs; get it wrong and breakers trip during the storm you bought it for.
How generator sizing actually works
Every appliance has two wattage numbers:
- Running watts — what it draws continuously.
- Starting (surge) watts — the spike when a motor starts, often 2–3× the running number.
Your generator needs to cover the sum of all running watts plus the single largest surge among them (motors rarely all start at the same instant — but one of them always starts at the worst one).
Typical appliance wattages
| Appliance | Running watts | Starting watts |
|---|---|---|
| Refrigerator/freezer | 700 | 2,200 |
| Sump pump (1/2 HP) | 1,050 | 2,150 |
| Furnace fan (gas heat) | 800 | 2,350 |
| Window AC (10,000 BTU) | 1,200 | 3,600 |
| Central AC (3 ton) | 3,800 | 11,000 |
| Well pump (1 HP) | 2,000 | 4,000 |
| Lights (LED, whole house) | 150 | 150 |
| Wi-Fi router + modem | 20 | 20 |
| CPAP machine | 60 | 60 |
| Electric water heater | 4,500 | 4,500 |
Check your actual nameplates — these are typical values, not your values.
Three sizing tiers
Essentials only (3,500–5,000W). Refrigerator, sump pump, lights, phone charging, Wi-Fi, CPAP. This is the portable-generator sweet spot and covers what keeps your home safe and your food cold.
Comfort backup (5,500–8,500W). Everything above plus a window AC or the furnace fan, microwave, and TV. Most popular tier for storm-prone areas; still portable, though heavy.
Whole house (10,000–24,000W). Central air, water heater, well pump, the works. At this size you’re usually looking at a standby generator with an automatic transfer switch — see our whole house generator cost guide for what installation really costs.
Worked example
Say an outage plan covers: refrigerator (700W), sump pump (1,050W), furnace fan (800W), lights (150W), Wi-Fi (20W), and a CPAP (60W).
- Running total: 700 + 1,050 + 800 + 150 + 20 + 60 = 2,780W
- Largest single surge: furnace fan at 2,350W means +1,550W above its running draw
- Requirement: 2,780 + 1,550 = 4,330W
- With 20% headroom: ~5,200W — a 5,500W generator fits perfectly.
Don’t skip the transfer switch
Any generator that connects to your home’s wiring — not just extension cords to appliances — legally and practically requires a transfer switch or interlock installed by an electrician. It prevents backfeeding the grid, which is lethal to line workers and will destroy the generator when power returns.
Bottom line
Add up your real running watts, add the single biggest surge, then add 20% headroom. For most homes that lands at 5,000–7,500 watts for essentials — and if the answer comes out above 10,000 watts, start comparing standby generators instead of portables.