What size generator do I need?
Generators have two ratings, and you need to check both:
- Running watts — what it can supply continuously. Add up everything you'll run at the same time.
- Starting watts — the short surge it can deliver while a motor starts. Fridges, pumps, furnace blowers and air conditioners draw two to three times their running power for a few seconds.
Generac's method, which the calculator follows: total running watts + the single highest starting surge = the starting watts you need. Motors rarely start at the exact same moment, so only the biggest one counts.
Generator wattage calculator: appliance chart
| Appliance | Running watts | Additional starting watts |
|---|---|---|
| Refrigerator/freezer | 700 | 2,200 |
| Sump pump, 1/2 hp | 1,050 | 2,150 |
| Well pump, 1/2 hp | 1,000 | 2,100 |
| Furnace fan blower, 1/2 hp | 875 | 2,300 |
| Window AC, 12,000 BTU | 3,250 | 3,950 |
| Central AC, 24,000 BTU | 3,800 | 4,950 |
| Electric water heater | 2,000 | 1,000 |
| Microwave, 1,000 W | 1,000 | — |
| Washing machine | 1,150 | 2,300 |
| Television, 27 in | 500 | — |
| Computer and monitor | 800 | — |
| Garage door opener, 1/2 hp | 875 | 2,350 |
From Generac's generator guide; Generac notes the figures are approximate. Newer ENERGY STAR refrigerators and window units often draw less — the nameplate (volts × amps) or a plug-in meter gives your real number.
Generator sizes and what they cover
| Generator (running / starting) | Typically covers |
|---|---|
| 3,250 / 3,750 W | Fridge, lights, phone charging, TV |
| 5,500 / 6,875 W | Add a sump pump or furnace fan |
| 6,500 / 8,125 W | Fridge, sump pump, furnace fan, microwave, electronics |
| 8,000 / 10,000 W | All of that plus a well pump |
| 15,000 / 22,500 W | Central AC or several big loads; usually a home standby unit |
Our advice: size for what must run, not for everything you own, and use a transfer switch or an interlock kit with a dedicated inlet so you can choose circuits without extension cords. Never back-feed a dryer outlet, and never run a generator in a garage, under a window or in an enclosed space — carbon monoxide kills.
If quiet, fuel-free backup appeals more, compare the solar battery calculator, which sizes battery storage for the same essentials. To check a single appliance's draw, the watts to amps calculator converts its nameplate.
