Generator size calculator
Pick the appliances you want to run at the same time. The calculator totals running watts and adds the biggest motor-start surge.
Assumption: a safety margin so the generator isn’t run at its limit. This is common practice, not a requirement from any source on this site.
Appliance wattages used by this calculator
The same sourced figures the calculator uses (also in plain HTML). Ranges are shown in full; the calculator uses the upper end. Starting watts = total watts while a motor starts. Full chart with notes.
| Appliance | Running W | Starting W | Basis | Source |
|---|---|---|---|---|
| Refrigerator or freezer (ENERGY STAR) | 132–192 | 1,200 | approx. | Honda Power Equipment |
| Refrigerator (typical range) | 150–400 | 800–1,200 | approx. | Champion Power Equipment |
| Freezer | 100–500 | 500–1,000 | approx. | Champion Power Equipment |
| Microwave oven, 650 W | 1,000 | 1,000 | approx. | Honda Power Equipment |
| Microwave oven, 1,000 W | 1,500 | 1,500 | approx. | Honda Power Equipment |
| Coffee maker (4-cup) | 600 | 600 | approx. | Honda Power Equipment |
| Dishwasher (cool-dry cycle) | 216 | 540 | approx. | Honda Power Equipment |
| Dishwasher (heated cycle) | 1,200–2,400 | — | approx. | Champion Power Equipment |
| Electric fry pan | 1,500 | 1,500 | approx. | Honda Power Equipment |
| Electric range, one 8-inch element | 2,100 | 2,100 | approx. | Honda Power Equipment |
| Window AC, 5,000 BTU | 450–600 | 900–1,200 | approx. | Champion Power Equipment |
| Window AC, 10,000 BTU | 1,500 | 2,200 | approx. | Honda Power Equipment |
| Central AC, 2-ton | 2,000–2,500 | 3,500–4,500 | approx. | Champion Power Equipment |
| Central AC, 3-ton | 3,000–3,500 | 5,000–6,000 | approx. | Champion Power Equipment |
| Central AC, 4-ton | 4,000–5,000 | 6,500–8,000 | approx. | Champion Power Equipment |
| Furnace fan (gas/oil), 1/8 hp | 300 | 500 | approx. | Honda Power Equipment |
| Furnace fan (gas/oil), 1/4 hp | 600 | 1,000 | approx. | Honda Power Equipment |
| Furnace fan (gas/oil), 1/3 hp | 700 | 1,400 | approx. | Honda Power Equipment |
| Furnace fan (gas/oil), 1/2 hp | 875 | 2,350 | approx. | Honda Power Equipment |
| Space heater (electric) | 750–1,500 | — | approx. | Champion Power Equipment |
| Electric water heater | 4,500 | 4,500 | approx. | Honda Power Equipment |
| Window fan | 50–200 | — | approx. | Champion Power Equipment |
| Ceiling fan | 15–75 | — | approx. | Champion Power Equipment |
| Sump pump, 1/3 hp | 800 | 1,300 | approx. | Honda Power Equipment |
| Sump pump, 1/2 hp | 1,050 | 2,150 | approx. | Honda Power Equipment |
| Well pump, 1/2 hp | 750–1,000 | 1,500–2,000 | approx. | Champion Power Equipment |
| Well pump, 1 hp | 1,000–2,000 | 2,000–4,000 | approx. | Champion Power Equipment |
| Garage door opener | 720 | 1,420 | approx. | Honda Power Equipment |
| Washing machine | 500–1,000 | 1,000–2,300 | approx. | Champion Power Equipment |
| Electric clothes dryer | 5,400 | 6,750 | approx. | Honda Power Equipment |
| Television, flat-screen 20" | 120 | 120 | approx. | Honda Power Equipment |
| Television, flat-screen 46" | 190 | 190 | approx. | Honda Power Equipment |
| Laptop computer | 200–250 | 200–250 | approx. | Honda Power Equipment |
| Desktop computer | 600–800 | 600–800 | approx. | Honda Power Equipment |
| Monitor (LCD) | 30 | 30 | approx. | Honda Power Equipment |
| Printer | 400–600 | 400–600 | approx. | Honda Power Equipment |
| Radio | 50–200 | 50–200 | approx. | Honda Power Equipment |
| Internet router / modem | 10–20 | — | approx. | Champion Power Equipment |
| Phone / laptop charger | 20–100 | — | approx. | Champion Power Equipment |
| Home security system | 15–40 | — | approx. | Champion Power Equipment |
| CPAP machine (no heated humidifier) | 30–60 | — | approx. | Champion Power Equipment |
| CPAP machine (heated humidifier) | 60–120 | — | approx. | Champion Power Equipment |
| LED light bulb (each) | 8–15 | — | approx. | Champion Power Equipment |
| Wi-Fi access point (not PoE-powered) | 6–12 | — | estimate | RACKDENSITY |
| Switch, 8-port unmanaged | 4–5 | — | estimate | RACKDENSITY |
| Router / firewall appliance | 12–18 | — | estimate | RACKDENSITY |
| Mini PC, Intel N100-class (light load) | 8–10 | — | estimate | MiniPCLab |
| Mini PC, 6–8 core | 12–50 | — | estimate | RACKDENSITY |
| NAS, 4 bays, HDDs spinning | 30–45 | — | estimate | RACKDENSITY |
How the calculation works
- Pick everything that will run together; add up the running watts.
- Motors (fridge, sump pump, furnace fan, AC) draw a brief surge on start. Because motors rarely start at the same instant, you only add the largest single start. Generac’s and Honda’s sizing guides use this same one-motor-at-a-time idea (Honda).
- Choose a generator whose rated (running) watts and starting (surge) watts both cover your totals.
Tip from Honda’s guide: you can lower the size you need by starting big loads one at a time and turning off non-essentials while a large motor starts.
Safety. Run gasoline generators outdoors, more than 20 feet from windows, doors and vents, and use a battery-powered CO detector indoors (CDC). Have an electrician install any transfer switch or inlet — never plug a generator into a wall outlet. 240 V appliances (dryer, range, central AC, many well pumps) need a generator with a matching 240 V outlet.
Related: appliance wattage chart · generator vs power station.