How we size generators
Two numbers
Every appliance has running watts — what it draws while on — and starting watts, the short burst a motor needs to get going, often several times its running power.[1][2] We store starting watts as the total at startup, never just the extra.
The formula
- Running = everything that runs together + the biggest item that takes turns.
- Peak = running + the single biggest startup jump. Motors don’t start in the same second, so we add only one.
- Minimum rated watts = running ÷ 0.8, so the generator isn’t pushed above about 80% for hours.
- Generator class = the smallest class that covers the rated watts and can start the peak, assuming starting capacity of about 1.25× rating.
Where the numbers come from
Manufacturers publish wattage charts, but they don’t agree — and they don’t even define “starting watts” the same way. Some list the additional watts on top of running, others the total. We convert every chart to totals, then take the median across official charts and round up to 50 W. When a motor has no published surge, we use the common rule of allowing three times its running watts. Retail charts are used only as a cross-check.
Your own appliance may differ. The label shows watts, or amps you can multiply by volts[3] — add it in the calculator as your own appliance.
| Appliance | Running | Starting (total) | Volts | Basis | Charts |
|---|---|---|---|---|---|
| Kitchen | |||||
| Refrigerator | 700 | 2,900 | 120 | Median of manufacturer charts | 7 |
| Chest freezer | 500 | 1,500 | 120 | Typical value — no chart lists this exact item | 1 |
| Upright freezer | 700 | 2,900 | 120 | Median of manufacturer charts | 5 |
| Microwave (1,000 W) | 1,000 | 1,000 | 120 | Median of manufacturer charts | 6 |
| Coffee maker | 1,300 | 1,300 | 120 | Median of manufacturer charts | 6 |
| Toaster | 1,100 | 1,100 | 120 | Median of manufacturer charts | 2 |
| Slow cooker | 250 | 250 | 120 | Median of manufacturer charts | 3 |
| Dishwasher | 1,500 | 1,500 | 120 | Typical value — no chart lists this exact item | 1 |
| Electric stove, 8-inch burner | 2,100 | 2,100 | 240 | Median of manufacturer charts | 5 |
| Heating & Cooling | |||||
| Furnace fan (1/2 HP) | 875 | 3,150 | 120 | Median of manufacturer charts | 5 |
| Furnace fan (1/3 HP) | 700 | 2,100 | 120 | Median of manufacturer charts | 3 |
| Window AC (10,000 BTU) | 1,200 | 2,200 | 120 | Median of manufacturer charts | 3 |
| Window AC (12,000 BTU) | 1,700 | 4,200 | 120 | One manufacturer chart | 2 |
| Central AC (3 ton) | 3,500 | 10,500 | 240 | Typical value — no chart lists this exact item | 4 |
| Heat pump | 4,700 | 9,200 | 240 | Median of manufacturer charts | 3 |
| Space heater | 1,500 | 1,500 | 120 | Median of manufacturer charts | 4 |
| Dehumidifier | 650 | 1,950 | 120 | Running from spec; starting = 3× (motor rule) | 1 |
| Box fan | 150 | 300 | 120 | Median of manufacturer charts | 4 |
| Ceiling fan | 75 | 75 | 120 | One manufacturer chart | 1 |
| Water | |||||
| Sump pump (1/2 HP) | 1,050 | 2,700 | 120 | Median of manufacturer charts | 4 |
| Sump pump (1/3 HP) | 800 | 2,100 | 120 | Median of manufacturer charts | 3 |
| Well pump (1/2 HP) | 1,000 | 2,550 | 240 | Median of manufacturer charts | 2 |
| Well pump (1 HP) | 2,000 | 6,000 | 240 | Median of manufacturer charts | 2 |
| Electric water heater | 3,900 | 3,900 | 240 | Median of manufacturer charts | 6 |
| Laundry | |||||
| Washing machine | 1,150 | 2,800 | 120 | Median of manufacturer charts | 5 |
| Electric dryer | 5,500 | 6,400 | 240 | Median of manufacturer charts | 3 |
| Gas dryer | 700 | 2,100 | 120 | Running from spec; starting = 3× (motor rule) | 1 |
| Everyday | |||||
| LED lights (10 bulbs) | 100 | 100 | 120 | One manufacturer chart | 1 |
| TV (LED 55") | 150 | 150 | 120 | Typical value — no chart lists this exact item | 3 |
| Wi-Fi router + modem | 20 | 20 | 120 | Median of manufacturer charts | 2 |
| Laptop | 150 | 150 | 120 | Median of manufacturer charts | 4 |
| Desktop computer + monitor | 800 | 800 | 120 | Median of manufacturer charts | 5 |
| Phone chargers | 25 | 25 | 120 | Median of manufacturer charts | 2 |
| Security system | 100 | 100 | 120 | One manufacturer chart | 1 |
| Hair dryer | 1,500 | 1,550 | 120 | Median of manufacturer charts | 2 |
| Garage door opener (1/2 HP) | 700 | 1,450 | 120 | Median of manufacturer charts | 3 |
| Medical | |||||
| CPAP (with heated humidifier) | 120 | 120 | 120 | Median of manufacturer charts | 2 |
| Oxygen concentrator (5 LPM) | 350 | 1,050 | 120 | Running from spec; starting = 3× (motor rule) | 3 |
Manufacturer charts we compared
- Generac GP6500E brochure — starting watts listed as additional
- Generac GP-Series owner’s manual — starting watts listed as total
- Champion owner’s manual wattage chart — starting watts listed as additional
- Honda wattage estimation guide — starting watts listed as total
- Westinghouse WGen12000 manual — starting watts listed as additional
- Briggs & Stratton generator brochure — starting watts listed as additional
- Firman wattage reference guide — starting watts listed as additional
- Predator 70143 quick start guide — starting watts listed as additional
- Portlandia Electric Supply sizing guide — starting watts listed as total
Values were last checked on October 10, 2026. Found a better source? Tell us and we’ll re-check.
Sources
- U.S. Department of Energy, Motor Systems Tip Sheet #10: Turn Motors Off When Not in Use
- Michigan State University, Electrical Tech Note 314: Induction Motor Characteristics
- Virginia Cooperative Extension, Estimating Appliance and Home Electronic Energy Use