What size generator do I need? Red Cross method + calculator
The American Red Cross gives a simple method. We turned it into a calculator, and added what rental companies in our directory actually state about sizes.
The Red Cross method
- Add up the power requirements of the appliances and devices you will want to use (check the label on the back or side).
- Add up the wattage of all the light bulbs you will want to use.
- Find the total amps you need by dividing watts by volts.
- Choose a generator that produces more amps than you need, because some machines draw up to 3 times as much power when starting up.
Most household devices run on 120 volts; most major appliances with a large plug require 240 volts (American Red Cross: Generator Safety).
What size generator do I need?
Pick what it's for, adjust what you'll power, and get a size range from published wattage tables. Confirm the final size with the rental company.
| What you'll power | Watts each (running) | Qty |
|---|---|---|
| Refrigerator Champion Power Equipment · starts at up to 1,200 W | 150–400 | 1 |
| Freezer Champion Power Equipment · starts at up to 1,000 W | 100–500 | 0 |
| Sump pump, 1/2 HP Champion Power Equipment · starts at up to 2,150 W | 800–1,050 | 0 |
| Well pump, 1/2 HP Champion Power Equipment · starts at up to 2,000 W | 750–1,000 | 0 |
| Well pump, 1 HP Champion Power Equipment · starts at up to 4,000 W | 1,000–2,000 | 0 |
| Window AC, 10,000 BTU Champion Power Equipment · starts at up to 2,400 W | 900–1,200 | 1 |
| Central AC, 3 ton Champion Power Equipment · starts at up to 6,000 W | 3,000–3,500 | 0 |
| Furnace fan, 1/2 HP Champion Power Equipment · starts at up to 1,600 W | 300–800 | 0 |
| Microwave | 600–1,200 | 1 |
| TV (32"–55") | 80–400 | 1 |
| LED light bulb (each) | 8–15 | 6 |
| Internet router / modem | 10–20 | 1 |
| Phone / laptop charger (each) | 20–100 | 2 |
| CPAP with heated humidifier | 60–120 | 0 |
| Space heater | 750–1,500 | 0 |
| Electric water heater | 3,000–4,500 | 0 |
| Washing machine Champion Power Equipment · starts at up to 2,300 W | 500–1,000 | 0 |
Running load 1,828–3,510 W, plus 1,200 W for the largest start-up surge, plus a 20% buffer.
- Type
- Portable generator (inverter if noise matters)
- Fuel
- Gasoline, propane or dual-fuel (ask what the unit takes)
- Ask for
- Transfer switch installed by an electrician if you want to power house circuits (never backfeed through an outlet)
This is an estimate from published averages, not a quote. Give the rental company your actual equipment list (labels show watts) and confirm the size. How we calculate
How the estimate is calculated, with every source
What rental companies state
Of the 231 rental companies in our directory, 190 state at least one generator size on their website and 111 mention towable units. Stated top sizes range from 0.6 kW to 4,000 kW. Each figure is quoted on the provider’s page.
For wiring a generator into a house, the Red Cross says to have an electrician install a power transfer switch (American Red Cross: Generator Safety).