What size generator do I need?
Short answer
To keep a fridge, furnace fan, sump pump, a few lights and the internet going, look for about 3,500 running watts and 4,400 starting watts. A fridge and lights alone need about 1,500. Running a whole house with central air takes a home standby generator of 20,000 watts or more.
Work it out for your numbers
Look for a generator of
3,500 running watts
- Starting watts at least
- 4,400 watts
- Your running load
- 2,850 watts
- Biggest startup surge
- Furnace blower (+1,550 W)
Running watts and starting watts
Every generator lists two numbers. Running watts is what it can supply all day. Starting watts is a brief extra burst for the second or two when a motor starts, since fridges, pumps and furnace fans draw two to three times their running power at startup. You need enough of both.
Common outage setups
| What you want to run | Look for (running) | Starting watts at least |
|---|---|---|
| Fridge, 6 LED bulbs, TV and internet | 1,500 W | 2,460 W |
| Fridge, furnace fan, sump pump, 10 LED bulbs, TV and internet | 3,500 W | 4,400 W |
| The same, plus a well pump | 5,000 W | 5,400 W |
| The same, plus a window air conditioner instead of the well pump | 5,000 W | 6,450 W |
| Two fridges or freezers, furnace fan, sump pump, well pump, microwave, 15 bulbs, TV | 7,000 W | 7,150 W |
The “look for” figure includes 20% headroom so the generator isn’t running flat out. Starting watts allow for the biggest single motor starting, since they rarely all start at once.
Typical appliance watts
| Appliance | Running watts | Starting watts |
|---|---|---|
| Refrigerator or freezer | 700 | 2,200 |
| Furnace blower | 800 | 2,350 |
| Sump pump | 1,050 | 2,150 |
| Well pump | 1,000 | 2,100 |
| Window air conditioner | 1,200 | 3,600 |
| Microwave | 1,000 | 1,000 |
| Space heater | 1,500 | 1,500 |
| Electric water heater | 4,000 | 4,000 |
These are typical figures from generator makers’ worksheets. The label on your own appliance gives the real number.
Safety first
- Run generators outside only, at least 20 feet from doors, windows and vents, as the CDC advises. Exhaust carbon monoxide kills within minutes. Put a battery CO alarm inside.
- Never plug a generator into a wall outlet to power the house. That “backfeeds” the grid and can electrocute utility workers. An electrician can install a transfer switch or interlock.
- Use heavy-gauge outdoor cords between the generator and appliances.
Common questions
Will a 2,000 watt generator run a refrigerator?
It depends on its starting rating. A typical fridge runs on about 700 watts but can need about 2,200 for a moment at startup, so look for a generator whose starting (peak) watts cover that, and check your fridge’s label.
What size generator runs a whole house?
With central air, an electric range and an electric water heater, usually a home standby generator of 20,000 watts or more. A 5,000 to 7,500 watt portable covers the essentials.
How far should a generator be from the house?
At least 20 feet from doors, windows and vents, according to the CDC.
Sources
Updated October 29, 2026. Figures are estimates for planning. Check product labels and local codes before you buy or build.