How to use this generator size calculator
- Enter the number of each appliance you need to run at the same time.
- Check the labels of your own appliances: wattages vary.
- Choose a generator whose running watts exceed the recommendation and whose starting (surge) watts exceed the peak.
- Connect generators to a house only through a transfer switch installed by an electrician.
Example
Running a refrigerator, sump pump, furnace fan, ten LED bulbs, a TV, a computer and a microwave needs 4,225 W with a peak of 5,725 W. A generator of 5,500 running watts with enough surge capacity covers it.
What to know before you buy
Portable, inverter or standby
Portable generators (3,000–10,000 W) power essentials during outages. Inverter generators are quieter and produce clean power safe for electronics. Standby generators are permanently installed, start automatically and can power a whole house.
Fuel and runtime
Gasoline is widely available but stores poorly. Propane stores indefinitely and burns cleaner. Dual-fuel generators run on both. Check runtime at 50% load to estimate how much fuel you need for a long outage.
Typical generator sizes
| What you want to run | Total running watts | Peak starting watts |
|---|---|---|
| Fridge + lights + TV | 1,000 W | 2,500 W |
| Essentials (fridge, sump, furnace fan, lights) | 3,225 W | 4,725 W |
| Essentials + window AC | 4,425 W | 5,925 W |
| Essentials + well pump + microwave | 5,225 W | 6,725 W |
Formula
Running watts = sum of all appliances. Starting watts = running watts + the largest single extra starting surge. Recommended size adds a 20% margin to running watts.
Tips from the pros
- Check the data plate on each appliance for its actual wattage.
- Stagger starting large motors so their surges do not overlap.
- Inverter generators provide cleaner power for computers and electronics.
Common mistakes to avoid
- Backfeeding a house through an outlet: it is illegal and dangerous for utility workers. Use a transfer switch.
- Running a generator indoors or in a garage: carbon monoxide can kill.
Frequently asked questions
What size generator runs a house?
Essentials (fridge, sump pump, furnace fan, lights) typically need 5,000–7,500 running watts; a whole house with central AC may need 15–22 kW.
What are starting watts?
Motors draw 2–3 times their running power for a few seconds when they start. The generator must handle that surge.
What size generator for a refrigerator?
A refrigerator needs about 700 running watts and up to 2,200 starting watts, so a 3,000 W generator handles it with room for lights.
Can a 5,000 watt generator run a house?
It can run essentials like the fridge, sump pump, furnace fan and lights, but not central air or electric water heaters.