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Generator Size Calculator: What Size Generator Do I Need?
Tick the appliances you want to run during a power outage. The calculator adds up running watts, allows for the extra surge when a motor starts, and recommends a portable or standby generator size.
Running watts vs starting watts
Every appliance draws its running watts while it works. Anything with a motor or compressor (fridges, pumps, furnace blowers, air conditioners) draws two to three times that for a few seconds when it starts: the starting or surge watts. A generator lists both numbers, and it must cover all running loads plus the surge of the biggest motor starting at the same time.
generator running watts ≥ total running watts × 1.2 · generator starting watts ≥ total running watts + largest extra surge
Only the single largest surge is added, because motors rarely start at exactly the same moment. Starting the biggest motor first and then adding other loads keeps it that way.
Worked example: essentials during an outage
A refrigerator (700 W), sump pump (1,050 W), furnace fan (800 W), lights (100 W), TV and internet (300 W) and chargers (100 W) need 3,050 running watts. The furnace fan has the biggest start-up surge, 1,550 W extra, so the peak is 4,600 W. With 20% headroom that calls for a generator with about 4,000 running watts and 5,000 starting watts.
Typical appliance wattage
| Appliance | Running W | Starting W |
|---|---|---|
| Refrigerator | 700 | 2,200 |
| Chest freezer | 500 | 1,500 |
| Sump pump (½ HP) | 1,050 | 2,150 |
| Well pump (½ HP) | 1,000 | 2,100 |
| Furnace fan (½ HP) | 800 | 2,350 |
| Window AC (10,000 BTU) | 1,200 | 3,600 |
| Lights (10 LED bulbs) | 100 | 100 |
| TV, modem and router | 300 | 300 |
| Phones and laptop | 100 | 100 |
| CPAP machine | 60 | 60 |
| Microwave (1,000 W) | 1,000 | 1,000 |
| Coffee maker | 800 | 800 |
| Electric burner | 1,500 | 1,500 |
| Dishwasher | 1,500 | 1,500 |
| Clothes washer | 1,150 | 2,250 |
| Electric clothes dryer | 5,400 | 6,750 |
| Electric water heater | 4,500 | 4,500 |
| Space heater | 1,500 | 1,500 |
| Garage door opener | 875 | 2,350 |
Typical figures. Check the nameplate on your own appliances: running watts = volts × amps. Central air conditioners vary widely (often 3,000 to 5,000 running watts and much higher starting); a soft-start kit can cut the surge by half or more.
Generator types
- Inverter generators (1,000 to 4,000 W) are quiet and give clean power for electronics. Good for a fridge, lights and devices.
- Portable generators (4,000 to 10,000 W) run essential circuits through a transfer switch or interlock.
- Home standby generators (10 to 26 kW) run on natural gas or propane, start automatically, and can power most or all of a house.
Safety
- Never run a generator indoors, in a garage, or near windows and vents. Keep it at least 20 ft from the house and use a carbon monoxide alarm.
- Do not plug a generator into a wall outlet ("backfeeding"). It can kill utility workers. Use a transfer switch or interlock installed by an electrician.
Want to know what those appliances cost to run normally? Try the electricity cost calculator.
Frequently asked questions
What size generator do I need to run a house?
Essentials (fridge, sump pump, furnace fan, lights, internet) need about 4,000 to 5,000 running watts. A whole house with central AC and electric appliances usually needs a 15 to 26 kW standby generator.
Will a 3,500 watt generator run a refrigerator?
Yes. A refrigerator needs about 700 running watts and up to 2,200 starting watts, so a 3,500 W generator can run it along with lights and a few small devices.
What is the difference between running and starting watts?
Running watts are what an appliance uses continuously. Starting watts are the short surge a motor needs to start, often two to three times higher.