Generator Size Calculator
Add up running and starting watts for a refrigerator, sump pump, furnace, well pump, air conditioner and lights to find the generator size in kW you need.
By CalcBuilt Editorial TeamUpdated September 9, 2026Formulas checked against manufacturer specs, see how we calculate.
Results
- Generator size
- 7 kW
- 7,000 W: starting watts x 1.2 margin, rounded up to the next 500 W
- Running watts
- 3,900 W
- Everything on at the same time, at steady state
- Starting watts
- 5,450 W
- Running watts plus the 1,550 W starting surge of the furnace blower, the largest motor
- Generator type
- Portable
- Portable generators run up to about 9 kW; a standby unit is optional at this size
- Planning estimate only
- Licensed electrician
- Connect through a transfer switch or interlock installed by a licensed electrician; never backfeed a panel through a dryer or range outlet
Generator Size formula
Running watts = Sum of (count x running watts) for each appliance + lights and electronics
Starting watts = Running watts + the largest single (starting - running) surge
Generator watts = Starting watts x 1.2, rounded up to the next 500 W
Generator kW = Generator watts / 1,000 (portable up to about 9 kW, standby 10 to 26 kW)Worked example
A rural home wants a refrigerator, a 1/2 HP well pump, a 10,000 BTU window air conditioner, a microwave, 400 W of lights and an electric water heater. Running watts: 700 + 1,000 + 1,200 + 1,000 + 400 + 4,500 = 8,800 W. Starting surges above running: refrigerator 1,500 W, well pump 1,100 W, window air conditioner 2,400 W; the largest is 2,400 W.
Starting watts = 8,800 + 2,400 = 11,200 W. Times 1.2 = 13,440 W, rounded up to 13,500 W, so a 13.5 kW unit, which is standby territory. Leaving the water heater off drops the need to 4,300 running and 6,700 starting watts, or 8,040 W rounded up to 8,500 W, within reach of a large portable.
Quick reference
| Loads on at once | Running W | Starting W | Generator |
|---|---|---|---|
| Refrigerator, lights | 1,300 | 2,800 | 3.5 kW |
| Refrigerator, sump pump, furnace blower, lights | 2,900 | 4,450 | 5.5 kW |
| Above plus microwave (defaults) | 3,900 | 5,450 | 7 kW |
| Defaults plus 1/2 HP well pump | 4,900 | 6,450 | 8 kW |
| Defaults plus 10,000 BTU window AC | 5,100 | 7,500 | 9 kW |
| Defaults plus 3 ton central AC | 7,400 | 11,900 | 14.5 kW |
| Defaults plus 3 ton central AC and electric water heater | 11,900 | 16,400 | 20 kW |
How to add up generator watts
List what must run during an outage, not everything in the house. Refrigerator, sump pump, furnace blower and some lights are the essentials for most homes; a well pump if you are not on city water. Add the running watts of all of them, then add the starting surge of the single largest motor, since motors start one at a time and the generator only has to absorb one surge on top of the steady load. Multiply by 1.2 so the generator is not running at its limit, which shortens its life and lets voltage sag.
Nameplates give the most accurate numbers. Where the label lists amps, multiply by volts (120 or 240) to get watts. Resistance loads such as a microwave, toaster or electric water heater have no surge but pull a lot of steady power, and the 4,500 W water heater is usually the first thing to leave off when sizing a portable.
Portable or standby, and connecting it safely
Portable generators top out around 9 kW and cover essentials plus a window air conditioner. Anything with central air, an electric water heater or a well pump on top of the basics pushes into standby territory: an air-cooled unit of 10 to 26 kW on a pad outside, fed by natural gas or propane, that starts itself when the power fails. Sizing every load in the house at once on a standby unit is rarely necessary; a load-management module can shed the air conditioner while the water heater runs.
This is a planning estimate. A licensed electrician installs a transfer switch or an interlock kit so the generator can never feed the utility lines, and local code decides where the unit sits and how it is fueled. Never plug a generator into a wall outlet or dryer receptacle: it backfeeds the grid, can kill a line worker, and destroys the generator when the power returns.
Key facts
- Motors draw two to three times their running watts for a second or two at startup, which is why a 700 W refrigerator needs about 2,200 W to start and only the largest motor is counted in the surge.
- Generators carry two ratings: running (rated) watts they can supply continuously and starting (peak) watts for a few seconds; size loads against both.
- NEC Article 702 requires a transfer switch or listed interlock for an optional standby system; backfeeding through a dryer outlet is illegal and can electrocute utility line workers.
- The CPSC advises running portable generators outdoors at least 20 ft from the house with the exhaust pointed away, because carbon monoxide from a generator can kill within minutes indoors.
- Air-cooled home standby generators come in sizes from 10 to 26 kW; a 3 ton central air conditioner alone needs about 3,500 running and 8,000 starting watts.
Frequently asked questions
What size generator do I need to run a house?
The essentials, a refrigerator, sump pump, furnace blower, microwave and 600 W of lights, need about 3,900 running and 5,450 starting watts, so a 7 kW portable. Add central air conditioning and you are at 14.5 kW or more, which means a standby generator.
Can a portable generator run central air?
Rarely. A 3 ton central air conditioner needs about 3,500 running and 8,000 starting watts on its own, and with the basics the total is around 14.5 kW, above the 9 kW that portables top out at. A soft-start kit on the compressor lowers the surge, but a standby generator is the usual answer.
How many watts does a refrigerator need on a generator?
About 700 W running and 2,200 W for the second or two the compressor starts. A 2,000 W inverter generator can run a refrigerator alone but stalls if the compressor starts while other loads are on.
What size generator for a sump pump and furnace?
A 1/3 HP sump pump, a furnace blower, a refrigerator and 600 W of lights draw 2,900 running watts and 4,450 starting watts, so a 5,500 W (5.5 kW) generator with the 1.2 margin.
Sources and references
- What Size Generator Do I Need? Sizing guide for running and starting watts (Generac)
- Generator Wattage Worksheet (running and starting watts by appliance) (Generac)
- NFPA 70, National Electrical Code (Article 702 Optional Standby Systems) (NFPA)
- What to Know About Generators and Carbon Monoxide (U.S. Consumer Product Safety Commission)
Next steps for this project
The usual order for a electrical & plumbing project. See all electrical & plumbing calculators.
Results are estimates based on standard formulas and typical product specifications. Confirm quantities with your supplier and local code before ordering.