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Generator Size Calculator
Use this free generator size calculator to work out how big a generator your home needs before you buy one or fire one up in a power outage. Check the appliances you plan to run, adjust their running and starting watts to match your own models, and the calculator adds up the load, adds the biggest motor surge, applies a safety margin, and gives you the recommended size in watts, kilowatts, and kVA.
Step 1: pick the appliances you will run at the same time. Edit any number to match the label on your appliance.
| Appliance | Running (W) | Starting (W) | ||
|---|---|---|---|---|
20% is the usual margin. It keeps your normal load at about 80% of the generator's rating, so the engine is not working at its limit for hours.
| Amount | Value |
|---|---|
| Total running watts | 3,450 W |
| Largest starting surge (extra) | 2,400 W |
| Running + largest surge | 5,850 W |
| Safety margin (20%) | 1,170 W |
| Recommended watts | 7,020 W |
| Recommended kilowatts | 7.02 kW |
| Recommended kVA (÷ 0.8 power factor) | 8.8 kVA |
| Next standard size up | 7.5 kW |
Appliances covered by this size:
How it works
Sizing a generator is a watts problem, and it has two kinds of watts. Running watts, also called rated watts, are what an appliance uses while it is running steadily: a refrigerator hums along at about 600 watts, a microwave at about 1,200 watts, a TV at about 150 watts. Starting watts, also called surge watts, are the short burst a motor draws for a second or two when it switches on. A refrigerator that runs at 600 watts can surge to 1,200 watts at startup, and a window air conditioner that runs at 1,200 watts can surge to 3,600 watts.
The calculator adds up the running watts of every appliance you checked, because all of them draw power at the same time. Then it looks at the extra surge of each motor appliance, which is its starting watts minus its running watts, and adds only the single largest one. The refrigerator adds 600 extra watts of surge, the window air conditioner adds 2,400 extra watts, and the air conditioner wins.
Largest surge = biggest (starting watts − running watts) among the checked appliances
Required watts = (total running watts + largest surge) × (1 + safety margin)
Kilowatts = watts ÷ 1,000 · kVA = kilowatts ÷ 0.8 (power factor)
After the surge, the calculator applies a safety margin, 20 percent by default. This margin does two jobs. First, it covers the small loads you forgot, a few extra lights or a phone charger, and the fact that nameplate watts are rounded. With a 20 percent margin your normal load sits near 80 percent of the generator's rating, which is where it is happiest.
Kilowatts and kVA are the two units generator sizes are sold in, and they are not the same. Kilowatts measure the real power your appliances actually consume. For home generators the two are linked by the power factor, conventionally 0.8, so kVA equals kW divided by 0.8. A 7.02 kW load needs about 8.8 kVA of generator capacity.
Finally, the calculator rounds up to the next standard generator size, because generators are sold in standard steps: 2 kW, 3 kW, 5 kW, 7.5 kW, 10 kW, 12 kW, 15 kW, 20 kW, 25 kW, and 30 kW. If your required size is 7.02 kW, the next standard size up is a 7.5 kW generator.
To get accurate numbers, read the label on each appliance, usually on the back or inside the door, and use its watts if it lists them. If the label lists only amps, multiply amps by volts, usually 120 volts for a standard outlet, to get watts. If you cannot find a label, the presets in the table are typical values for common models, and the worked example below shows how the math runs with them.
Worked example
You want backup power for a refrigerator, a window air conditioner, a microwave, house lights, and a TV during an outage.
Step 1: Add the running watts — 600 (refrigerator) + 1,200 (air conditioner) + 1,200 (microwave) + 300 (lights) + 150 (TV) = 3,450 watts.
Step 2: Find the largest extra surge — the refrigerator surges 1,200 − 600 = 600 watts, and the air conditioner surges 3,600 − 1,200 = 2,400 watts, so the largest surge is 2,400 watts.
Step 3: Add the surge — 3,450 + 2,400 = 5,850 watts.
Step 4: Apply the 20 percent margin — 5,850 × 1.20 = 7,020 watts.
Step 5: Convert — 7,020 ÷ 1,000 = 7.02 kW, and 7.02 ÷ 0.8 = 8.78, or about 8.8 kVA.
Step 6: Round up to the next standard size — 7.02 kW rounds up to a 7.5 kW generator.
The result is a 7,020 watt, 7.02 kW, 8.8 kVA requirement, covered by a 7.5 kW generator.
FAQ
What size generator do I need for a house?
It depends on what you run at the same time. A few essentials such as a refrigerator, lights, a TV, and phone chargers usually need a 2,000 to 3,000 watt generator. Add a window air conditioner or a well pump and you are in the 5,000 to 7,500 watt range. A whole house with central air, an electric water heater, and a well pump often needs 10,000 to 20,000 watts. List the appliances you actually run together in the calculator above and it will size the generator for your list, not a guess.
Why do you only add the largest starting watts, not all of them?
Because motor-driven appliances almost never start at the same instant. A refrigerator compressor, an air conditioner, and a sump pump each draw a big surge for a second or two when they start, but they start at different times. Adding every surge together would size a generator far larger than you need. The safe rule is to add the running watts of everything, then add only the single biggest extra surge on top, because that covers the worst moment: everything running while the biggest motor starts.
What is the difference between kW and kVA?
Kilowatts (kW) measure the real power your appliances actually use, while kilovolt-amps (kVA) measure the apparent power the generator must supply, including power that motors borrow and return each cycle. For home generators the two are linked by the power factor, usually taken as 0.8. So kVA equals kW divided by 0.8. A 7.02 kW load needs about 8.8 kVA of generator capacity. Generator nameplates often show kVA, so this calculator shows both.
Can I run my generator at full load all the time?
No, and you should not plan to. Most portable generators are rated for a maximum wattage and a lower continuous or running wattage, often about 80 to 90 percent of the maximum. Running at the maximum for hours overheats the engine, shortens its life, and risks a shutdown under load. Size your generator so your normal load sits at 70 to 80 percent of its rated running watts. That is exactly what the 20 percent safety margin in this calculator does for you.
What happens if my generator is too small?
The generator bogs down when a motor tries to start: voltage drops, lights dim, the motor hums without starting, and the generator's breaker trips or the engine stalls. Repeated low voltage can damage refrigerator compressors, air conditioners, and electronics. If the calculator's recommended size is larger than the generator you own, either buy a bigger generator or run fewer appliances at once, and never start two large motors at the same time.