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Generator size calculator
Add up running and starting watts for your fridge, sump pump, furnace, well pump and more to find the generator size you need for a power outage.
Results
| Generator running (rated) watts needed | 2,693 W |
|---|---|
| Starting (peak/surge) watts needed | 3,719 W |
| Total running load | 2,244 W |
- Largest start-up surge: Furnace blower (1/2 HP) (+1475 W). Start big motors one at a time, the biggest first.
- Inverter generators deliver cleaner power for electronics. Conventional open-frame generators give more watts per dollar.
About this calculator
Generators are sold with two numbers. Running (rated) watts is what the generator can sustain. Starting (peak) watts is a short burst for motor start-up. A home's outage loads are full of motors: fridges, freezers, sump and well pumps, the furnace blower. Each can draw two or three times its running watts for a second or two as it starts.
This calculator follows the method generator manufacturers use. Add up the running watts of everything that will run together, then add the single largest extra start-up surge, since you can stagger motor starts. It includes headroom so the generator isn't running near full load for hours.
How to use it
- Add each appliance you'll power at the same time. Presets use typical nameplate values, but your own appliance labels are better.
- For motors, enter both running and starting watts. Resistive loads like heaters and lights have equal values.
- Keep 20% headroom unless you know your loads precisely.
- Choose a generator whose rated watts meet the first result and whose starting watts meet the second.
The formula
Running load = Σ running W × quantity.
Starting watts needed = running load + the largest (starting − running) difference among your appliances.
Recommended rated watts = running load × (1 + headroom).
Worked example (default values)
Inputs
| What you want to power at the same time | Refrigerator (running while compressor on): 150 W; Sump pump (1/2 HP): 1050 W; Furnace blower (1/2 HP): 875 W; LED light bulb: 9 W × 6; Wi-Fi router + modem: 15 W; TV (50-inch LED): 100 W |
|---|---|
| Running headroom | 20 % |
Results
| Generator running (rated) watts needed | 2,693 W |
|---|---|
| Starting (peak/surge) watts needed | 3,719 W |
| Total running load | 2,244 W |
Tips and common mistakes
Well pumps are the big one. A 1 HP submersible well pump can need around 4,000-6,000 starting watts, and most are 240 V. Check whether your portable generator's 240 V outlet can supply it before relying on it.
Central AC with a typical 3-ton compressor often needs a 10 kW+ generator, or a soft-start kit, which can cut start-up current substantially.
Frequently asked questions
What size generator do I need to run a house?
Essentials only (fridge, sump pump, furnace fan, lights, router): usually 3,500-5,500 running watts. Add central AC or an electric water heater and you're at 10-20 kW standby territory.
Can a 2000W inverter generator run a refrigerator?
Usually yes, on its own. A modern fridge runs at about 100-250 W and starts at about 800-1,500 W. A second motor load starting at the same time can trip it.
Running watts vs starting watts?
Running watts is the continuous rating. Starting (surge) watts is a short peak, typically lasting a few seconds or less, for motor start-up.
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Estimates for planning only, based on the assumptions shown. Real-world performance varies with equipment, temperature and installation. Updated October 19, 2026. See our methodology.