Generator Size Calculator
Work out how many watts of generator capacity you need to run everything at once, plus the startup surge from whichever appliance needs it most. Add, remove or edit appliances to match your home.
How this calculator works
Most appliances only need their running watts most of the time, but motor-driven appliances briefly spike to a higher starting watts figure when they switch on. A generator only has to cover one appliance's surge at a time — assuming they don't all start in the same instant — so the calculator checks every appliance's surge differential and sizes for whichever one creates the largest total demand:
total running watts = sum of every appliance's running watts for each appliance: candidate peak = total running watts − its own running watts + its starting watts peak demand = the largest candidate peak across all appliances recommended generator size = ceil( peak demand × 1.2 ) // 20% buffer
This matters because the appliance with the highest starting watts isn't always the one that creates the biggest peak demand — what matters is the gap between an appliance's starting and running watts, not its starting watts alone.
Worked example
Using the default appliance list — fridge, sump pump, furnace fan, well pump, and lights:
- Total running watts: 800 + 800 + 500 + 1,000 + 200 = 3,300 W.
- Surge gap per appliance: fridge 2,000 − 800 = 1,200; well pump 2,000 − 1,000 = 1,000; sump pump 1,300 − 800 = 500; furnace fan 1,000 − 500 = 500. The fridge has the largest gap, even though the fridge and well pump share the same starting watts figure.
- Peak demand: 3,300 − 800 + 2,000 = 4,500 W.
- Recommended size: 4,500 × 1.2 = 5,400 W.
Running vs starting watts — typical figures
| Appliance | Running (W) | Starting (W) |
|---|---|---|
| Refrigerator | 800 | 2,000 |
| Sump pump | 800 | 1,300 |
| Furnace fan | 500 | 1,000 |
| Well pump | 1,000 | 2,000 |
| Lights / misc electronics | 200 | 200 |
These are typical figures for reference only — check the nameplate or manual for your specific appliances, since actual wattage varies by model and age.
Frequently asked questions
What is the difference between running watts and starting watts?
Running watts is the power an appliance draws once operating normally. Starting watts, or surge watts, is the higher, brief spike many motor-driven appliances need to start up — often two to three times their running watts. A generator has to cover everything running at once, plus the surge from whichever single appliance is starting at that moment.
How do I size a generator for a well pump or sump pump?
Both have motors with a real starting surge, so size around their starting watts rather than running watts, especially if either is likely to be the largest surge in your list. Check the specific pump's nameplate or manual for its actual figures rather than assuming a generic default.
Portable vs standby generators — what's the general tradeoff?
Portable generators are cheaper and moveable but need manual connection and refueling, and must always run outdoors. Standby generators install permanently, start automatically during an outage, and run on natural gas or propane, but cost significantly more and need professional installation.
Why should a generator never run indoors?
Portable generators produce carbon monoxide, a colorless, odorless gas that can build up to lethal levels in enclosed or partially enclosed spaces — including garages, even with the door open. Always run a generator outdoors, well away from windows, doors and vents, and use a carbon monoxide detector inside the home.