Generator Size Calculator — What Size Generator Do I Need?
What size generator do you need for your home? Select the appliances you want to power during an outage — fridge, sump pump, furnace, well pump, AC — and get the exact running watts, peak surge watts, and recommended generator size. Free.
Generator sizing comes down to two numbers: total running watts (everything on at once) and peak watts (running load plus the single largest motor-starting surge — motors draw 2–3× their running wattage for a moment at startup). A fridge, sump pump, furnace fan, and lights typically need a 5,000–7,500 W portable; add central air conditioning and you're into 10 kW+ or whole-home standby territory. Select your appliances to get both numbers plus a recommended size class with a 10% safety margin built in.
Inputs
Add running watts for anything else (medical equipment, freezer, EV…)
Results
Recommended Generator Size (W)
4,100
Size Class
3,500–4,500 W mid-size portable
Total Running Watts
2,100
Peak (Starting) Watts
3,650
Understanding This Calculator
Learn how to size a backup generator from running watts and motor-starting surges — big enough to hold your critical loads, without paying for capacity you'll never use.
How the math works
Every appliance has two numbers: running watts (continuous draw) and starting watts (the brief surge when a motor spins up — a fridge compressor may need 2,200 W to start but only 700 W to run).
The sizing rule: sum the running watts of everything you'll power, then add the single largest starting surge (starting − running for that one appliance). Motors do not all start at the same instant, so you only budget for the biggest one. The result is multiplied by a 1.1 safety margin — generators shouldn't run flat-out continuously — and mapped to a standard size class, from a 2 kW inverter unit up to an 18–24 kW whole-home standby.
Worked example
Say an outage kit covers a fridge (700 W running / 2,200 W starting), a sump pump (1,050 / 2,200), and lights, TV, and internet (600 / 600):
- Running total = 700 + 1,050 + 600 = 2,350 W
- Largest single surge delta = fridge: 2,200 − 700 = 1,500 W (sump pump's is 1,150)
- Peak = 2,350 + 1,500 = 3,850 W
- With the 1.1 margin ≈ 4,300 W → a 3,500–4,500 W mid-size portable
Adding central air conditioning (3,800 / 8,000 W) would push the same household into standby-generator territory.
How to read the result
The recommended wattage is a floor, not a bragging target. Slightly oversized is good (capacity for the next appliance, quieter operation under partial load); heavily oversized wastes fuel and money, and lightly loaded generators run inefficiently. If the result lands just above a size class, either trim a load or step up — never plan to run a generator at 100% for hours.
- Well pumps and central AC dominate sizing; decide early whether they are truly essential during an outage.
- Electric resistance heat, ovens, and dryers are usually impractical on portable generators.
- Any connection to house wiring requires a transfer switch installed by a licensed electrician — backfeeding through an outlet is illegal and can kill line workers.