Generator Wattage Calculator
Generator size you need from your running and starting loads.
Input sheet
DIY at your own risk. Calcora's calculators and guides are general estimates and information only — not professional, engineering, legal, or safety advice. Always verify local building codes and permit requirements, and hire a licensed pro for electrical, gas, plumbing, structural, or any work you're not fully comfortable doing yourself.
Buy a generator too small and it stalls when the fridge kicks on; too large and you overpay and waste fuel. The right size covers everything running at once plus the single biggest motor's startup surge.
How it works
Add total running watts to the largest single starting surge; that peak sets the generator size.
Motor-driven appliances (refrigerators, well pumps, air conditioners) draw a brief starting surge two to three times their running wattage. The trick is that only one such motor typically starts at any instant, so you size for total running load plus the largest single surge — not the sum of every surge.
The result rounds up to a clean wattage and suggests a unit with headroom, because running a generator near its limit shortens its life and leaves nothing in reserve for an added load.
Generator size = total running watts + largest single starting (surge) watts.
Worked examples
3000 W of running loads with a 2200 W largest motor surge. → 5200 starting watts needed — look at a 5500 W or larger unit.
3000 + 2200 = 5200; round up for headroom.
1800 W of essentials plus a sump pump that surges to 1500 W. → 3300 starting watts needed.
1800 running + 1500 surge = 3300.
Tips & gotchas
- List only what you truly need during an outage — fridge, a few lights, a furnace fan — rather than your whole house, and the generator (and fuel bill) shrinks.
- Add the single largest starting surge once, not every appliance's surge; motors rarely start simultaneously.
- Leave 10–20% headroom; running a generator at full capacity continuously wears it out and raises the risk of a stall.
- If you plan to back-feed a furnace or well pump, confirm those motor surge ratings specifically — they are common culprits for an undersized unit.
FAQ
What is the difference between running and starting watts?
Running (rated) watts is the steady draw an appliance needs to keep operating; starting (surge) watts is the brief, higher spike a motor pulls at the instant it turns on. Generators list both ratings.
Do I add the surge for every motor?
No — only the single largest surge. Since motors rarely start at the same moment, adding every surge would oversize the generator and waste money.
Should I size up for future needs?
A modest buffer is wise if you might add loads, but oversizing wastes fuel and runs the engine inefficiently at light load. Match the realistic essentials plus headroom.
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