Generator Wattage Calculator

A generator wattage calculator sizes the generator you need from two numbers: the total running watts of everything you will power at once, plus the largest single starting surge (motors briefly pull far more than they run on). Size for both and you avoid the classic mistake of a generator that stalls when the fridge kicks in.

Your measurements

Result

Enter your running total and the biggest surge, add headroom, and it gives the minimum generator size — in watts and approximate kVA — to buy.

How it is calculated

Required size = (total running watts + the single largest starting surge) × a headroom factor. Only the biggest motor's surge is added, since appliances rarely start at the same instant. Motors (fridges, pumps, AC, well pumps) surge roughly 2-3× their running watts for a moment. Do not size to run continuously at 100%.

Steps

Add up the running watts of everything you will run together.
Find the largest single starting surge (usually a motor — fridge, pump, AC).
Enter both, plus a safety headroom.
Read the minimum generator size in watts.
Pick a generator rated at or above it, and use a transfer switch for home backup.

Worked example

3,000 running watts plus a 2,200 W surge, with 20% headroom, needs about a 6,300 W generator.

Common questions

What size generator do I need?

Enough for your total running watts plus the largest starting surge, with headroom. 3,000 W running + a 2,200 W surge needs about 6,200 W.

What are starting (surge) watts?

The brief spike a motor draws to start — often 2-3× its running watts. Only the largest one matters, since they rarely start together.

Can I run my whole house on a generator?

Only if it covers the combined running and surge loads; most portables run essentials (fridge, lights, furnace fan). Use a transfer switch or interlock — never backfeed.

Should I size with extra headroom?

Yes — running a generator near 100% continuously shortens its life and leaves no margin. 20% headroom is sensible.