Generator Size Calculator
Size a backup generator: enter your total running watts and the largest motor's starting surge to get the recommended generator size in kilowatts.
How this calculator works
A backup generator has to cover two different loads. First is the steady running draw of everything switched on at once. Second is the brief surge a motor pulls the instant it starts, which sits well above its running draw.
This calculator finds your peak demand by adding the total running watts to the single largest starting surge, then adds 20% headroom so the generator never has to run flat out. Enter your running total and your largest surge, and it returns the recommended size in watts and kilowatts.
Formula: Peak demand = total running watts + the single largest starting surge; recommended size = peak × 1.2 (20% headroom).
Worked example
A load of 4000 W running with a largest starting surge of 2000 W:
- Total running watts: 4000 W
- Largest starting surge: 2000 W
- Peak demand = 4000 + 2000 = 6000 W
- Recommended size = 6000 × 1.2 = 7200 W = 7.2 kW
Notes
Only the largest motor’s surge is added to the running total. Motors almost never start at the same instant, so stacking every surge together would oversize the generator far beyond what you actually need.
A generator should never run at 100% of its rating for long. Running near the ceiling strains the engine and burns extra fuel, which is why the recommended size builds in 20% headroom above peak demand.
How to use
Add up the running watts of every device you plan to power at the same time, and note the single largest starting surge among them. Enter both figures, and the calculator returns your peak demand plus a recommended generator size with headroom already included. Match that size to a unit’s rated output when you shop.
Electrical work is code-governed and safety-critical. Confirm circuit, breaker, and conductor sizing with a licensed electrician and your local code.
Frequently asked questions
How do I size a backup generator?
Add up the running watts of everything you want to power at once, then add the single largest starting surge on top of that. The result is your peak demand. Multiply peak demand by 1.2 to leave 20% headroom, and that gives the recommended generator size. In the worked example, 4000 W running plus a 2000 W surge reaches a 7.2 kW recommendation.
Why do you add only the largest starting surge?
Motors draw a brief burst of extra power the instant they start, well above their running draw. But your appliances almost never start at the exact same moment, so adding every surge together would badly oversize the generator. Adding only the single largest surge to your running total captures the worst realistic peak without paying for capacity you will never use.
What does the 20% headroom factor do?
A generator should never run flat out at 100% of its rating for long stretches. Running near the limit strains the engine, raises fuel use, and shortens its life. Multiplying peak demand by 1.2 leaves roughly 20% spare capacity, so the unit runs comfortably below its ceiling and has room for a load you forgot to count.
What is the difference between running watts and starting watts?
Running watts is the steady power a device draws while operating. Starting watts, also called the surge, is the short spike a motor pulls at the moment it kicks on, such as in a fridge, pump, or air conditioner. Purely resistive loads like lamps and heaters have no surge, so their starting and running watts are the same.
Estimates only. Verify quantities with your supplier before purchasing.