Generator Size Calculator
Add up the running watts of everything you want to power and the starting surge of the biggest motor to size a portable or standby generator.
How to use the Generator Size Calculator
- Enter the running watts of each item – Example: refrigerator 700, furnace blower 600, sump pump 500, lights 300.
- Enter the extra starting watts of the largest motor – Starting watts minus running watts; often 1–3× the running watts.
- Enter the safety margin.
- Press Calculate. The result updates automatically as you type.
About the Generator Size Calculator
Generators carry two ratings: running (rated) watts they can supply continuously, and starting (surge) watts they can deliver for a few seconds while a motor spins up. Refrigerators, well pumps, sump pumps, furnace blowers and air conditioners can draw two to three times their running power at startup, so the largest motor often decides the size.
List each item's running watts from its label or the manufacturer's data. A margin of 20–25% keeps the generator out of overload, leaves room for things you forgot, and improves fuel efficiency and engine life. Whole-house standby units are often rated in kW or kVA; for a home standby system, have an electrician perform a load calculation and install a transfer switch.
Formula
Running load = sum of running watts
Starting watts needed = Running load + extra surge of the largest motor
Recommended rated watts = Running load × (1 + margin)
Only the single largest surge is added, because motors rarely start at the same instant. Example: 2,100 W running + 1,500 W surge = 3,600 W starting; with a 20% margin you want about 2,600 W of continuous output.
Example calculation
Using the default values:
- Running watts of each item = 700, 600, 500, 300
- Extra starting watts of the largest motor = 1500 W
- Safety margin = 20%
the calculator returns:
- Recommended running (rated) watts: 2,600 W
- Required starting (peak) watts: 3,600 W
- Total running load: 2,100 W
- Equivalent standby rating at 0.8 PF: 3.25 kVA
Frequently asked questions
What size generator do I need to run a house?
Essentials such as a refrigerator, sump pump, furnace fan and some lights usually need 3,000–5,000 running watts. Adding central air or an electric range pushes it to 10–20 kW or more.
What is the difference between starting and running watts?
Running watts are what a device uses continuously. Starting watts are the brief extra power motors need to get going, typically lasting one to three seconds.
Can I run a refrigerator on a 2,000 W generator?
Usually yes: a typical refrigerator runs at 150–800 W and needs 1,200–2,200 W to start. Check that the generator's surge rating covers the starting watts.
Last updated: · Category: Electrical calculators