How to use the generator size calculator
- Add up the running watts of every appliance you will run at the same time. Use the nameplate or manual for each one.
- Enter the starting (surge) watts of the largest motor, such as a refrigerator, sump pump or air conditioner, and its running watts.
- Choose a safety margin.
List appliances from their nameplates rather than from memory. Decoratoradvice has found running and starting watts vary a lot between models.
Generator formulas
Running watts needed = total running watts × (1 + margin)
Starting watts needed = (total running watts + largest starting watts − its running watts) × (1 + margin)
Only one motor is assumed to start at a time. If two large motors could start together, add both surges.
Worked example
Appliances running together total 3,500 W. The largest motor runs at 700 W and needs 2,200 W to start. 10% margin:
Running: 3,500 × 1.10 = 3,850 W
Starting: (3,500 + 2,200 − 700) × 1.10 = 5,500 W
Without the margin, 5,000 running watts and a motor surging from 1,000 to 3,000 W needs 7,000 W of starting capacity.
Where to find wattage
- The appliance nameplate or manual lists watts, or volts and amps. Watts ≈ volts × amps.
- Starting watts for motors are often several times the running watts; check the manufacturer's information.
- A plug-in watt meter measures running watts for plug-in appliances.
Safety
The U.S. Consumer Product Safety Commission advises:
- Use portable generators outside only, at least 20 feet from doors, windows and vents.
- Never run a generator indoors, including garages, basements, crawlspaces and sheds, even with doors or windows open.
- Install battery-operated carbon monoxide alarms, or plug-in alarms with battery backup.
Connecting a generator to house wiring needs a transfer switch installed to local code.
Tips
- List only what you truly need during an outage, such as the refrigerator, sump pump, lights and phone charging.
- Stagger motor start-ups so only one surges at a time.
- Room air conditioners and electric water heaters are large loads; see the BTU calculator and water heater size calculator.
- Running costs on grid power can be estimated with the electricity cost calculator.
Common mistakes
- Ignoring starting watts. A generator that covers running watts can still stall when a motor starts.
- Adding every appliance in the house. Size for what runs at once.
- Running a generator in a garage. CPSC warns against it even with the door open.
Frequently asked questions
What size generator do I need to run a refrigerator?
Add its running watts to everything else on at the same time, then make sure the starting capacity covers its surge. Check the refrigerator's nameplate for both figures.
What is the difference between running and starting watts?
Running (rated) watts are what a generator can supply continuously. Starting (surge) watts are the short burst it can supply while a motor starts.
Do I need a safety margin?
A margin leaves room for appliances that draw more than their label and avoids running the generator at its limit.
How do I convert amps to watts?
Multiply amps by volts. A 120 V appliance drawing 5 A uses about 600 W.
How far from the house should a generator be?
At least 20 feet from doors, windows and vents, according to the CPSC.
Key takeaways
- Running watts = everything on at once, plus a margin.
- Starting watts = running total plus the largest motor's extra surge.
- Use nameplate figures for each appliance.
- Run portable generators outside only, at least 20 ft from openings.