Generator Size Calculator
Find out how big a generator you need. Add up the running watts of everything you want to power, include the largest starting surge, then add a safety margin to get the recommended size in watts and kilowatts.
How to size a generator
- Add up the running watts of every device you will run at once.
- Find the single device with the biggest starting surge and enter that.
- Set a safety headroom, often around 20 percent.
- Read the peak load and the recommended generator size.
Examples
3000 running plus 2000 starting watts
3000 running watts, 2000 starting watts, 20% headroom
5000 W peak, recommend about 6000 W or 6 kW
Frequently asked questions
What is the difference between running and starting watts?
Running watts is the steady draw while a device operates. Starting watts is the short surge when a motor first spins up, often two to three times the running figure.
Why only add the largest starting surge?
Motors rarely start at the exact same instant, so only the single biggest surge is added on top of the total running watts.
How much headroom should I add?
Around 20 percent is common. It stops the generator running flat out, which helps it last longer and handle small spikes.
Can I run a generator at its maximum rating?
Only briefly. Generators list a peak and a lower continuous rating. Sizing with headroom keeps you in the continuous range for steady use.
How do I find the watts for each device?
Check the rating label for watts, or multiply volts by amps. Motor-driven tools and fridges also list a higher starting or surge figure.
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