Generator Size Calculator
Work out the generator size to buy from the loads you actually plan to run. Add up the running watts of everything on at once, enter the surge of the single hardest-starting motor, and choose a safety headroom. The tool returns the peak load, the recommended watts with headroom, and the size in kilowatts. Use it before a camping trip, a job site setup, or picking a standby unit for outages.
How to size a generator
- Add up the running watts of everything that will run at the same time and enter it in Running watts.
- Enter the surge of the single device with the biggest start, such as a fridge or pump, in Largest starting watts.
- Set Headroom to the safety margin you want, often around 20 percent.
- Read the large recommended size in kilowatts at the top.
- Check the Peak load and Recommended watts below, then press Copy result.
Examples
Camping load, 3000 running plus a 2000 watt surge
Running 3000 W, largest starting 2000 W, headroom 20%
5,000 W peak, recommend 6,000 W (6 kW)
Small job site with a hard-starting tool
Running 5000 W, largest starting 3600 W, headroom 25%
8,600 W peak, recommend 10,750 W (10.75 kW)
A few lights and a laptop with no motor surge
Running 1200 W, largest starting 0 W, headroom 10%
1,200 W peak, recommend 1,320 W (1.32 kW)
Frequently asked questions
What is the difference between running and starting watts?
Running watts is the steady draw a device pulls while it operates. Starting watts is the brief surge when a motor first spins up, often two to three times the running figure. This tool adds the running total and one starting surge to find the peak the generator must handle.
Why does it only add the largest starting surge?
Motors rarely start at the exact same instant, so stacking every surge would oversize the generator. The realistic worst case is that your other loads are already running when the single hardest device kicks in, so the calculator adds just that one biggest surge to the running total.
How much headroom should I add?
Around 20 percent is a common default and it is what the tool starts with. Headroom keeps the generator from running flat out, which helps it last longer, run cooler, and absorb small spikes. Raise it if your loads are unpredictable or you want quieter operation.
How is the recommended size calculated?
Peak watts is running watts plus the largest starting watts. Recommended watts is that peak multiplied by one plus the headroom percent. So 5,000 peak at 20 percent headroom becomes 6,000 watts, which the tool also shows as 6 kilowatts.
Can I run a generator at its maximum rating?
Only briefly. Generators list a higher peak or surge rating and a lower continuous rating. Sizing with headroom keeps your steady load inside the continuous range, so reserve the peak rating for short motor starts rather than all-day use.
How do I find the watts for each device?
Check the rating label for a watts figure, or multiply volts by amps to get watts. Motor-driven items like fridges, pumps and power tools usually list a separate higher starting or surge figure, which is the number to use for the largest starting field.
Does the tool recommend a specific generator model?
No. It gives a target wattage and kilowatt figure to shop with. Compare that against a unit's continuous rating rather than its peak, and remember that altitude, temperature and fuel type can lower a generator's real output.
What happens if I leave the starting watts at zero?
That is fine for loads with no motor surge, such as lights, chargers and electronics. Peak then equals your running watts, and only the headroom is added. Running watts itself must be greater than zero or the tool asks for a positive number.
Is my input private?
Yes. The math runs locally in your browser, so the wattage figures you enter stay on your device and are never sent to a server.
Learn more
- Running vs Starting Watts for Generators
Why motors surge on startup, how to add up a realistic peak load, and how much headroom to leave so a generator runs safely for years.
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