Generator Sizing Calculator
Size a generator for your running load plus motor startup surge. This free online home, garden & energy calculator runs entirely in your browser — no signup, no data sent anywhere.
Inputs
Results
How It Works (Formula & Method)
A generator must supply the continuous running watts of everything on at once, plus the extra surge the largest motor draws for a second or two at startup. Peak load is running watts plus that surge; a 20% headroom is added for the recommended size.
Worked Example
Below is a worked example using the calculator's default values. The same numbers are pre-filled in the form above so you can press Calculate and see the result without typing anything.
Inputs used:
- Total Running Watts: 4000
- Largest Motor Starting Surge (W): 2200
With these inputs, the calculator computes the metrics shown in the Results panel. Change any value and press Calculate again to see how the result responds — the live widget and the chart both update instantly.
Frequently Asked Questions
What does the Generator Sizing Calculator compute?
The Generator Sizing Calculator takes 2 input values and returns 3 results. Estimate the generator size you need from the total running wattage of your appliances plus the starting surge of the largest motor.
Is my data sent to a server?
No. The Generator Sizing Calculator runs entirely in your browser using static JavaScript. Your inputs are never transmitted to CalculatorHive or any third party, and nothing is stored after you close the page.
Is this calculator free to use?
Yes. Every calculator on CalculatorHive is completely free. There is no signup, no paywall, and no usage limit. The site is supported by display advertising and affiliate partnerships, which are clearly labeled.
How accurate is the result?
The math is mathematically exact for the inputs you provide. Real-world accuracy depends on how accurately your inputs reflect the real situation — defaults are typical values, not guarantees.