Generator Size Calculator
Fill in the values and the answer updates as you type.
How the Generator Size Calculator works
Required watts = (running watts + the extra surge of the biggest motor) × (1 + margin). Motors such as air conditioners and pumps can draw 2–3 times their running power for a second or two at start. kVA = watts ÷ 1000 ÷ power factor. Example: with the default values, recommended generator size comes to 9,000 W.
Inputs and results
The Generator Size Calculator takes 4 inputs and gives 3 results.
What you enter
- Total running watts (W) — starts at 4500.
- Extra starting watts of largest motor (W) — starts at 3000.
- Safety margin (%) — starts at 20.
- Generator power factor — starts at 0.8.
What you get
- Recommended generator size (W)
- Recommended size (kVA)
- Share of capacity used when running
Example: Generator Size Calculator with the starting values
With total running watts 4500 W, extra starting watts of largest motor 3000 W, safety margin 20 % and generator power factor 0.8, the calculator returns:
| Recommended generator size (W) | 9,000 |
|---|---|
| Recommended size (kVA) | 11.3 |
| Share of capacity used when running | 50% |
Change any value above and the results update straight away.
Generator Size Calculator FAQ
How do I use the Generator Size Calculator?
Enter total running watts, extra starting watts of largest motor, safety margin and generator power factor in the fields, then read recommended generator size (w), recommended size (kva) and share of capacity used when running in the result panel. The answer updates as you type, and you can press Calculate or Enter to refresh it. Use “Copy link” to share your exact numbers with someone else.
What does the Generator Size Calculator calculate?
Choose a generator size from running and starting loads. It returns recommended generator size (w), recommended size (kva) and share of capacity used when running from the 4 values you provide.
What values does the Generator Size Calculator start with?
It opens with total running watts 4500 W, extra starting watts of largest motor 3000 W, safety margin 20 % and generator power factor 0.8 so you can see a working result straight away. Replace them with your own numbers, or press Reset to return to these starting values.
Is the Generator Size Calculator free and private?
Yes. It is free, needs no account, and runs in your browser, so nothing you type is sent to or stored on our servers. See the Privacy Policy for details.
Can I rely on the Generator Size Calculator for important decisions?
Treat the result as a well-rounded estimate. Real situations often have details a general calculator cannot know, so confirm important decisions with a qualified professional. See the Disclaimer for more.