Calculators
Generator Sizing Calculator
Size a portable or standby generator from appliances' running and starting watts, with a safety margin.
- Runs in your browser
- Nothing uploaded
The tool
Enter your numbers below. Results update as you type, and nothing leaves your browser.
Everything you enter stays on your own machine. Nothing is sent to us or to anyone else, which you can confirm in your browser network tab.
What people use it for
- Choosing a portable generator
- Sizing a standby generator
- Planning which circuits to back up
- Off-grid power planning
About this tool
What the Generator Sizing Calculator does
A generator must supply the running watts of everything on at once, plus the extra surge when a motor starts. Because motors rarely start at the same moment, sizing usually adds only the single largest extra starting surge to the total running load, then a margin for headroom.
Formula
- Running load = Σ running watts × quantity
- Peak = running load + largest (starting − running) surge
- Recommended = peak × (1 + margin)
Worked example
Using the starting values in the calculator above:
| Input | Value |
|---|---|
| Appliances | 6 entries |
| Safety margin | 20% |
| Result | Value |
|---|---|
| Recommended generator size | 6,840 |
| In kilowatts | 6.8 |
| Total running watts | 4,150 |
| Peak with largest motor start | 5,700 |
How to read the result
Use appliance nameplates or manuals for accurate wattage; motors such as pumps and compressors have the largest starting surges.
Connecting a generator to house wiring requires a transfer switch or interlock to protect utility workers; never backfeed through an outlet.
For planning only. Electrical, gas and refrigerant work must follow current codes and be carried out or checked by a licensed professional.
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Questions
Questions about this tool
The things clients actually want to know — costs, timelines, ownership and what happens after launch.
Enough for the total running watts of what you will use at once plus the largest starting surge, with some margin.
The brief extra power a motor needs when it starts, often two to three times its running watts.
Large standby units can; portable units usually power selected essentials.
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