Appliances & Utilities calculator
What Generator Size Do I Need?
Find generator running and surge rating from total simultaneous running watts, largest additional startup surge watts, and generator headroom.
Enter your measurements
This page is built for one question: what generator size do i need? Supply total simultaneous running watts, largest additional startup surge watts, and generator headroom, then use the returned generator running and surge rating to screen unsuitable options.
- Total simultaneous running watts — Required
- Largest additional startup surge watts — Required
- Generator headroom — Required
Generator running and surge rating
Enter the measurements above to calculate generator running and surge rating.
How the calculation works
The engine follows this rule: requiredRunning=runningWatts×(1+headroom); requiredPeak=(runningWatts+largestStartingWatts)×(1+headroom). It returns generator running and surge rating from the configured inputs.
Worked example
Example: Total simultaneous running watts = 600 W; Largest additional startup surge watts = 1200 W; Generator headroom = 20%. Result: Required Running = 720 W; Required Peak = 2160 W.
Before you decide
Use the finished, usable dimensions after trim, handles, skirting, packaging or other projections where relevant. Check the exact product documentation for ratings, clearances, installation and safety requirements before relying on the result. For what generator size do i need, the limiting result on this page is generator running and surge rating.
Manufacturer or safety specification required for the final decision. The CMS formula handles the stated measurement calculation only; installation clearances, ratings, certification and safety instructions must come from the exact product documentation.
Frequently asked questions
What does the Generator Size calculator tell me?
It returns generator running and surge rating from the measurements entered on this page. The calculation is specific to generator size, so the result should be compared with the exact product or specification you are considering.
Which measurements should I take for generator size?
Measure total simultaneous running watts, largest additional startup surge watts, generator headroom. Use clear, usable dimensions rather than nominal room or product labels.
Should I round the generator running and surge rating?
Keep the calculated value for comparison, then choose a real product that meets or stays within the returned generator running and surge rating while allowing for the clearances, tolerances or ratings shown by its manufacturer.