Generator Size Calculator
Estimate generator wattage from running load and startup surge
Suggested minimum generator rating
How we calculated it
About the Generator Size Calculator
Generator Size Calculator estimates the electrical capacity needed to run a group of appliances or equipment. It must consider both steady running watts and short startup surges from motors, pumps and compressors.
Selecting a generator from running watts alone can cause overload during startup. At the same time, choosing a unit far larger than needed can increase cost and may reduce efficient operation at light load.
How to Use the Generator Size Calculator
- List loads that may operate simultaneously.
- Enter running watts for each load.
- Enter startup or surge watts for motors and compressors.
- Calculate continuous and peak requirements, then add a reasonable design margin.
How the Calculation Works
Continuous requirement is the sum of simultaneous running watts.
Peak requirement is based on the realistic combination of running loads plus the startup surge of equipment that can start at the same time. A design margin can then be applied to avoid operating continuously at the nameplate limit.
Worked example
Generator Size Calculator Worked Example
2,500 W running load plus pump startup adding 1,800 W
If all other loads remain on, peak demand is about 4,300 W. A generator must satisfy both the 2,500 W continuous requirement and the short 4,300 W startup event.
Result guide
Understanding Your Generator Size Calculator Result
The correct generator should handle sustained load and the largest realistic surge. Nameplate watts alone do not tell you fuel consumption or runtime.
Before you rely on the answer
Assumptions and Limitations
- Load ratings are accurate.
- Simultaneous-use assumptions are realistic.
- Startup surges reflect the actual equipment.
Practical tip
Using This Result in Practice
Check whether the generator rating is continuous, rated or maximum surge output. A short-duration maximum rating should not be treated as continuous capacity.
Related calculations
Useful Calculations to Continue With
Once generator size is selected, use the Generator Runtime Calculator to estimate fuel-supported operating time.
If a load is specified by volts and amps instead of watts, use the Electrical Power Calculator first.
Common questions
Generator Size Calculator FAQ
Why do I need surge watts?
Motors and compressors can draw substantially more power during startup than while running.
Should I add every surge together?
Only if those loads can realistically start at the same time.
Is maximum generator wattage the same as continuous rating?
Not always. Check the manufacturer’s rated continuous output.