Generator Size Calculator
Estimate generator capacity from running load, surge load and design margin.
Interactive Electrical Calculator
Generator Size Calculator
Estimate generator capacity from running load, surge load and design margin.
Result
Enter values and calculate.
About the Generator Size Calculator
Use the Generator Size Calculator below to work through sizing generators and UPS systems for backup power in seconds, right in your browser. Estimate generator capacity from running load, surge load and design margin. An undersized generator or UPS will stall, trip or shut down under real load, usually exactly when it is needed most.
It's aimed at homeowners and facility managers planning backup power, and installers sizing standby generators or UPS units, who need a fast, reliable number without redoing the math by hand each time.
How the calculation works
The calculator is built around this relationship:
Generator W = (running + surge) × (1 + margin).
Where each input maps to a field in the calculator above:
- Running Load in W.
- Largest Starting Surge in W.
- Design Margin in %.
You can adjust any of these values as many times as you like; the result updates on every change with nothing sent off your device.
A worked example
For example, start with 5000 W for running load, 2000 W for largest starting surge and 20 % for design margin. Those defaults are only a starting point — replace them with your own measured or nameplate values, then press Calculate to get a result specific to your situation.
When to use the Generator Size Calculator
Typical reasons people reach for the Generator Size Calculator:
- Sizing a portable or standby generator for a home or job site.
- Estimating how long a UPS will support a given load during an outage.
- Checking whether a generator can handle the starting surge of a well pump, AC unit or compressor.
- Planning backup power for critical loads like refrigeration, medical equipment or a home office.
Good practice: Fuel or battery runtime estimates from a manufacturer usually assume a specific load percentage, so check that assumption before relying on a quoted runtime.
Also worth knowing: Add up the starting (surge) wattage of the largest motor load plus the running wattage of everything else when sizing a generator, not just the sum of running wattages.
Frequently asked questions
What does the Generator Size Calculator calculate?
Estimate generator capacity from running load, surge load and design margin.
Is the result exact enough to build from?
The formula above is applied exactly to the values you enter, so the math itself is precise. Real-world results can still differ due to component tolerances, temperature, installation method, equipment efficiency and local code requirements, so treat the output as a strong planning estimate and verify final designs against manufacturer data and the electrical code that applies to your installation.
Does this calculator store or send my data anywhere?
No. The Generator Size Calculator runs entirely in your browser, and nothing you type is transmitted or stored anywhere.
Why not just estimate this instead of calculating it?
Motor-driven loads (pumps, compressors, AC units) draw a surge current at startup that is often several times their running current, and backup equipment has to be sized for that surge, not just the running load.
Will this work for both metric and everyday values?
The Generator Size Calculator works on any modern browser, phone or desktop, and recalculates as soon as you press Calculate. If a result looks off, double-check that every field is filled in with the unit shown next to it (mixing up units, such as W with kW, is the most common cause of an unexpected answer) and re-enter the values.