UPS Size Calculator
Estimate minimum UPS VA rating from load, power factor and headroom.
Interactive Electrical Calculator
UPS Size Calculator
Estimate minimum UPS VA rating from load, power factor and headroom.
Result
Enter values and calculate.
About the UPS Size Calculator
The UPS Size Calculator handles sizing generators and UPS systems for backup power without needing a spreadsheet, app download or manual lookup table. Estimate minimum UPS VA rating from load, power factor and headroom. 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.
It's built for homeowners and facility managers planning backup power, and installers sizing standby generators or UPS units, and every field updates instantly so you can try a few scenarios before committing to a real installation.
How the calculation works
Here is the relationship the calculator solves for you:
UPS VA = (W / PF) × (1 + margin).
Where each input maps to a field in the calculator above:
- Load in W.
- UPS Power Factor.
- Headroom 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 800 W for load, 0.9 for ups power factor and 25 % for headroom. Press Calculate and the UPS Size Calculator returns the result instantly — then change one field at a time to see how sensitive the answer is to that particular input.
When to use the UPS Size Calculator
Typical reasons people reach for the UPS 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: UPS sizing should include a safety margin above the connected load, since running a UPS at its absolute maximum rating shortens its service life.
Also worth knowing: Fuel or battery runtime estimates from a manufacturer usually assume a specific load percentage, so check that assumption before relying on a quoted runtime.
Frequently asked questions
What does the UPS Size Calculator calculate?
Estimate minimum UPS VA rating from load, power factor and headroom.
How accurate is this calculator?
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 UPS 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.
What should I do if the result looks off?
The UPS 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.