About the VA to Watts Calculator
Uninterruptible power supplies and small generators are often labelled in volt-amperes, while the devices they power are labelled in watts. The two are linked by the power factor: watts equal volt-amperes times the power factor. This calculator does the multiplication for you, then goes further, showing the reactive power and the phase angle between voltage and current. It also tells you what VA rating you would need to supply a load of that many watts, which is the sum people usually need when choosing a UPS for a computer or a network cabinet.
Updated October 2026
How to use the VA to Watts Calculator
- 1Enter the apparent power in VA.
- 2Enter the power factor. Check the data sheet, or try 0.6 to 0.9 for a UPS.
- 3Read the real power in watts and kilowatts.
- 4Look at the reactive power and phase angle.
- 5See the VA needed to run that many watts.
Worked example
A server rack draws 1,200 watts. The UPS has a power factor of 0.9. The VA required is 1,200 divided by 0.9, which is 1,333 VA. Adding a 25 percent margin gives 1,667 VA, so a 2,000 VA unit would be a sensible choice. If the UPS had a power factor of 0.6, you would need 2,000 VA before the margin, and 2,500 VA with it.
Be careful when comparing products from different brands. Some manufacturers advertise only the VA figure in large print and bury the watt figure in the small print, which can make a unit look twice as powerful as it really is. Always compare watts with watts, and check what happens at the edge of the rating: many units can only run at full load for a short time, and runtime on battery falls sharply as the load rises toward the limit.
Choosing a power factor
- Check the UPS or generator data sheet first.
- Many line-interactive UPS units use 0.6 to 0.7.
- Modern online UPS models are often 0.9 to 1.0.
- Induction motors run at 0.7 to 0.9.
- Use 1 for heaters, kettles and incandescent lighting.
Related tools
The Amps to kVA Calculator and kVA to kW Calculator cover larger systems, the Power Factor Calculator helps you correct a poor value, the Battery Charge Time Calculator shows how long a battery takes to fill and the mAh to Wh Calculator converts battery capacity to energy.
Features
- Watts from VA and power factor
- Reactive power and phase angle
- VA needed for a given load
- Kilowatts shown as well
- Checks for impossible power factors
- Instant, private, free
Frequently asked questions
What is the formula for VA to watts?
Watts equal VA times the power factor. A 1,000 VA UPS with a power factor of 0.8 can supply 800 watts. If the power factor is 1, the UPS can supply 1,000 watts.
Why is a UPS rated in VA?
VA shows the maximum current the unit can supply at its output voltage. The real power it can deliver depends on the power factor of the load, which varies with the equipment. Newer UPS models quote both VA and watts.
How many watts is a 1500 VA UPS?
At a power factor of 0.6, it is 900 watts; at 0.9, it is 1,350 watts; at 1.0, it is 1,500 watts. The watt rating is on the label or data sheet and is the figure to compare with your equipment.
How do I choose a UPS for my computer?
Add up the watts of everything you want to power, then divide by the UPS power factor to get the VA you need. Allow a margin of 20 to 25 percent. A 400 watt PC and monitor with a 0.6 power factor UPS needs at least 667 VA, so choose 850 or 1,000 VA.
What is reactive power?
It is the part of the apparent power that swings back and forth between the source and the load without doing useful work. It is measured in volt-amperes reactive (var), and it grows as the power factor falls.
Is VA the same as watts for a resistive load?
Yes, when the power factor is 1 the two are the same, as with a heater or a filament bulb. For computers with active power-factor correction, the power factor is also close to 1.