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Free calculator

Use this calculator to

  • Convert equipment watts into the VA rating a UPS or inverter must supply
  • Size small UPS units for tills, servers and network cabinets
  • Check both the VA and watt limits of a UPS against your connected load
  • See how power factor changes the apparent power a circuit carries

Watts to VA Calculator

Convert real power (W) to apparent power (VA).

W
PF
Result
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Formulas

  • VA = W / PF

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Common scenarios

Select one to run it in the calculator above.

For business

Why this matters for businesses

Small commercial supplies (retail units, single-site SMEs, branch offices) live in the volt-amp world for inverters, UPS units and lighting circuits. A 1000VA UPS does not give 1000W of real power output. At a power factor of 0.7, the same unit delivers around 700W, which is the figure that matters when the till system, the router, the CCTV and the alarm panel all need to ride through a brownout. Reading the nameplate as VA without converting back to watts is how owners end up with kit that fails when it matters.

For lighting and IT loads, the watts to VA conversion is also the basis of cable and breaker sizing on the final circuit. A 5kW lighting load at a power factor of 0.95 draws 22.9A at 230V single-phase. Specify the cable on watts alone and the design is technically out of spec under BS 7671. SMEs adding LED lighting, expanding IT racks, or installing small EV charge points need this conversion to make sure the local design engineer signs off the work cleanly the first time.

Purely Energy works with SMEs from £5k a year of energy spend upwards. We handle the supply contract, advise on capacity changes when the business grows, and connect clients with vetted electrical contractors when they need on-site design work. Our quick-quote flow gets a comparable rate from a panel of 30+ suppliers in under three minutes, and our account team handles the renewals so the owner-operator does not have to track contract end dates across a panel of brokers.

Common questions

How do I convert watts to VA?

Divide the real power in watts by the power factor: VA = W / PF. A 650 W load at power factor 0.95 is 650 / 0.95, about 684 VA. Because the power factor of real equipment is always 1 or below, the VA figure is always at least as large as the watt figure. The two are only equal for purely resistive loads such as heaters, where the power factor is effectively 1.

What is the difference between watts and volt-amps?

Watts measure real power, the energy actually converted into useful work or heat. Volt-amps measure apparent power, the product of the voltage and the total current flowing, including the reactive component that oscillates between source and load without doing work. The power factor is the ratio between them: W = VA x PF. Cables, transformers and UPS inverters have to carry the full apparent power, which is why they are sized in VA, not watts.

Why are UPS systems rated in both VA and watts?

A UPS has two separate limits: its inverter and battery electronics cap the real power in watts, and its output current capacity caps the apparent power in VA. A unit sold as 3,000 VA / 2,700 W must not exceed either figure. Convert your connected load both ways: total the equipment watts against the watt rating, and divide by power factor to check the VA rating. Whichever limit you hit first is the one that sizes the UPS.

What power factor should I assume for IT equipment?

Modern servers, switches and PCs with active power factor correction typically run at 0.95 or better, and many datasheets quote 0.98 to 0.99. Older or cheaper power supplies without correction can sit anywhere from 0.6 to 0.7, which inflates the VA demand sharply for the same watts. Check the equipment datasheet where you can; if the fleet is recent, 0.95 is a sound planning figure, and 0.9 adds a margin of caution for mixed kit.

How much headroom should I leave when sizing a UPS?

Common practice is to keep the steady-state load at or below about 80 percent of the UPS rating in both VA and watts. The headroom absorbs inrush when equipment starts, leaves room for growth without an immediate replacement, and improves runtime on battery, since autonomy falls quickly as load approaches the rating. Convert your watt total to VA at a realistic power factor first, then apply the headroom to whichever of the two ratings is the more constraining.

Related calculators

The conversions that procurement and engineering use alongside this one.

Watts to VA Calculator | Purely Energy