Watts ↔ amps ↔ VA calculator
Convert between watts, amps and volt-amps with the right voltage and power factor — the everyday conversions for sizing a UPS, a generator, a PDU or a circuit. Type into any box and the others update instantly.
Real power — the work done & heat made
Apparent power — UPS & generator rating
Current — for breakers & cabling
Type into any box — the others recalculate. VA = W ÷ PF, A = W ÷ (V × PF). Sizing a UPS for this load? →
Three numbers, one power factor
Watts, VA and amps are all linked by the supply voltage and the power factor. Get the power factor wrong and your UPS or generator can be undersized even when the wattage looks fine — because they are rated in VA, not watts.
Use this to check a breaker rating from a nameplate, convert a kVA UPS rating to amps, or work out the current a new server will pull. To size a whole UPS from a list of equipment, use the UPS calculator.
Watts, amps & VA — common questions
How do I convert watts to amps?
For a single-phase supply, amps = watts ÷ (volts × power factor). For example, a 1,000 W load on a 230 V supply at a power factor of 0.9 draws 1000 ÷ (230 × 0.9) ≈ 4.83 A. Enter the watts, voltage and power factor above and the current is calculated instantly.
How do I convert watts to VA (volt-amps)?
VA = watts ÷ power factor. Watts is real power (the work done); VA is apparent power (what a UPS or generator is rated on). A 1,000 W load at a 0.9 power factor is about 1,111 VA. At a power factor of 1.0 they are equal.
What is power factor and what value should I use?
Power factor (PF) is the ratio of real power (W) to apparent power (VA), between 0 and 1. Modern IT power supplies with active PFC are typically 0.95–0.99; a UPS output is usually rated at 0.9; older or motor loads can be 0.8 or lower. If you are unsure for IT equipment, 0.9 is a safe planning figure.
What is the difference between watts and VA?
Watts measure real power — the energy actually converted to work and heat. VA measure apparent power — the product of voltage and current the source must supply. When current and voltage are out of phase (power factor below 1) the VA is larger than the watts, which is why UPS and generators are sized in VA, not watts.
Does this work for three-phase?
Yes — switch the phase toggle to three-phase and pick a line-to-line voltage (400 V is standard UK three-phase). The converter then applies the √3 factor: line current = watts ÷ (√3 × line-to-line volts × power factor). For example, 10 kW at 400 V three-phase and 0.9 power factor draws about 16 A per line. Use the single-phase setting for UK 230 V and common 120/208/240 V supplies.