Watts, volt-amps and amps describe the same electricity in three different ways, and the link between them — power factor — is where most UPS and circuit sizing mistakes happen. This guide explains all three plainly, with the formulas, so you can convert with confidence (or just use our watts ↔ amps ↔ VA calculator).
Real power vs apparent power
Watts (W) are real power — the energy actually doing work and producing heat. Volt-amps (VA) are apparent power — the product of the voltage and the current the supply must deliver. When the current and voltage are perfectly in step they are equal; when they are not, VA is larger than W.
For IT planning this matters because the work and heat track watts, but the supply, the UPS and the generator all have to provide the VA.
Power factor — the link
Power factor (PF) is simply watts ÷ VA, a number between 0 and 1. A PF of 1.0 means all the supplied power does work; a PF of 0.8 means only 80% does and the other 20% sloshes back and forth, still occupying capacity in the cable and the UPS.
Modern IT power supplies use active power-factor correction (PFC) and sit at 0.95–0.99. Older equipment and motor loads can be 0.8 or lower. UPS outputs are usually rated at 0.9, with many online models now at unity.
Why UPS and generators are rated in VA
Because a UPS or generator is limited by the current it can push, not just the work done, it is rated in VA (or kVA). A 5 kVA UPS at 0.9 PF can deliver 4.5 kW of real power. Quote it a load in watts only and you can accidentally exceed its VA limit — which is why our UPS calculator sizes against both.
The conversions (single-phase)
VA = W ÷ PF. W = VA × PF. Amps = W ÷ (V × PF), or equivalently VA ÷ V. Example: 1,000 W at 230 V and 0.9 PF is 1,111 VA and 4.83 A.
These are the exact formulas behind the converter — type any one value and the other two update. For three-phase, switch the converter to three-phase: the line current becomes A = W ÷ (√3 × V_LL × PF), where V_LL is the line-to-line voltage (400 V is standard UK three-phase).
Where it bites in practice
Breaker and cable sizing depend on amps, so a low power factor means more current for the same watts — and a bigger breaker. UPS sizing depends on VA. Energy bills depend on watts (and large sites can be charged for poor power factor).
Get the three straight and the rest of your power planning — UPS, PDU, circuit, cooling — falls into place. The cooling calculator then turns the watts into the heat you must remove.