Electrical calculation guides

A generator or UPS rated in kVA doesn't tell you how many kW you'll actually get – that depends on one number most spec sheets don't explain. Here's what it means.

Convert your own figureskVA to kW converterEnter the apparent power in kVA and the power factor to get the real power in kW, or the other way round.Go to the kVA ↔ kW converter

Basics

BasicsOhm's law explained: U = R x I, with worked examplesOhm's law is one equation – U = R × I – but it answers three different questions depending on which of the three values you already know. Here's how to use it, and how it connects to power.7 min read

Technique

TechniqueHow to size a cable: current, length and voltage dropA cable that's too thin for its length doesn't just risk overheating – it can lose enough voltage over the run that equipment at the far end underperforms or trips out. Here's the calculation method BS 7671 is built around, and where its limits are.8 min read

TechniqueResistors in series and parallel: formulas and worked examplesTwo resistors combine completely differently depending on how they're wired: in series, resistance always goes up; in parallel, it always goes down – and the two formulas aren't the same one rearranged.8 min read

Conversion

ConversionkVA to kW: what the difference actually meansA generator rated 100 kVA doesn't give you 100 kW. The gap between the two numbers is the power factor, and getting it wrong is the single most common reason people undersize or oversize a generator, UPS or transformer.6 min read