Why transformers are rated in kVA, not kW
A transformer's heating depends on the current through its windings, and current depends on apparent power — kVA — not only on the useful power, kW. Motors, compressors and fluorescent fittings draw current that does no useful work, so the kVA they need is higher than their kW. The ratio between the two is the power factor.
Step 1: add up the load
List everything that may run at the same time and add up the power ratings from the nameplates, in kW. A motor rated in HP converts at roughly 0.746 kW per HP.
Do not add equipment that never runs together — the geyser and the welding set, say — unless it might. The honest figure is the worst realistic hour, not the sum of every socket.
Step 2: convert to kVA
Divide the kW total by the power factor. For mainly resistive loads such as heaters and incandescent lamps, the power factor is close to 1. For mixed loads with motors, 0.8 is a common working figure. So 16 kW of mixed load is about 16 ÷ 0.8 = 20 kVA.
Step 3: allow for motor starting
An induction motor started direct-on-line typically draws five to seven times its running current for a few seconds. One large motor starting on a small transformer can pull the voltage down enough to trip other equipment. If a single motor is a big share of your load, size so the transformer can carry that start, or use a soft starter or star-delta starter to reduce it.
Step 4: leave headroom
A transformer that runs at its full rating all day runs hot, and heat is what ages insulation. A margin of 20–25% above the calculated load is a common rule of thumb, and more if you already know the load will grow.
- Worked example: 12 kW of lighting and fans + one 7.5 HP pump (≈ 5.6 kW) = 17.6 kW.
- At a power factor of 0.8: 17.6 ÷ 0.8 = 22 kVA.
- With 25% headroom: 22 × 1.25 ≈ 27.5 kVA — so you would look at the next standard rating up.
General guidance only — every installation is different. For work on live equipment, use a licensed electrician. Last reviewed 28 September 2026.