Sizing a transformer: kVA, kW and power factor

Three powers, three roles

• kW — active power. The part that does real work: heat, motion, light, computation. This is what your energy meter reads.
• kvar — reactive power. Exchanged with the grid by motors, transformers and electronics to build their magnetic fields. It produces nothing, but it drives current.
• kVA — apparent power. The combination of both: the total current the equipment must actually carry.

Why a transformer is rated in kVA

What heats a transformer is the current through its windings — whether that current "works" or not. Two 800 kW loads don't draw the same current: at cos φ = 1 you need 800 kVA; at cos φ = 0.8 you need 1,000 kVA, and the transformer carries the current of those 1,000 kVA even though only 800 kW does work. That's why its nameplate shows kVA: a current capacity, independent of the load.

Worked example (400 V installation)

• Workshop drawing 800 kW with a motor fleet, measured cos φ = 0.8;
• Apparent power needed: 800 ÷ 0.8 = 1,000 kVA;
• With the usual headroom (~20%), aim for 1,250 kVA — the next standard IEC rating.

Ordering "800 kVA because we use 800 kW" would have put the transformer in permanent overload from day one.

Watch the harmonics

Power electronics (drives, servers, UPS, EV chargers) distort the current: beyond cos φ, the K-factor then governs sizing. If your load is electronics-heavy, that's a separate specification — flag it in the configurator and we size for it.

General sizing guidance; the final rating depends on your installation and should be validated against your specification.