Your installation consumes kW, but a transformer is bought in kVA. The difference between the two comes down to a single number, cos φ, and ignoring it leads straight to undersizing.
What heats a transformer is the current through its windings, whether that current "works" or not. Two 800 kW loads do not impose the same current: at cos φ = 1, 800 kVA are needed; at cos φ = 0.8, 1 000 kVA, and the transformer genuinely carries the current of the 1 000 kVA even though only 800 kW work. That is why its nameplate shows kVA: a current capability, independent of the nature of the load.
The three-phase 400 V formula: S (kVA) = √3 × U × I ÷ 1000. A feeder of 1 443 A at 400 V ≈ 1 000 kVA. Our calculators convert both ways.
Had you ordered "800 kVA because we consume 800 kW", the transformer would have been in permanent overload from day one. The full method lives in the sizing guide.
Capacitor banks (reactive power compensation) raise cos φ towards 0.95–1. A double benefit: the distributor bills reactive energy beyond a threshold (depending on your contract and tariff period), and a better cos φ frees kVA capacity on the existing transformer. Compensation is sized with your design office; state it in your request, it influences the choice.
Power electronics (drives, servers, UPS, charging points) distort the current: beyond cos φ, the K-factor then enters the sizing. If your load is electronics-heavy, read that guide next.
General sizing pointers; reactive billing thresholds depend on your contract and are confirmed with your supplier / DSO.
It depends on cos phi: kW = kVA x cos phi. At cos phi = 1, 1 kVA = 1 kW; at cos phi = 0.8, 1 kVA delivers only 0.8 kW of useful power.
The transformer is limited by current (hence kVA). A generating set is limited both by its alternator (kVA) and by its engine (kW), hence the display of both values.
Absent a measurement, 0.8 is the classic prudent assumption for a mixed load with motors. A heavily compensated or very resistive installation can approach 0.95 to 1. A site measurement remains the best answer.
Yes, twice: reactive energy can be billed by the distributor beyond a contractual threshold, and it occupies kVA capacity; transformer, cables and protections must be larger for the same useful power.