The phone call always starts the same way. “Your generator is underrated. It keeps tripping.” The caller had bought a 500 kVA set and was pulling 450 kW through it. In his head, 500 kVA meant 500 kW, and we’d sold him a machine that couldn’t do what the number on the side said. He wasn’t entirely wrong to be angry. He was just angry at the wrong number.
Here’s the thing nobody explains at the point of sale. A generator set carries two power numbers, and they measure two different things. Mix them up and you either buy too much machine or — worse — you cook a good one. Customers ask me about this constantly, so let me answer the three questions I actually get, in the order I get them.
No. And this is the single most expensive misunderstanding in the generator business.
kVA is apparent power — volts times amps. It’s the total current sloshing through the alternator’s copper windings. kW is real power — the part that actually does work, turns shafts, makes heat, lights lamps. The ratio between them is power factor: kW divided by kVA.
The industry rates generator sets at 0.8 power factor, lagging. So a 500 kVA set is a 400 kW set. 500 × 0.8. That’s the math the brochure hopes you won’t do.
An old hand I learned commissioning from put it in one sentence I’ve never improved on: “kVA is what the copper feels. kW is what the diesel feels.” The alternator is limited by current — too many amps and the windings cook. The engine is limited by real power — too many kilowatts and it runs out of torque, fuel, and cooling. Two machines in one box, two different limits.
At exactly 0.8 power factor, both limits arrive at the same time. That’s the whole reason 0.8 is the rating point. It’s not a law of your building. It’s a test-lab compromise the industry agreed on decades ago — roughly the power factor of a typical mixed load of motors and lighting. Your actual loads will land somewhere else, and that somewhere else decides which half of your generator runs out first.
Tempting logic. At unity power factor, kVA equals kW, so why not take the full 500? Because the engine in that box was built to make about 400 kW of shaft power, plus a modest margin. The alternator’s copper could carry 500 kVA all day. The engine cannot produce 500 kW. Feed it that and it lugs, exhaust temperatures climb, and the protection trips — or, on a set with lazy protection, it just cooks slowly over a season until something expensive lets go.
I got called to exactly this on a winter job years back. A contractor had a 200 kVA set — 160 kW on the engine side — and he’d hung 180 kW of site heaters off it. Heaters are resistive. Power factor 1.0. Beautiful, clean load. The current was only about 260 amps on a 400-volt system; the alternator was rated for nearly 290 and was, frankly, bored. But the engine was being asked for 180 kW from a 160 kW block. The breaker kept tripping on overload and he was furious at the alternator manufacturer.
The alternator was innocent. He’d bought a 200 kVA machine to do a 180 kW job, and the kW number was the one that mattered.
So the working rule: size the engine on kW, check the alternator on kVA. Add up your loads both ways. If you’ve got 450 kW of load at 0.8 power factor, that’s 562 kVA — you need a 625 kVA set, not a 500. Anyone who quotes you the kVA number after you asked for kW is either sloppy or selling. Sizing a set properly means doing both columns of that math, not one.
Hurts. A bit, anyway.
Motors run lagging — typically 0.8 to 0.85 once they’re up to speed, much worse while starting. That drags your power factor down, which pushes you toward the alternator’s current limit while the engine still has room to spare. Take 350 kW of motor load at 0.7 power factor: that’s 500 kVA. The engine sees 350 kW and shrugs — it’s rated 400. The alternator sees 500 kVA and it’s maxed out, roughly 720 amps per phase at 400 volts, with nothing left for the next motor start. You’ve got kW headroom and no kVA headroom. The nameplate’s big number lied to you again, in the other direction this time.
And motor starting is its own special case — a motor can pull five or six times its running current for the first second or two. That starting kVA is an alternator problem, not an engine problem. The engine only has to carry the running kW once the motor’s up to speed. This is why reading the rating properly matters more than memorizing it: the kVA column is where starting currents live.
Lagging power factor — motors, transformers — is normal. Every generator expects it. Leading power factor is the weird one, and it comes from capacitive loads: UPS input filters, long cable runs sitting unloaded, and power-factor correction capacitor banks that somebody forgot to switch out when the mains failed.
Here’s what happens. A capacitive load actually pushes voltage up at the alternator terminals. The AVR sees the rising voltage and backs the excitation down to compensate. Backs it down further. Then further. Eventually the regulator runs out of room — it can’t reduce excitation any more — and voltage starts swinging on its own. I’ve watched a set hunt ±10% on voltage with a perfectly steady engine, all because a capacitor bank was still connected from the utility days. The fix took ten minutes: open the capacitor breaker on generator supply. The diagnosis took a day, because nobody thought to ask what was connected.
If your site has a UPS, this deserves extra attention — UPS systems and generators fight over exactly this territory. And if your loads are full of drives and switching power supplies, harmonic distortion rides along with the power factor problem as a package deal.
So next time a quote says 500 kVA, do the one multiplication the salesman hopes you’ll skip. 500 × 0.8. That’s your machine. Everything else is copper.
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