What follows is enough to have an informed conversation about it, without needing an electrical background.
kVA and kW are not the same number
Generators are rated in kVA. Appliances are usually labelled in kW. They are related but not interchangeable, and the gap between them is where most sizing mistakes live.
kW is the power that actually does work — the heat, the light, the turning. kVA is the total the generator has to supply, including the part that circulates without doing work. For a typical mixed commercial load the difference is around twenty percent, which is why a set is rated in the larger number.
The practical takeaway: never assume a 100 kVA set delivers 100 kW of useful output. It does not, and designing as though it does is how installations end up short.
Motors are what actually set the rating
A motor — a pump, a compressor, a lift, an air conditioner — draws several times its running current for the moment it starts. That surge lasts a second or two, but the generator must supply it without the voltage sagging enough to disturb everything else on the circuit.
This means the largest motor in the building, and whether several can start at the same time, often matters more than the total connected load. Two sites with identical electricity bills can need genuinely different sets for this reason alone.
- Lighting and electronics
- Draw close to their rated figure with no meaningful surge. Easy to add up.
- Motors
- Surge hard on starting. The biggest one, and how many start together, drives the rating.
- Heating elements
- Kettles, geysers, ovens. Heavy but steady, with no starting surge.
- Sensitive equipment
- Servers, medical devices, audio. Small draw, but they need the supply to stay clean and steady.
Bigger is not safer
The instinct is to add margin and buy larger. Past a point, that instinct causes problems of its own.
A diesel engine running at a small fraction of its rating does not reach proper operating temperature. Over time that leads to unburnt fuel and carbon deposits accumulating where they should not — a known condition that shortens engine life and increases servicing. Fuel efficiency is also worst at very light load.
A set sized to run comfortably in its efficient band, with enough headroom for the surges and for planned expansion, will outlast and out-earn a set bought two sizes too large.
What we need from you to size it properly
Sizing is quick once the information exists. Four things make it straightforward:
- The load list
- Everything that must keep running during an outage, with ratings from the nameplates where you can read them.
- The motors
- Pumps, lifts, compressors and air conditioners — what they are and whether they can start simultaneously.
- How long, how often
- Short standby outages and long daily running lead to different choices about fuel capacity and duty rating.
- Where it goes
- The location decides ventilation, exhaust routing, and how much acoustic treatment the site needs.
Common questions
- What does kVA mean on a generator?
- kVA is the total electrical capacity the generator can supply, including the portion that circulates without doing useful work. Appliances are usually rated in kW, which is only the useful part — for a typical mixed load the difference is around twenty percent, so a set never delivers as many kW as its kVA figure.
- Is it better to buy a bigger generator than I need?
- Some headroom is sensible; a lot is counterproductive. A diesel engine run persistently at very light load does not reach proper operating temperature, which leads to carbon build-up, shorter engine life and poor fuel efficiency. Size it to run comfortably in its efficient band with room for surges and planned growth.
- Why does my water pump need so much extra capacity?
- Motors draw several times their running current for the moment they start. The generator has to supply that surge without the voltage dropping enough to disturb other equipment, so the largest motor on site often sets the rating.