Getting the size right is the single decision that determines whether a rental works. Undersize and the set trips or damages equipment; oversize and you pay for capacity you never use while running the engine at a load too light to be healthy for it.
What the ratings mean
- kW (kilowatt) — real power, the work actually delivered.
- kVA (kilovolt-ampere) — apparent power, the total the alternator has to supply.
- MW (megawatt) — 1,000 kW, used for utility, industrial and large temporary-power projects.
- Power factor — the ratio between them. Check the power factor stated on the actual generator rating; do not assume every set uses the same value.
To convert: kW = kVA x power factor, and kVA = kW / power factor. At 0.8, a 500 kVA set is a 400 kW set.
Prime, standby and continuous ratings
The same physical machine carries different ratings depending on duty. A standby rating assumes occasional emergency use; a prime rating assumes variable load for extended periods; a continuous rating assumes constant full load. Comparing a standby number against a prime number makes one unit look larger than it is. Ask which rating the quote uses.
Why starting current decides the size
Induction motors — pumps, compressors, lifts, HVAC, refrigeration — can draw several times their running current for the first seconds of start-up. A load list that only totals running watts will point you at a generator that cannot start the equipment it is supposed to run. Starting order matters too: staggering large motor starts can meaningfully reduce the size you need.
Building a usable load list
- List every load with its running watts or amps, and its voltage.
- Note starting requirements for anything with a motor or compressor.
- Mark which loads run simultaneously and which never do.
- Record voltage, phase and connection type for each.
- Ask the sizing specialist to allow for any planned additional loads.
Voltage, phase and connection
A correctly sized generator that cannot be connected is no use. Confirm single-phase or three-phase, the voltage the equipment needs, the receptacle or termination on the unit, and cable sizing for the run length. Voltage drop over a long cable run is a real derating factor.
When you need an engineered load study
Bring in an electrical engineer or the provider's application team when the job involves life-safety systems, medical or data-centre load, paralleled sets, utility-interactive operation, large motor starting, non-linear load such as variable-frequency drives, or anything permit-conditioned. A load study is not upsell; it is what stops an expensive outage on a live site.
An important limit on any rule of thumb
No kW figure can be promised to power a specific site without a load calculation. Charts and calculators — including any on this site — narrow the range and prepare you for the conversation. The provider's sizing check and, where required, a qualified electrical review, is what confirms it.
Common mistakes
Comparing a kVA rating against a kW requirement, sizing to nameplate totals rather than realistic simultaneous load, ignoring starting surge, and forgetting that a lightly loaded diesel run for weeks develops its own maintenance problems.
Questions people ask
Sources
- Considerations for Generator Set Selection — Cummins (retrieved 2026-10-30)