Jul 18, 2026
Choosing the right generator size is the most important decision when buying or renting. Too small and it overloads; too big and you waste fuel and risk wet-stacking. Here's how to size a diesel generator correctly.
kVA vs kW — what's the difference?
kW is real power (the actual work done); kVA is apparent power. They're linked by the power factor, usually 0.8: kW = kVA × 0.8. Generators are rated in kVA.
Step 1 — List your loads
Add up the running wattage of everything the generator must power: lights, motors, AC units, pumps, tools and IT equipment.
Step 2 — Account for starting loads
Motors and compressors can draw 3–6× their running current at start-up. Size for the peak, not just the average.
Step 3 — Add a safety margin
Add about 20–25% headroom for future growth and to avoid running the set at 100% all the time.
Standby, prime or continuous?
- Standby — backup only, variable load
- Prime — main power, variable load, unlimited hours
- Continuous — constant load, unlimited hours
Avoid under-loading (wet-stacking)
Running a diesel generator below ~30% load causes unburnt fuel to build up. Right-sizing prevents this, and a load bank can test and clean the set.
Not sure what you need? Send us your load list and our engineers will recommend the right kVA. Browse our product range.