10 October 2026
How to choose the right diesel generator
A plain-English guide to sizing a diesel generator: what to add up, why motors matter, and how to pick between silent and open sets.
Buying a generator that's too small means it trips when everything starts at once. Buying one that's too big means paying more for the machine and running it under-loaded. Here's how to get it right.
1. List what you need to run at the same time
Write down every piece of equipment the generator must power together — lighting, heating, IT, pumps, compressors, kitchen kit. Note each one's power rating in kW (it's on the rating plate or in the manual).
Add them up. That total is your running load.
2. Allow for starting loads
Electric motors, pumps and compressors draw several times their running power for a moment when they start. If you have large motors, the generator needs headroom to cover that surge — or the motors need soft starters.
As a rule of thumb, add a margin of 20–25% on top of your running load so the set isn't working flat out.
3. Convert kW to kVA
Generators are usually sold in kVA. For most three-phase sets, kW = kVA × 0.8. So a 100 kW load needs roughly a 125 kVA generator. See our guide on kVA vs kW for more.
4. Decide: standby or prime power
If the generator only runs when the mains fails, size on the standby rating. If it is your main power source, size on the prime rating, which is lower.
5. Silent or open?
Silent sets sit in an acoustic canopy and suit offices, hospitals, homes and built-up sites. Open-frame sets are for plant rooms and containers where the building handles the noise and weather.
Not sure? Ask us
Send us a list of what you need to power and we'll size it for free and recommend an in-stock model.