To size a diesel generator: total the loads that run simultaneously, add the starting surge of the largest motor, divide by the power factor (≈0.8) to get kVA, derate for altitude and heat, then add 10–20% reserve. That one sentence is the whole method — the rest of this guide walks through each step with the numbers and the traps.
Choosing the right diesel generator is the difference between reliable power and a unit that overheats, wet-stacks, or simply can’t start your motors. The two most expensive mistakes in procurement are undersizing (the genset trips under load) and oversizing (you pay more upfront and then run the engine at light loads it was never designed for).
Step 1 — List every load and its running power
Write down each appliance or machine the generator must run, with its running (continuous) power in watts or kW. Check the nameplate, not your memory. For a mixed site, total the loads that will run at the same time, not the sum of everything you own — a workshop rarely runs the welder, the compressor, and every light simultaneously.
Step 2 — Account for the starting surge
Electric motors (pumps, compressors, AC units) draw 3–7× their running current at startup. This inrush, not the running load, usually sets the generator size. As a rule of thumb, add the largest motor’s starting requirement on top of the running total of everything else already spinning. A 5 kW pump may need 15–20 kW available the instant it starts.
If several motors start at once, stagger them — starting sequence is free capacity. Sites that can’t stagger (automatic pump arrays, for instance) must size for the worst simultaneous start.
Step 3 — Convert kW to kVA
Generators are rated in kVA, but loads are often given in kW. Convert using the power factor (PF):
kVA = kW ÷ PF (typical PF for mixed commercial loads ≈ 0.8)
So a 16 kW load at 0.8 PF needs 20 kVA.
Step 4 — Add headroom for altitude and temperature
Diesel engines lose output in thin air and high heat — critical for highland and tropical sites. Typical derating is roughly:
- ~3% per 300 m above ~1,000 m altitude
- ~2% per 5 °C above ~40 °C ambient
Sites at 3,000–4,000 m can lose 15–20% of rated output. Size for the derated figure, then add 10–20% reserve for future loads. The full mechanics — and what turbocharging does and doesn’t fix — are in our guide to why diesel engines lose power at altitude.
Step 5 — Choose phase, frequency, and rating type
- Single-phase for light commercial/residential; three-phase (230/400 V) for industrial motors and larger sites.
- 50 Hz is standard across most export markets (Latin America, Europe, Africa, Middle East); 60 Hz applies in some countries — confirm before ordering, the two are not interchangeable.
- Prime rating for continuous main power; standby for backup only. The same machine carries different numbers for each duty — see generator power ratings explained before you compare datasheets.
Worked example
A small farm workshop: lighting and tools 6 kW running, a 4 kW water pump (starts under load), a 3 kW cold-room compressor.
- Running total when everything is on: 6 + 4 + 3 = 13 kW
- Largest motor start: the pump may demand ~3× → add ~8 kW of surge headroom → 21 kW peak
- Convert: 21 ÷ 0.8 = 26 kVA
- Site at 2,500 m: derate ~15% → need the derated output to cover 26 kVA → look at ~31 kVA rated
- Add reserve → a 33–35 kVA prime-rated, three-phase set is the sensible bracket.
Skipping step 4 is how a “correctly sized” generator ends up tripping every afternoon in the highlands.
Open-frame vs silent vs welding
| Type | Best for |
|---|---|
| Open-frame | Construction sites, lowest cost, easy access |
| Silent (enclosed) | Occupied areas, < 75 dB, weather-resistant |
| Welding genset | Field repair and fabrication — power + welding in one |
The mistakes we see most often
- Buying to the standby number and running it as daily prime power — early overhaul guaranteed.
- Ignoring motor starting because “the running load fits.”
- Sizing at sea level for a highland site.
- No reserve margin, so the first new machine added to the site overloads the set.
Quick reference
- Total running load → 2. Add largest motor’s starting surge → 3. ÷ 0.8 to get kVA → 4. Derate for altitude/heat → 5. Add 10–20% reserve.
Not sure which model fits your duty cycle? Browse the diesel generator range, then send us your application, power requirement, altitude, and the loads you need to start — our engineers will recommend the right HTX set and confirm lead time within 24 business hours.