The mistake that overloads "oversized" generators
Most generators sold in Kenya are rated at the international reference condition of sea level and 25-40 degC. The trouble is that almost nobody in this country operates there. A diesel engine breathes air, and the thinner air of the Kenyan plateau carries less oxygen, so the engine simply makes less power.
The result is a quiet, expensive surprise: a factory buys a "100 kVA" set, installs it in Eldoret, and then cannot understand why it trips on overload every time the compressors and chillers run together. The capacity was never there. It was de-rated away by physics the supplier ignored.
The derating rule of thumb
For a naturally-aspirated diesel engine:
- Altitude: lose roughly 3-4% of rated output for every 300 m above 1,000 m.
- Temperature: lose a further ~1-2% for every 5-6 degC above the reference (often 25-40 degC).
A useful approximation of the usable power at your site is:
P_site ≈ P_rated × (1 − altitude_derate) × (1 − temperature_derate)
Worked example — a 100 kVA set in Eldoret (~2,100 m, 30 degC): altitude loss ≈ 7.7%, temperature loss ≈ 1%, so usable output ≈ 100 × 0.923 × 0.99 ≈ 91 kVA, before any canopy airflow restriction.
Derate across major Kenyan towns
| Location | Elevation | Approx. altitude derate | A 100 kVA set delivers ≈ |
|---|---|---|---|
| Mombasa / Diani | ~15 m | 0% | 100 kVA |
| Kisumu | ~1,130 m | ~1% | 99 kVA |
| Nairobi | ~1,795 m | ~5.5% | 94 kVA |
| Nakuru | ~1,850 m | ~6% | 94 kVA |
| Nyeri | ~1,750 m | ~5% | 95 kVA |
| Eldoret | ~2,100 m | ~7.7% | 92 kVA |
(Indicative, naturally-aspirated diesel. Turbocharged engines lose less; always confirm against the engine manufacturer's derate curve.)
How to size correctly for Kenyan conditions
- Do a load study first. Size in kW against your real demand profile, including the largest motor that can start while everything else runs.
- Apply the site correction. Use the altitude and worst-case radiator air-on temperature (not shade temperature) for your location.
- Allow for the canopy. An acoustic enclosure with restricted airflow adds its own small derate.
- Then choose the rating (standby / prime / continuous per ISO 8528) that matches your duty.
The bottom line
A generator's brochure kVA is a sea-level promise. In Nairobi, Nakuru or Eldoret the machine you actually own is several percent smaller — and that gap is exactly the margin that disappears on the hottest afternoon when your heaviest loads coincide.
Send us your location, altitude and largest motors and we will return a site-corrected sizing — free, and in writing. Call +254 768 860 665 or +254 782 914 717.