A size chart tells you the neighborhood, not the address. Below is the working chart for small-to-medium diesel generators — what each kVA class typically runs — followed by the two corrections (motor starting and altitude) that decide whether the “right” class from the chart actually works at your site.
The chart (50 Hz, 0.8 power factor)
| Class | ≈ kW available | Typically runs |
|---|---|---|
| 5 kVA | 4 kW | House or small shop: lights, fridge, TV, fans, small tools — one at a time on the heavy items |
| 8–10 kVA | 6.5–8 kW | Small workshop or farmhouse: lighting + a welder or a compressor, small pump, kitchen loads |
| 12–15 kVA | 10–12 kW | Shop, small restaurant, poultry shed: refrigeration + lighting + a mid-size motor with care |
| 20–30 kVA | 16–24 kW | Small factory line, irrigation pump station, telecom site, mid construction site |
| 40–60 kVA | 32–48 kW | Multi-machine workshops, cold storage, larger pump stations, building backup |
| 80–100+ kVA | 64–80+ kW | Industrial base load, large sites — usually multiple units or grid-parallel territory |
Read ”≈ kW available” as the honest number: generators are rated in kVA, loads are billed in kW, and the bridge is the power factor — kW = kVA × 0.8 for typical mixed loads. A “10 kVA” set is an 8 kW machine in real life.
Correction 1 — Motor starting shrinks the chart
The chart assumes resistive-ish loads. Every electric motor (pump, compressor, cold-room unit) demands 3–7× its rated current at start, and for those seconds it owns the generator. One 4 kW pump starting direct-on-line can momentarily ask for what a 12–15 kVA class delivers. That’s why the same load list lands one or two classes higher once starting is counted — the full arithmetic, with a worked example, is in how to size a diesel generator.
Rule of thumb for the chart: if your biggest single motor is more than a quarter of the class’s kW, step up one class.
Correction 2 — Altitude and heat shrink it again
Chart numbers are quoted at reference conditions near sea level. Thin air and high ambient temperatures derate real output — roughly 3% per 300 m above 1,000 m, plus about 2% per 5 °C above 40 °C. A highland site at 3,000+ m loses 15–20%: your “20 kVA” is a 16 kVA machine up there. Details and a worked example: why diesel engines lose power at altitude.
Don’t forget which rating the chart number is
The same generator carries a standby, a prime, and a continuous rating — and the biggest number on the datasheet is the standby one, valid only for short emergency use. Daily-duty machines must be chosen against the prime rating. If you compare charts from different sellers without checking which rating they quote, you’re comparing apples to advertising — see generator power ratings explained.
How to use the chart properly (60 seconds)
- Total the loads that run simultaneously, in kW.
- Add the starting surge of the largest motor.
- Divide by 0.8 → kVA.
- Derate for your altitude/temperature.
- Add 10–20% reserve → pick the class, prime-rated.
A quick sanity check of the result against the chart’s “typically runs” column catches most order-of-magnitude mistakes before they cost money.
Two quick questions buyers ask about the chart
“Why not just buy two sizes up and be safe?” Because a diesel that spends its life at 20–30% load suffers for it — glazing, wet-stacking, wasted fuel and capital. Oversizing is a slower mistake than undersizing, but it is still a mistake. Size for the corrected load plus 10–20%, not “plus a category for peace of mind.”
“Is one big set better than two smaller ones?” For most small sites, one correctly sized set is simpler and cheaper. Two sets earn their keep when the load swings hugely between day and night, or when downtime is unacceptable — questions worth raising with your supplier once your load list is real.
Single-phase or three-phase?
Classes up to ~10–12 kVA are commonly single-phase; industrial classes above that are usually three-phase (which also feeds your single-phase loads). If anything on your site is a 380–415 V machine, the phase question is already answered — the decision logic is in single-phase or three-phase generator.
Have your load list ready? Browse the diesel generator range — open-frame, silent, and welding sets — then send us the list with nameplate data, your altitude, and your duty or message us on WhatsApp. Our engineers will confirm the exact class and configuration within 24 business hours.