A size chart gives you a starting point, not a final answer. Below is the working chart for small and medium diesel generators, showing what each kVA class typically runs. After it come two corrections, for motor starting and for altitude, which decide whether the class the chart suggests 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; heavy items one at a time |
| 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-size 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 several units or grid-parallel operation |
50 Hz or 60 Hz? The table is quoted at 50 Hz (3,000 rpm). The same set built for 60 Hz runs at 3,600 rpm and delivers roughly 10% more output. So a class that looks marginal at 50 Hz may fit comfortably in a 60 Hz market (Peru, Ecuador, Colombia, Brazil, Mexico). HTX generator sets are available in both versions; ask for the 60 Hz rating of the model you are considering.
The ”≈ kW available” column is the useful figure. Generators are rated in kVA, loads are rated in kW, and the power factor links the two: kW = kVA × 0.8 for typical mixed loads. A “10 kVA” set is an 8 kW machine in practice.
kVA to kW, to watts and to amps: the three conversions
The difference between kVA and kW
kVA is apparent power: what the alternator, the cables and the breaker must carry. kW is real power: what the engine must produce and what you pay for in fuel. The power factor links them, and generator sets are rated at a power factor of 0.8 under ISO 8528. That is why a “10 kVA” set is an 8 kW machine.
The formulas
| To go from… | Formula | Worked at 20 kVA |
|---|---|---|
| kVA to kW | kW = kVA × 0.8 | 20 × 0.8 = 16 kW |
| kW to kVA | kVA = kW ÷ 0.8 | 16 ÷ 0.8 = 20 kVA |
| kVA to watts | W = kVA × 1,000 × 0.8 | 16,000 W |
| kVA to amps (three-phase) | A = kVA × 1,000 ÷ (1.73 × V) | at 380 V: 30.4 A |
| kVA to amps (single-phase) | A = kVA × 1,000 ÷ V | at 220 V: 90.9 A |
The current table, already worked out
| kVA | kW (at 0.8) | A at 220 V 1-ph | A at 380 V 3-ph | A at 400 V 3-ph |
|---|---|---|---|---|
| 5 | 4 | 22.7 | 7.6 | 7.2 |
| 10 | 8 | 45.5 | 15.2 | 14.4 |
| 15 | 12 | 68.2 | 22.8 | 21.7 |
| 20 | 16 | 90.9 | 30.4 | 28.9 |
| 30 | 24 | — | 45.6 | 43.3 |
| 60 | 48 | — | 91.2 | 86.6 |
| 100 | 80 | — | 151.9 | 144.3 |
Single-phase is unusual above roughly 20 kVA, because the current becomes too high for practical cables and breakers. That is why larger installations use three-phase.
Three points a generic converter will not tell you
- The 0.8 power factor belongs to the generator, not to your load. If your load is mostly resistive (heating, ovens, incandescent lighting), the engine’s kW becomes the real limit and the alternator has capacity to spare. If your load is mostly motors, the site power factor can drop to 0.7, and you will need more kVA than the calculation suggests.
- These are running currents, not starting currents. A motor draws 3–6 times its rated current for a few seconds when it starts. Sizing from the current column while ignoring the start is the mistake described in the next section.
- Voltage affects this calculation more than frequency does. At 380 V and at 400 V the same kVA gives different currents. Use the actual voltage of your installation.
Correction 1: motor starting
The chart assumes mainly resistive loads. Every electric motor (pump, compressor, cold-room unit) draws 3–7× its rated current at start, and for those seconds it takes most of the generator’s capacity. One 4 kW pump starting direct-on-line can briefly demand what a 12–15 kVA class delivers. For this reason, the same load list often needs one or two classes more once starting is counted. The full calculation, with a worked example, is in how to size a diesel generator.
Rule of thumb for the chart: if your largest single motor is more than a quarter of the class’s kW, choose the next class up.
Correction 2: altitude and heat
Chart figures are quoted at reference conditions near sea level. Thin air and high ambient temperature reduce real output by 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%, so a “20 kVA” set delivers about 16 kVA there. Details and a worked example are in why diesel engines lose power at altitude.
Check which rating the chart figure is
The same generator has a standby, a prime and a continuous rating. The largest figure on the datasheet is the standby rating, which is valid only for short emergency use. Choose a machine for daily duty by its prime rating. Charts from different sellers are only comparable if they use the same rating; see generator power ratings explained.
How to use the chart (60 seconds)
- Add up the loads that run at the same time, in kW.
- Add the starting surge of the largest motor.
- Divide by 0.8 → kVA.
- Derate for your altitude and temperature.
- Add 10–20% reserve, then choose the class by its prime rating.
Compare the result with the chart’s “typically runs” column. This quick check catches most order-of-magnitude mistakes before they cost money.
To run these five steps with your own figures, use the generator size calculator. It shows each step and which of our models cover the result.
Two quick questions buyers ask about the chart
“Why not buy two sizes up to be safe?” Because a diesel engine that runs most of its life at 20–30% load wears badly, through cylinder glazing and wet-stacking, and it wastes 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 one extra class for peace of mind.
“Is one large set better than two smaller ones?” For most small sites, one correctly sized set is simpler and cheaper. Two sets make sense when the load differs greatly between day and night, or when downtime is unacceptable. Discuss this with your supplier once your load list is final.
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 supplies your single-phase loads. If any machine on your site runs on 380–415 V, you need three-phase. The decision logic is in single-phase or three-phase generator.
Have your load list ready? Browse the diesel generator range of 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.