To size a diesel generator, add up the loads that run at the same time and 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. The rest of this guide goes through each step with figures and common mistakes.
A correctly sized generator runs reliably. A wrongly sized one overheats, suffers wet-stacking, or cannot start your motors. The two most expensive purchasing mistakes are undersizing and oversizing. An undersized set trips under load. An oversized set costs more at purchase, then runs at light loads the engine was not 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. Take the figure from the nameplate, not from memory. For a mixed site, add up the loads that will run at the same time, not everything you own. A workshop rarely runs the welder, the compressor and every light together.
Step 2: Allow for the starting surge
Electric motors (pumps, compressors, air-conditioning units) draw 3–7× their running current at startup. This inrush current, not the running load, usually decides the generator size. As a rule of thumb, add the starting requirement of the largest motor to the running total of everything already on. A 5 kW pump may need 15–20 kW available at the moment it starts.
If several motors can start at once, start them one after another. A staggered start sequence reduces the capacity you need at no cost. If your site cannot stagger starts, as with automatic pump arrays, 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 with 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, which matters at 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 explanation, including what turbocharging does and does not fix, is in our guide to why diesel engines lose power at altitude.
Step 5: Choose phase, frequency and rating type
- Single-phase for light commercial and residential use; three-phase (230/400 V) for industrial motors and larger sites.
- Frequency depends on the country, not the continent. Europe, most of Africa, the Middle East and parts of South America (Chile, Argentina, Bolivia, Paraguay, Uruguay) use 50 Hz. North America and much of Latin America, including Peru, Ecuador, Colombia, Brazil, Mexico, Venezuela, Central America, use 60 Hz. Check your own grid rather than a regional generalisation. Voltage is a separate question: Peru is 220 V at 60 Hz, while Ecuador and Colombia are nearer 120/240 V at the same frequency. Confirm phase, voltage and frequency before you order.
- The same engine can be built for either frequency, but the choice is made at build time; it is not a switch. At 60 Hz the set runs at 3,600 rpm instead of 3,000 and delivers more output. HTX generator sets are available in 50 Hz and 60 Hz versions; tell us your market and we will quote the matching configuration.
- Use the prime rating for continuous main power and the standby rating for backup only. The same machine has different figures for each duty; see generator power ratings explained before you compare datasheets.
Worked example
A small farm workshop has lighting and tools of 6 kW running, a 4 kW water pump that starts under load, and a 3 kW cold-room compressor.
- Running total with everything 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% → the derated output must cover 26 kVA → look at ~31 kVA rated
- Add reserve → a 33–35 kVA prime-rated, three-phase set is the right range.
If you skip step 4, a generator that looked correctly sized at sea level will trip regularly at a highland site.
Open-frame, silent or welding
| Type | Best for |
|---|---|
| Open-frame | Construction sites, lowest cost, easy access |
| Silent (enclosed) | Occupied areas, < 75 dB, weather-resistant |
| Welding generator set | Field repair and fabrication: power and welding in one machine |
The most common mistakes
- Buying by the standby figure and running the set as daily prime power, which leads to an early overhaul.
- Ignoring motor starting because the running load fits.
- Sizing at sea level for a highland site.
- No reserve margin, so the first 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.
The generator size calculator runs the same five steps with your figures and shows which of our models cover the result.
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 the lead time within 24 business hours.