How to Size a Diesel Generator: kVA, Starting Surge, Altitude and Heat

A generator that is slightly too small does not fail gracefully. It starts, it carries the load for a while, and then it starts doing the things that cost money: voltage dips that trip equipment, wet stacking from chronic underloading, alternator overheating, and a service life measured in months instead of years.

The good news is that sizing is not guesswork. It is four steps, and three of them are the ones buyers skip.

Step 1: Convert your real load from kW to kVA

Your load list is in kilowatts. The generator on the nameplate is in kVA. The gap between them is power factor.

A generator set is rated in kVA because the alternator has to carry current, not just deliver real work. That current depends on the power factor of whatever you plug in. For most mixed commercial and agricultural loads, 0.8 is the standard planning value.

So a farm workshop with 80 kW of connected load needs a 100 kVA set at a minimum. If your load is dominated by motors rather than heaters and lighting, confirm the actual power factor rather than assuming 0.8 — some older single-phase motor loads run lower.

Step 2: The number that actually decides the size — starting surge

This is the step that gets skipped, and it is the reason generators come up short.

When an electric motor starts direct-on-line, it draws several times its running current for a few seconds. Your generator has to supply that peak, and it has to do it without the voltage collapsing far enough to trip everything else already running. Resistive loads — heaters, incandescent lighting — have no surge at all.

Load type	Starting surge multiplier	Note
Resistive heating, incandescent lighting	1×	No surge; power factor ≈ 1.0
LED lighting with driver	1–1.5×	Non-linear; may need a harmonic-rated alternator
Industrial motor, direct-on-line	3–6×	The single largest sizing factor in industrial applications
Industrial motor with soft starter	1.5–2×	Cuts the peak dramatically; check compatibility
Water pump (DOL, 1–5 HP)	3×	Submersible pumps can surge up to 5×
Air conditioner (direct-on-line)	3×	The most common cause of oversizing in buildings
Air conditioner (inverter / VFD)	1.1–1.3×	Far easier on a generator
UPS system	1–1.5×	Non-linear; often sized at 1.5–2× UPS kVA for harmonic margin
Welding equipment	1–2×	Size for peak arc demand; inverter welders are gentler

Work out which single load has the largest starting surge, then check whether it starts while everything else is already running. A 30 kW submersible pump kicking in on a site that is already drawing 60 kW is a completely different machine from the same pump starting on an otherwise idle site.

Step 3: Derate for altitude

Altitude derating is an engine problem, not an electrical one. As elevation rises, air pressure falls, each cylinder gets less oxygen per stroke, and the engine makes less power. A turbocharger fights back by spinning faster in thin air, which is why turbocharged engines lose far less than naturally aspirated ones.

Most manufacturers apply no derating at all below 1,000 m. Above that, the widely used planning rule is roughly 1% per 100 m for a turbocharged diesel and 1.5% per 100 m for a naturally aspirated one.

Site altitude	Turbocharged (approx.)	Naturally aspirated (approx.)
0 – 1,000 m	0%	0%
1,500 m	~5%	~7.5%
2,000 m	~10%	~15%
2,500 m	~12%	~18%
3,000 m	~15%	~22%

These are planning figures. Manufacturer curves vary — Cummins commonly starts derating above about 915 m at roughly 3% per 300 m, Perkins uses around 4% per 300 m above 1,000 m. Always pull the specific engine's curve from its datasheet.

Above 3,000 m, a standard turbo is usually not enough. High-altitude turbo kits, matched pistons or dedicated high-altitude engine models are factory configuration decisions, which is why site altitude has to be known at quote time, not discovered at commissioning.

Step 4: Derate for heat — and note that the alternator cares more than the engine

Heat derating runs on two different curves, and this is where people trip up.

The engine derates on hot intake air: less dense air, less efficient combustion. The common reference is about 1% per 10 °C above 25 °C. Gentle.

The alternator derates faster, and against a different reference — a 40 °C cooling-air inlet, not 25 °C. Above that, the winding insulation runs hotter and output has to come down to keep it within its temperature-rise limit. Stamford's application guidance (AGN 012) gives roughly 3% per 5 °C above 40 °C:

Cooling-air inlet	Derating factor
40 °C	1.00
45 °C	0.97
50 °C	0.94
55 °C	0.91
60 °C	0.88

The alternator also loses a little with altitude — about 0.4% per 100 m above 1,000 m — because thinner air cools the windings less effectively. Small, but it stacks on top of the engine's much larger penalty.

The two factors are multiplicative. Both apply at the same time.

Worked example. A site needs 129 kVA of effective capacity. It sits at 1,500 m altitude (factor 0.94) and sees 40 °C peak ambient (factor 0.97).

129 ÷ (0.94 × 0.97) = 129 ÷ 0.912 = 141 kVA nameplate minimum. You select the next standard size up: 150 kVA.

Notice what happened. The site needs 129 kVA, and the honest answer is 150 kVA — a 16% step up, purely from air and heat. A supplier who skips these two steps will quote you 130 kVA, and that set will struggle on the hottest afternoon of the year.

Then two things that are not on the sizing sheet but belong in the enquiry

50 Hz or 60 Hz, and single or three phase

This sounds trivial and it is not. A 60 Hz machine running a 50 Hz load will spin motors faster than designed and overheat them; a 50 Hz machine on a 60 Hz system does the opposite. Confirm the grid frequency of the destination country and whether your load is single-phase or three-phase before you ask anyone for a price. Both are build-configuration items, not afterthoughts.

Prime, standby or continuous

ISO 8528 defines the rating classes, and the same physical set carries a different number in each. Prime power (PRP) is for supplying a variable load for an unlimited number of hours per year. Emergency standby (ESP) is for running during a mains failure, for a limited number of hours per year, and is normally the higher number on the spec sheet. Continuous (COP) is for a constant load, unlimited hours.

If a quote does not say which class the kVA figure refers to, the number is not comparable to anyone else's.

What to send a supplier so the quote is actually right

Four pieces of information, and you will get a usable answer instead of a generic brochure:

1. Your load list — every significant load, its kW, and how many start at once.
2. Site altitude, in metres. Not "highland" — the number.
3. Peak ambient temperature, in °C. The hottest afternoon, not the annual average.
4. Duty class — prime, standby or continuous, plus grid frequency and phase.

A supplier who quotes before asking any of these is guessing. A supplier who asks is doing the job.

A note on what this guide is

Everything above is general engineering practice, and the derating figures are planning values — your specific engine and alternator combination will have its own published curves, and those govern. Treat this as the method and the questions to ask, not as a substitute for the datasheet.

If you are sizing a set for a farm, a mine site, a workshop or a backup application and want a second read on the numbers, send us the four items above. We export generator sets from a partner factory in China and we would rather get the size right before quoting than after.