Generator Sizing Calculator

Undersize a generator and it trips every time a motor starts. Oversize it and you burn fuel running a set that never reaches proper operating temperature. This calculator sizes against both your running load and your starting surge, and derates for the ambient temperatures we actually get here.

How this calculation works

Generator sizing is two questions, not one, and the bigger answer wins.

Running demand is what the set has to supply continuously. Add up every load in kilowatts, divide by power factor to get kVA, then apply a diversity factor — because on a real site, not everything runs at once. A construction site typically runs at 70–80% diversity; a data centre runs at 100%.

Starting demand is the spike when the largest motor starts. A direct-on-line motor draws roughly six times its running current for a few seconds. That surge has to be met without the voltage collapsing, or the set trips and everything else drops with it.

The important detail, and the one most calculators get wrong: you size against your total running load plus the surge from the single largest motor — not the sum of every motor’s starting current. Motors don’t all start simultaneously, and sizing as though they do produces a generator two frame sizes too big.

Then comes derating. Generator nameplate ratings are quoted at standard reference conditions, typically 25–27°C. At 45°C an engine produces meaningfully less power. As a working rule, allow roughly 1% capacity loss per degree above 40°C. A 500 kVA set on a July afternoon in Mussafah is not a 500 kVA set.

kVA is not kW. A generator’s kVA rating is apparent power. The useful work it does is kW, and for a standard 0.8 power factor set, kW = kVA × 0.8. A 100 kVA generator delivers 80 kW. Specifications that quote one and order the other are the most common sizing error we see.

Worked example

A contractor fitting out a mid-rise commercial tower in Abu Dhabi:

Load	Type	Rating	Notes
Tower crane	Motor, DOL	45 kW	The largest starting load
Site lighting	Resistive	12 kW	Runs continuously
Welding plants ×4	Welding	8 kW each	Intermittent
Site accommodation A/C	Air conditioning	25 kW	Runs most of the day
Small power and tools	Electronic	15 kW

Running load comes to roughly 129 kW. At 0.85 power factor that’s about 152 kVA, and with an 80% diversity factor the running demand is around 121 kVA.

The crane at 45 kW is about 53 kVA running. Started direct on line at six times, it draws roughly 318 kVA — a surge of 265 kVA above its own running figure. Starting demand is therefore 121 + 265 = 386 kVA.

Starting governs. Add 20% reserve for load growth: 463 kVA. Derate 5% for a 45°C ambient: 488 kVA required. The recommendation is a 500 kVA set.

Notice what happened. The running load was only 129 kW — a 200 kVA set would carry it comfortably. The crane’s starting surge more than doubled the requirement. This is why a soft starter or VFD on the largest motor is often cheaper than the next two frame sizes of generator, and it’s worth asking the question before you order.

Common mistakes

Sizing on running load alone. Works fine until the crane starts and the site goes dark. Always check the starting case.

Adding up every motor’s starting current. Massively oversizes. Only the largest single surge matters.

Ignoring ambient derating. Every generic calculator online assumes a temperate climate. In the UAE that’s a 5–10% error in the wrong direction.

Confusing kW and kVA. Ordering “500 kW” when you meant 500 kVA gets you a set 25% larger than you needed, and a bill to match.

Oversizing “to be safe.” A generator running below about 30% load for extended periods wet-stacks: unburnt fuel accumulates in the exhaust, glazes the cylinder bores, and fouls the injectors. The healthy operating band is roughly 50–80% of rated output. Oversizing is not free insurance — it’s a maintenance problem you pay for later.

FAQ

What size generator do I need for a construction site? It depends far more on your largest motor than on your total load. A site with a 45 kW tower crane started direct on line will typically need 400–500 kVA even if the running load is under 150 kW. Run your actual load list through the calculator above.

What’s the difference between kVA and kW? kVA is apparent power, kW is real power — the work actually done. They’re related by power factor. Standard generators are rated at 0.8 power factor, so a 100 kVA set delivers 80 kW. Generators are sold in kVA; loads are usually specified in kW.

Does the UAE climate affect generator sizing? Yes, significantly. Engine output falls as intake air temperature rises. Allow roughly 1% capacity loss per degree above 40°C, which means a 5–10% derating through the UAE summer. This calculator applies it automatically.

Should I size for prime power or standby? Prime rating is for continuous running with variable load — the right rating for construction sites and events. Standby rating is for emergency backup only, typically limited to a few hundred hours a year. A set rated 500 kVA standby is usually around 450 kVA prime. Check which rating a quotation refers to.

Can I run two generators instead of one large one? Yes, with synchronising panels. Two sets in parallel give you redundancy and let you shut one down at low load, which avoids wet stacking. It costs more in equipment but often makes sense for long projects or critical loads.