What Size Generator Does Your Construction Site Need?

The most expensive generator on any construction site is the wrong-sized one. Undersized, it trips at the worst possible moment — usually mid-lift, with the tower crane holding a load it now can't land — and every trip is lost production across the whole site. Oversized, it hums along at a fraction of its capacity, burning diesel inefficiently and slowly damaging itself in the process. Sizing a generator for a construction site is not about picking a big number to be safe; it's a short engineering exercise, and this guide walks through it step by step.

Step 1: List every electrical load on the site

Start with paper, not catalogs. Walk the site plan phase by phase and list everything that will draw power from the generator. On a typical Egyptian building site that list looks like:

Two rules make this list honest. First, read the nameplate, not your memory — the data plate on each machine states its rated power, and guessing from machine size is how sites end up undersized. Second, note which loads are actually electric: on many sites the compressor and the lighting towers are diesel-driven units of their own and never touch the generator.

Step 2: Separate running power from starting power

Motors are the whole story here. A resistive load — lighting, heaters — draws the same power from the moment it switches on. A motor does not: started direct-on-line, it briefly draws several times its running current while it comes up to speed. That surge lasts seconds, but the generator has to survive it without stalling or collapsing voltage across the site.

The practical consequences:

• Identify your largest motor — usually the crane hoist or the material hoist. The generator must carry the site's normal load plus that motor's starting surge at the moment it kicks in.
• Starting method matters. Modern cranes and hoists with variable-frequency drives (VFDs) or soft starters draw far gentler starting currents than direct-on-line motors. If your big machines are electronically driven, your generator can be sized closer to the running load; if they start direct-on-line, you need meaningful headroom. This single detail — checked on the equipment spec sheet — swings generator sizing more than any other.
• Stagger what you can. If the dewatering pumps, the hoist, and the welding bay don't all need to start in the same minute, sequence them. Good site discipline is free generator capacity.

Step 3: Convert kW to kVA honestly

Generator sets are rated in kVA; most equipment nameplates state kW. The bridge between them is power factor. Construction loads — heavy on motors and welding — conventionally run around 0.8, which is also the power factor most generator sets are rated at. The arithmetic is one line: kVA = kW ÷ power factor. A site whose realistic peak demand works out to a given kW figure needs that number divided by 0.8 to land in kVA terms — a 25 percent uplift. Skipping this conversion is one of the two classic sizing errors (the other is ignoring starting current), and it always errs in the dangerous direction.

Step 4: Apply a diversity factor — but a defensible one

Not everything on your list runs at once, and sizing for the arithmetic sum of every nameplate builds a generator for a site that never exists. What you want is the realistic worst quarter-hour: the crane lifting, the hoist running, the welding bay live, the offices cooling, the pumps pumping. Build that scenario deliberately — by phase, because peak structure-phase demand differs from finishing-phase demand — and size for it.

Be honest in both directions. Diversity is real, but "it'll never all run together" is how undersized sites talk before the first summer afternoon proves otherwise, with every office AC flat out while the crane works overtime to close a slab pour.

Step 5: Derate for Egyptian conditions

A generator's rating assumes reference operating conditions, and an Egyptian site in August is not those conditions:

• Heat. Sustained high ambient temperatures reduce the output a diesel engine and alternator can continuously deliver. Manufacturers publish derating guidance for temperature — apply it rather than discovering it in July.
• Dust. Site dust loads air filters and radiators; a choked radiator derates the machine unofficially and then damages it. This is a maintenance-schedule issue as much as a sizing one, but chronic dust argues for margin.
• Rating class. Generator sets carry different ratings for standby, prime, and continuous duty. A set powering a working site all day, every day needs to be selected and sized on its prime power rating — not the larger standby figure on the brochure, which assumes occasional emergency use.

Step 6: Stay inside the healthy loading band

Here is the counterintuitive part: bigger is not safer. Diesel engines want to work. Run for long periods at a small fraction of capacity, a diesel generator never reaches proper operating temperature and suffers wet stacking — unburned fuel accumulating in the exhaust system, glazing cylinders, fouling injectors, and shortening the machine's life while it burns diesel inefficiently the whole time.

As a working rule, a site generator is happiest spending most of its day in the middle of its capacity range — comfortably loaded, with headroom for starting surges, and never idling for hours at a token load. If your calculated demand leaves a candidate set loafing near the bottom of its range all day, choose a smaller set — or split the load, which is the next point.

One large set, or two smaller ones?

A construction site's demand profile is lopsided: heavy by day when the crane, hoist, and workshops run; light by night when only lighting, security, offices, and perhaps dewatering pumps remain. One large set sized for the daytime peak spends every night in exactly the low-load condition that causes wet stacking.

The pattern used on well-run larger sites is a day set and a night set: the main generator carries the working day, and a much smaller set carries the night load, each operating in its healthy band. The second set also buys you redundancy — on a site where a dead generator stops the crane, the hoist, and the pour schedule at once, a backup is not a luxury. Very large projects go further, running multiple synchronized sets that share load and back each other up, but the day/night pairing captures most of the benefit at ordinary scale.

A worked example of the method (illustrative numbers)

To make the sequence concrete — with deliberately round, illustrative figures, not a price list or a rating chart:

1. The load list for the structure phase totals, say, 210 kW of connected equipment.
2. The realistic worst-case scenario — crane plus hoist plus welding plus offices — comes to 140 kW of simultaneous running load.
3. The largest direct-on-line motor adds its starting surge on top; with electronically started machines the allowance is modest, with direct-on-line starting it is substantial. Assume the analysis lands total peak demand at 170 kW.
4. Convert to kVA at 0.8 power factor: 170 ÷ 0.8 ≈ 213 kVA.
5. Apply derating for summer heat per the manufacturer's data, then select the next standard set size that keeps normal running load in the healthy middle of the range.

The exact numbers belong to your site; the sequence — list, scenario, starting surge, power factor, derate, band-check — belongs to every site.

Fuel and the practicalities

Whatever size you land on, the generator is only as good as its fuel logistics. Consumption scales with load, so your day-set/night-set decision also shapes the fuel budget. Plan storage and delivery cadence early — on remote sites that means fuel bowsers and a refuelling routine, not a phone call the morning the tank runs dry — and treat filter and oil service intervals as schedule items with the same seriousness as crane maintenance, because in dusty conditions they arrive fast.

Whether generators are worth owning outright or better rented per project follows the same utilization logic as any other machine on site — the framework in renting vs buying heavy equipment in Egypt applies to power plant word for word. And since site power interacts with everything else you're mobilizing, the same early-planning rule from excavator rental cost applies: specify your loads and dates before the peak season books the capacity.

When you have your load list and scenario in hand, you can browse available generators or send the numbers for a sizing recommendation and quote.

Size the generator to the site you'll actually run — not the biggest brochure, and not the sum of every nameplate. The machine will repay you with quiet, boring reliability, which on a construction site is the highest compliment power equipment can earn.