Generator Sizing Guide
Most rented generators in the Kingdom are either too big — wasting money and diesel — or too small, stopping the site at the worst possible moment. This guide walks through the arithmetic that actually decides the answer: how to total your loads, how to convert kW to kVA, why motor starting current usually governs the size, and how much headroom you need at 45–50°C.
Figures on this page are typical planning and market numbers. Always verify against your own equipment nameplates and the generator manufacturer's data before committing to a size.
There is no single magic formula, but there is a sequence engineers use on almost every site. Follow it in order and write your assumptions down — when the scope changes in two months, you will need to know where the number came from.
Walk the site and write down every device that will be plugged into the set: motors, pumps, hoists, welders, lighting, site-office air conditioning, water coolers, kitchen equipment. Take the figure from the nameplate, not from memory. Motors are usually stamped in kW (mechanical output) while welders, UPS units, and transformers are stamped in kVA — keep the two columns separate for now so you don't add unlike units.
Almost nothing on a construction site runs at 100% simultaneously. The demand factor is your honest estimate of what fraction of the connected load is actually drawing power at the same moment. On a busy site with continuous concrete and steel work, 0.7–0.8 is typical. On a site where the crane, the welders, and the benders take turns, 0.5–0.65 is more realistic. Get this wrong upward and you rent — and fuel — a set twice the size you need.
This is the step people get wrong. An induction motor started direct-on-line (DOL) draws roughly 6–7 times its running current for a few seconds, and it draws that current at a very poor power factor. A 22 kW hoist motor that runs at about 27 kVA can demand well over 150 kVA at the instant it starts. Your genset must ride through that step without the voltage or frequency collapsing. Size for the running load plus the starting surge of the single largest motor you will start on that set.
Scope grows. A second welding bay appears, the site office doubles, someone adds a dewatering pump after the first rain. A 20–25% margin on the running demand covers that growth, keeps the alternator out of its overload band, and leaves room for the derating you will lose to Saudi summer ambient temperatures. Headroom is not the same as oversizing: it is calculated on the running load, not stacked on top of an already-inflated number.
Gensets are not built in every size. Rental fleets are stocked in standard classes — 20, 30, 40, 60, 80, 100, 150, 200, 250, 400, 500, 630, 800, 1000, 1250, 1500 kVA and up. Take the larger of (running demand + headroom) and (running demand + largest starting surge), then pick the next standard class above it. Asking for a non-standard number just means the owner quotes you the next class anyway, with a delay while they check.
Confusing the two units is the single most common error in rental enquiries. A kilowatt (kW) is real power: the actual work your equipment does. A kilovolt-ampere (kVA) is apparent power: the total current that has to physically flow through the alternator, the cables, and the breakers. Inductive loads — motors, transformers, welders, discharge lighting — draw more current than the useful work suggests, and the gap between the two numbers is the power factor.
Diesel generator sets are conventionally rated at a power factor of 0.8 — so a 100 kVA set delivers 80 kW, and a 500 kVA set delivers 400 kW.
That is why generators are sold in kVA: the alternator is limited by the current its windings can carry, not by useful work. If you total your loads in kW and then ask for a generator with the same number, you will get a unit roughly 20% smaller than you need. And if your site power factor is genuinely poor — many older motors sit at 0.7 or below at part load — the gap widens, and power factor correction at the board can save you a whole class.
The same machine carries three different numbers on its nameplate, and the difference between them is not marketing — it is engineering and contract. Always ask which rating a quote refers to.
The highest number on the nameplate, and the one most often quoted in advertising. It applies to emergency backup only — a variable load, a limited number of hours per year, and no sustained overload capability. Do not size a full-time construction site against a standby rating.
Unlimited running hours at a variable load, with a typical 10% overload allowance for one hour in twelve. This is the rating that matters for construction sites, events, and any application where the genset is the only source of power. Prime is usually around 90% of the standby figure.
Unlimited hours at a constant load with no overload allowance — used for remote camps, telecom sites, and anything running as a base-load power station. It is the most conservative rating and therefore the lowest kVA number for the same machine.
The numbers below are reasonable planning figures; the point is to show the arithmetic. Swap in your own nameplate values and follow the same steps.
Concrete mixer, poker vibrator, water pump, site lighting, two office cabins, and hand power tools.
Demand factor 0.7 (everything rarely runs at once): 24.7 × 0.7 ≈ 17.3 kW.
Convert to kVA at PF 0.8: 17.3 ÷ 0.8 ≈ 21.6 kVA of running load.
The largest motor is the mixer: 5.5 kW ≈ 6.9 kVA running. Started direct-on-line at 6×: 6.9 × 6 ≈ 41 kVA momentarily. Peak demand = 21.6 − 6.9 + 41 ≈ 56 kVA.
Headroom of 25% on the running load: 21.6 × 1.25 ≈ 27 kVA. Here the starting surge (56) governs, not the headroom (27) — so the next standard class is 60 kVA.
A tower crane on VFD drives, six cabins, three welders, a rebar bender, lighting towers, and a pump.
Demand factor 0.65 (the crane has an intermittent duty cycle and the welders do not all strike at once): 129 × 0.65 ≈ 84 kVA of running load.
The crane's VFDs ramp up gently, so the largest direct-on-line motor is the rebar bender at 7.5 kW ≈ 9.4 kVA. At 6×: ≈ 56 kVA momentarily. Peak demand = 84 − 9.4 + 56 ≈ 131 kVA.
Headroom of 25%: 84 × 1.25 = 105 kVA. The 131 kVA peak governs — the next standard class is a 150 kVA prime set, loaded at about 56% while running, which is a healthy band.
Had the same crane been started direct-on-line instead of on VFDs: 45 kW ≈ 56 kVA running, and at 6× that is close to 337 kVA momentarily — pushing you toward a 400 kVA class or a staged start sequence. That is the practical difference between renting in the 150 class and renting in the 400 class.
Price the transport before you lock the size — larger sets need heavier haulage and wider site access. Transport calculator
Typical planning figures only, for a first-pass estimate. Real values vary by manufacturer, model, and age — always verify against your actual equipment nameplates.
Look at the last column: a relatively large welder is barely a starting problem, while a single 15 kW air compressor started direct-on-line can dictate a larger class all by itself.
Generator sets are rated at reference conditions defined in ISO 8528 / ISO 3046 — roughly 25°C, near sea level. A Riyadh or Jubail summer afternoon at 45–50°C is far outside that reference, and both the engine and the alternator lose output. The loss is machine-specific, commonly in the range of 5–15% at those ambients, and altitude adds to it (Taif and Abha sit high enough for altitude derating to matter on top of the heat). Ask the owner for the manufacturer's derate curve for the exact model, and check that the set has a high-ambient radiator. Plan your headroom on the derated figure, not the brochure figure.
Sand and airborne dust are the single biggest cause of shortened service intervals in the Kingdom. Sets working in open desert or near earthmoving typically need pre-cleaners or cyclonic air filters and far more frequent air-filter changes than the manual's default interval. Radiator cores clog with dust and lose cooling capacity, which compounds the heat derating above. Before you sign, agree in writing who performs servicing, at what interval, and whether filters and consumables are inside the rate or billed separately.
For budgeting, a diesel genset burns roughly 0.20–0.28 litres of fuel per kWh produced. A 150 kVA set running at 75% load is producing about 90 kW, which works out to roughly 20–25 litres per hour, or around 250 litres in a ten-hour shift. Use that only for planning and confirm against the manufacturer's fuel curve for the model on offer. Fuel terms are agreed directly with the owner: most rentals are self-fuelled, some owners offer scheduled delivery or all-inclusive packages on monthly contracts. Settle tank capacity, refuelling access, and bunding before the set arrives.
Open-set and standard canopy generators typically run around 65–75 dB(A) at 7 metres; soundproofed canopy models run around 55–65 dB(A). Near residential neighbourhoods, hospitals, schools, or hotels — and on most urban Riyadh and Jeddah sites — the quieter canopy is worth the premium, and your project specification or municipal permit may set a limit for night work. Position the set away from bedroom-facing façades, keep exhaust outlets pointed away from occupied areas, and if your site requires noise documentation, ask the owner for what they hold before booking.
Grid electricity in the Kingdom is supplied by the Saudi Electricity Company (SEC). Temporary construction supply has to be applied for, and until it is energised most sites run entirely on diesel generation — which is why the sizing exercise above matters so much on early-phase work. If the generator will later back up or transfer with an SEC supply, that changeover needs a properly specified transfer switch or interlock, and the arrangement is the responsibility of your licensed electrical contractor and consultant. RentYourMachine does not certify equipment, installations, or connections: owners supply the documents they hold for their machine, and we collect them before a rental is confirmed.
A diesel engine is designed to work under load. Run one for long stretches below about 30% of its rating and the combustion chamber never reaches operating temperature, so unburnt fuel passes into the exhaust — wet stacking: black soot, deposits on valves and turbo, glazed bores, and rising oil consumption. The result is premature servicing and possibly a deduction from your deposit.
Higher fuel burn at part load
Bigger transport and handling cost
More site footprint and access needed
Risk of service charges from carbon build-up
An undersized generator does not fail quietly. The first big motor to start drags the voltage down, contactors drop out, control systems reboot, and a pour or a lift stops halfway. Over time, overload and under-frequency protection trips repeatedly, sensitive loads suffer from unstable voltage, and you end up swapping the set — paying a second transport charge and losing a day.
Voltage dip on every motor start
Protection trips and sudden stoppages
Unstable welding arc and poorer quality
Stress on the alternator and control panel
A second transport charge and a lost day
The practical rule: target an average loading of 50–70% of the prime rating after heat derating, and make sure the set can absorb your largest starting step. If you fall between two classes, consider a soft starter or a staged start sequence before jumping straight to the bigger class.
Classes, specs, and market rates
Browse listed units
What builds a final quote
Estimate the cost of getting it there
The same method applied to excavators
When each option makes sense
Send your largest motor and how it starts, your city, and the rental duration — and we'll source a real quote from RentYourMachine and partner suppliers. Delivery is arranged with the owner, and documents are collected before the rental is confirmed.
Add up the nameplate ratings of everything that will be connected, apply a demand factor of about 0.5–0.8 depending on how much runs at once, convert kW to kVA by dividing by a power factor of about 0.8, add 20–25% headroom, then check that the set can also absorb the starting surge of your largest motor. Round up to the next standard class. A small villa site often lands between 40 and 60 kVA; a mid-rise site with a tower crane, site offices, and welding bays usually lands between 150 and 250 kVA.
kW is real power — the work actually done. kVA is apparent power — what the alternator and cables have to carry. They are linked by power factor: kVA = kW ÷ PF. Diesel generator sets are almost always rated at a power factor of 0.8, so a 100 kVA set delivers 80 kW. Generators are sold in kVA because the alternator is limited by current, not by useful work, and an inductive load draws more current for the same output.
A motor started direct-on-line draws about 6–7 times its running current for a few seconds at a poor power factor. On a grid connection that surge is absorbed by the network; on a generator it has to come from one engine and one alternator. If the set is too small the voltage dips, contactors drop out, protection trips, and the site stops. Soft starters typically cut the surge to around 2–3 times running current and variable frequency drives reduce it further, which often lets you rent a smaller, cheaper set.
Generator sets are rated at ISO reference conditions of roughly 25°C near sea level, so a 45–50°C site is well outside the reference and both engine and alternator lose output. The exact loss is model-specific and commonly falls in the 5–15% range at those temperatures, with additional altitude derating in higher cities such as Taif and Abha. Ask the owner for the manufacturer's derate curve for the specific model and confirm the set has a high-ambient radiator, then base your headroom on the derated figure.
Two smaller sets give you redundancy and let you shut one down during light periods so the running set stays in a healthy load band — useful when day and night loads differ sharply, or when a stoppage is expensive. One large set is simpler to fuel, connect, and supervise, and usually costs less per kVA. If you split the load, either keep the circuits separate or use sets and controllers designed for paralleling; do not connect two generators together without a proper synchronising panel.
Send the kVA class you calculated, the largest motor and how it is started (direct-on-line, soft starter, or VFD), the city and site address, the rental duration, whether you need a soundproofed canopy, the voltage and phase you need, and who will supply fuel. That is enough for an owner to come back with a real quote instead of a range. Delivery is arranged with the owner when you book, and the documents the owner holds for the machine are collected before the rental is confirmed.