150 kVA isn’t 150 kW: How to choose a diesel generator for your site

A contractor buys a 100 kVA diesel generator for a UAE construction site. The site offices, lights and workshop tools work normally. Then a large water pump starts, the voltage drops and some equipment shuts down.

The running load was only part of the decision. The pump needed much more current for a short time, and the buyer had not checked whether the generator could handle that start.

The opposite mistake also costs money: buying a much larger set that spends most of its working life at a very light load. The right choice comes from three checks: real electrical output, operating duty and motor-starting demand. Here is how to apply them before buying a used diesel generator.

kVA vs kW: what 150 kVA really delivers

kVA describes the apparent electrical power carried by the generator. kW describes real power used by the load. The relationship between them depends on power factor (PF). Most three-phase diesel generator ratings use 0.8 PF, but the actual site load may have a different power factor.

150 kVA × 0.8 = 120 kW

At its stated 0.8 power factor, a 150 kVA set rated for 120 kW has two limits: the engine must produce enough real power, and the alternator must carry the electrical current. A load with a lower PF can reach the alternator’s kVA limit before reaching 120 kW.

For a simplified 150 kVA / 120 kW set, the limits look like this:

This is why a buyer should ask for both kVA and kW, plus the duty rating. A generator advertised as 150 kW is not equivalent to one rated at 150 kVA.

The distinction matters with other brands too. The TPC TPC7000 specifications show 175 kW gross engine power, while its generator listing uses a 175 kVA headline. The Fuji Galaxy FD research page similarly shows 110 kW gross engine power. Gross engine power is not the same as verified electrical kW available from the alternator. Confirm the generator-set output before comparing these models.

Prime vs standby ratings: which one should you buy?

A generator providing power for 10 hours every workday faces a different duty from one that starts only when a building loses grid electricity. The same engine and alternator may have different permitted output ratings for these uses.

For the ISO-based Prime definition described in Caterpillar’s generator rating guidance, a set may provide its full Prime output for limited periods, but the average operating load is normally limited to 70% of that Prime rating over the defined averaging period. Manufacturer-specific rules govern.

That distinction has a direct buying consequence. If a generator is genuinely rated at 150 kVA Prime / 120 kW, 70% of its rated real output is 84 kW. A site that needs 85 kW continuously is already slightly above that typical average-load threshold, even before motor starting and local conditions are assessed. It would be misleading to call the same generator comfortably sized just because 85 kW is below 120 kW.

A set advertised at 150 kVA standby may have a lower Prime rating. Never assume that a 150 kVA headline represents 150 kVA Prime. Ask for the exact nameplate, manufacturer data sheet and duty class.

Why a water pump can overload a correctly sized generator

A pump motor draws more current when it starts than while it is running. Direct-on-line (DOL) induction-motor starting current can be several times the normal running current. A motor drawing 50 A in operation might briefly draw about 300 A at startup, depending on its design.

That current surge can cause a voltage dip in the alternator. Lights may flicker, contactors may drop out, and other equipment can stop working. The voltage dip a site can tolerate depends on the connected loads: there is no single percentage at which every system fails.

Operators have several ways to improve starting performance without automatically buying a much larger generator:

·       Start the largest motor first. Keeping cabins and workshop loads off during pump startup reduces the load already on the generator.

·       Use a correctly selected soft starter or variable-frequency drive. These can reduce starting current, but the motor, pump, generator and power quality must be checked together.

·       Consider star-delta starting only when suitable. It lowers initial current but also reduces starting torque substantially; a pump may fail to accelerate, and the change to delta can create another current peak.

·       Check the alternator’s motor-starting curve. Voltage-dip performance, excitation and available engine torque matter more than a simple kVA multiple.

The restrictions on star-delta starting are described in ABB’s Softstarter Handbook. Caterpillar’s load-starting guidance also recommends starting large loads in stages and assessing acceptable voltage dip. The final motor-starting decision should come from the relevant motor and generator data, not a fixed rule of thumb.

100 vs 150 kVA for a job site: a worked example

Consider a UAE contractor running a pump, air-conditioned site cabins, lighting and an electric compressor. The following figures are assumed for illustration, not actual measurements from a specific site. The pump’s 30 kW motor is assumed to be 90% efficient, giving approximately 33 kW of electrical running demand.

The site’s calculated running requirement is 85 kW and approximately 99 kVA. These are separate limits: adding the individual kVA values is more useful here than dividing total kW by an arbitrary 0.8 PF. All loads are assumed to operate together in the table.

A 100 kVA / 80 kW set is too small for this assumed full running load: the 85 kW requirement exceeds its nominal real-power rating even before any motor starts. A 150 kVA / 120 kW set offers more nameplate capacity, with the site using about 71% of rated kW and 66% of rated kVA.

But a 150 kVA Prime set is only a candidate, not an approved final size. If 85 kW is the average demand for most operating hours, it is slightly above the typical 70% average-load limit discussed earlier. If pumps and workshop loads cycle on and off, the actual operating average could be lower. The duty profile must be measured or estimated over time.

Now start the pump. With an illustrative DOL surge of six times its approximately 39 kVA running demand, the pump may require about 230 kVA during startup. The other equipment accounts for around 60 kVA of running demand. Adding those figures gives a rough screening total close to 290 kVA, not a final transient sizing result: motor-start power factors, phase relationships and the alternator’s short-term response matter.

A star-delta starter may lower the initial current, but it does not prove the pump will start or that the 150 kVA machine will ride through the changeover. The contractor should check the pump’s starting torque, consider a suitable starter or drive, arrange a sensible starting sequence and have the supplier assess voltage dip with the generator’s motor-starting data.

Buying decision: Rule out the hypothetical 100 kVA set for the all-on running case. Shortlist the 150 kVA class only after checking the load profile, Prime rating, ambient-temperature capability and motor-start performance. If the duty or starting requirements remain too high, evaluate a larger or differently rated generator rather than making a recommendation from the headline kVA alone.

UAE heat and the cost of buying too big

A generator’s published output depends on its declared reference conditions. Do not assume that every model is rated at 25°C: a Perkins unit represented on Makana, for example, documents its 150 kVA / 120 kW base rating at 40°C ambient. Another generator may have different conditions or derating limits.

If the installation regularly reaches 45°C, the buyer must ask whether the engine, alternator and radiator can deliver the required output there. Cooling airflow, dust-clogged filters and the location of the enclosure can also affect performance. Request the manufacturer’s ambient-temperature rating and derating guidance instead of applying a universal percentage.

Oversizing can be costly as well. A 400 kVA set rated around 320 kW at 0.8 PF would be carrying just 27% of its rated kW if the site used 85 kW. For some diesel generators, prolonged very light loading increases the risk of wet stacking, exhaust deposits and poor fuel use per unit of electricity. The safe operating range is engine-specific; the often-cited 30% threshold is a warning sign, not a rule for every engine.

Instead of adding an automatic 20–25% margin, assess the real load profile, starting peaks, seasonal temperatures, future expansion and manufacturer limits. For sites with large day-to-night load swings, staged operation or multiple appropriately sized sets may be worth evaluating.

Find used diesel generators for sale in UAE on Makana

After calculating the expected running load and starting requirements, shortlist generators by verified electrical kW/kVA, 50 Hz compatibility, duty rating and condition. Makana lists stationary diesel generators for sale as well as stationary generators through its machinery auctions; available stock and auction status change over time.

Listings may include inspection information, operating hours, photographs and operational videos. Review what is actually documented for the machine and arrange a suitable load test where necessary. If no listed generator matches the project’s required duty and electrical configuration, buyers can submit the details through Makana’s machinery procurement service.

The right diesel generator is not simply the one with the largest number on its label. It is the one that can support the site’s actual loads, start its motors and work reliably within its rated conditions.

FAQs about diesel generator sizing

How many kW is a 150 kVA generator?

At a rated power factor of 0.8, 150 kVA corresponds to 120 kW. Check the exact machine’s nameplate and whether that figure is Prime, standby or another duty rating.

How do I convert a kW load to kVA?

Divide the running kW by the load’s actual power factor. For example, 85 kW at 0.85 PF is about 100 kVA. If the PF is unknown, an assumed value can help with early planning, but it cannot replace the actual load data or a motor-starting check before purchase.

What is the difference between Prime and standby generator ratings?

Prime power is intended for regular operation under the manufacturer’s load and operating limits. Standby power is for backup use during outages, often with different permitted output and running-hour limits. Compare equivalent duty ratings when buying.

What size generator do I need to start a water pump?

Use the pump motor’s running demand, starting method, required torque, other simultaneous loads and the generator’s motor-starting performance. A 150 kVA nameplate alone cannot show whether a particular pump will start successfully.

Is a bigger diesel generator always safer?

No. A larger generator may cost more and run inefficiently at a very light load. The better size depends on normal demand, starting peaks, permitted duty, ambient conditions and the manufacturer’s recommended operating range.