Build a load list, tell us how your motors start, and get the generator set size your site actually needs — sized against the three limits that matter: steady load, transient voltage dip, and the engine's ability to accept the step.
Tap the things you need below. They get added to your list, and you can change the numbers afterwards. Do not worry about being exact — you can adjust anything later.
If you are not sure, leave the first option selected. It is the most common one, and it is the safest guess because it asks the most from the generator.
Two quick questions. The answers already selected suit most sites in Iraq, so if you are unsure you can simply carry on.
Defaults suit a typical turbocharged industrial genset on a 400 V / 50 Hz Iraqi site. Replace them with the figures from your supplier's datasheet when you have them.
This is the generator size your list needs. Send it to us for a price, or go back and change anything to see how the number moves.
This calculator gives an indicative size for budgeting and shortlisting. It is not a design document. Generator, alternator and protection selection must be confirmed by a qualified electrical engineer against the manufacturer's derating curves, the actual load characteristics, and the wiring rules that apply at your site. Mega Standard accepts no liability for decisions made on these figures alone.
Where the calculator interpolates or simplifies a published table, it is stated in the method notes below. Manufacturer data always takes precedence.
Most undersized generators on site were sized by adding up the kilowatts on the nameplates and adding twenty percent. That method fails because a generator is not limited by one thing — it is limited by three, and any one of them can be the binding constraint.
A generator set has to do three separate jobs. It has to carry the steady load indefinitely without overheating. It has to hold the voltage up when a large motor starts and demands six times its running current. And its engine has to absorb a sudden block of load without the frequency sagging far enough to drop out contactors. This calculator checks all three and shows you which one decided the answer, because that tells you what to change if the number comes back higher than your budget.
Every sizing exercise is only as good as its load list. Two things are routinely got wrong here.
Motor nameplates state shaft output, not electrical input. An 11 kW motor at 90 % efficiency draws 12.2 kW from the supply, and at 0.85 power factor that is 14.4 kVA. Adding the nameplate figure straight into the total quietly loses that difference on every motor in the list. This calculator converts shaft output to electrical input using the efficiency you set.
Demand factor is not optional padding. If you have six air conditioners but the building only ever runs four at once, enter a demand factor rather than six units at 100 %. Sizing for a load that never occurs is how sites end up with a set that spends its life at 25 % load, glazing its bores.
Apparent power does not add arithmetically. A 10 kVA load at 0.5 power factor and a 10 kVA load at 1.0 power factor do not make 20 kVA — they make about 18.0 kVA, because their reactive components are not in phase with each other. The calculator sums active and reactive power separately and recombines them:
P_total = Σ (P_load × qty × demand factor) Q_total = Σ (Q_load × qty × demand factor) S_total = √(P_total² + Q_total²) PF_avg = P_total ÷ S_total
On a mixed industrial load this is usually worth 3–8 % against a naive kVA sum. On a load list dominated by welding sets and rectifiers, it is worth considerably more.
An induction motor started direct on line draws roughly six to seven times its rated current for the first few seconds, at a power factor of around 0.3. That current is almost entirely reactive, and reactive current is exactly what makes an alternator's terminal voltage collapse.
The calculator assumes the worst realistic case: every other load is already running, and then the single largest motor starts. That is what happens at 06:00 on a Sunday when the plant comes up in sequence.
| Starting method | Starting current | Starting torque | Where it fits |
|---|---|---|---|
| Direct on line | 6–7 × In | 100 % | Small motors, stiff supplies. Brutal on a generator. |
| Star-delta | ~2.3 × In | 33 % | Cheap and effective, but only for loads that start unloaded — fans, unloaded compressors. |
| Soft starter | ~3 × In | Adjustable | Pumps, conveyors. Controlled ramp, no torque step. |
| Autotransformer | ~3.2 × In | 42 % at 65 % tap | Large motors needing more starting torque than star-delta gives. |
| Variable frequency drive | ~1.1 × In | Full, from zero speed | The kindest option for a generator, and the most expensive. |
The practical consequence is worth stating plainly: changing one large motor from direct-on-line to star-delta will often let you buy a generator one or two frame sizes smaller. The starter costs a fraction of the difference. If this calculator tells you the voltage dip is the governing constraint, that is the first thing to look at.
Voltage dip is set by the alternator's sub-transient reactance, X"d. When a starting load of S_start is applied, the dip approximates to:
ΔU ≈ (X"d × S_start) ÷ (S_gen + X"d × S_start)
ISO 8528-5 performance class G2 permits a transient deviation of −15 %, and class G3 tightens it to −10 %. Those are the numbers to hold your supplier to. Note that ordinary contactors start dropping out somewhere around −20 % to −30 %, so a dip that merely looks ugly on paper can actually trip your plant.
Load acceptance is an engine limit, not an alternator one. When a block of real power appears, the engine has to find the fuel and air for it before the governor loses speed. A naturally aspirated diesel can usually take close to 100 % of its rating in one step. A turbocharged engine has to spool the turbo first, and is typically limited to 60–70 % for class G2. This is why an oversized alternator on an undersized engine does not solve a starting problem.
Published generator ratings are quoted at reference conditions. ISO 3046-1 sets those at 25 °C and 100 kPa; most manufacturers publish site ratings at 40 °C. Above that, output falls away as intake air gets thinner and hotter.
The rules of thumb this calculator applies are about 2 % loss per 5 °C above 40 °C, and about 1 % per 100 m above 1000 m altitude.
For an Iraqi site this is not a rounding error. Kirkuk, Baghdad and Basra all run past 48 °C in July and August. A set specified at 40 °C and installed in a poorly ventilated block room can lose 5–8 % of its output at exactly the moment every air conditioner in the building is running. Size for the hottest day, not the annual average, and pay attention to the radiator discharge path — a generator recirculating its own hot air is derated far more severely than any table predicts.
Take the default load list on this page: two 7.5 kW submersible pumps starting direct on line, six 2.2 kW split air conditioners, a 5 kW lighting circuit and 8 kW of general small power.
The electrical input works out at roughly 39 kW and 24 kVAr, so about 46 kVA running at a power factor near 0.85. On steady load alone, a 60 kVA set at 80 % loading would cover it.
But one 7.5 kW pump starting direct on line pulls around 57 kVA on its own, on top of everything already running. Hold the dip to 15 % with a typical X"d of 0.13 and the alternator needs considerably more capacity than the steady load suggests — which is why the calculator returns a larger set, and flags the voltage dip as the governing constraint.
Fit soft starters to those two pumps and the answer drops. That is the trade this tool exists to make visible.
There is no shortcut that does not involve listing your loads. The size depends on the total running load, on how the largest motor starts, and on the ambient temperature at your site. Enter your load list above and the calculator will show you the required kVA together with which of the three constraints determined it.
kW is the real power that does the work. kVA is the apparent power the generator must actually supply, which is larger whenever the load is not purely resistive. Generators are rated in kVA at a stated power factor, almost always 0.8 — so a 100 kVA set is an 80 kW set. Your alternator is sized by the kVA and your engine by the kW, and either can be the limit.
Standby (ESP) is for a set that runs only when the utility fails, roughly up to 200 hours a year, with no overload capability. Prime (PRP) is for unlimited running hours at variable load, provided the 24-hour average stays at or below 70 % of the rating. In most of Iraq, where the generator carries the site for many hours every day, PRP or continuous rating is the honest choice — a standby-rated set run as a prime mover will not last.
Roughly 2 % of output for every 5 °C above 40 °C, so a site at 50 °C loses about 4 % before you account for anything else. Poor ventilation compounds it badly: if hot radiator air recirculates into the intake, the effective intake temperature can be well above the outdoor shade temperature, and the losses climb accordingly.
Sometimes, but it is the most expensive way to do it. Direct-on-line starting draws six to seven times rated current at a very poor power factor, and sizing the generator to hold voltage through that surge can easily cost more than a soft starter or a star-delta panel would. Use the calculator to compare — change the starting method on your largest motor and watch the required kVA move.
No. It gives you a defensible number for budgeting, for shortlisting suppliers and for checking a quotation that looks wrong. Final selection needs the actual alternator and engine datasheets, the real derating curves, a proper look at the harmonic content of your load, and compliance with the wiring rules in force at your site. That is a qualified engineer's job.
Mega Standard has supplied industrial power and electrical infrastructure across Iraq since 2012, from our base in Kirkuk. Send us the result from this calculator and we will come back with a specification, a price and a lead time — including the transfer switch, protection and cabling, not just the machine.