Sizing tool
Generator kVA Calculator
Enter the loads you expect to run and the calculator returns a recommended rating in kVA. It uses the arithmetic a quotation is actually built on: the connected load, a demand factor for the fact that not everything runs at once, an allowance for the largest motor starting, a power factor of 0.8 and a margin. Nothing is sent anywhere until you press the button.
The result is a starting point for a quotation, not a substitute for one. Altitude, ambient temperature, humidity and the enclosure all reduce what a set can deliver, and a load list with several large motors starting together can move the answer by a whole class.
Step 1
Enter the loads
Put a quantity against each load you expect to run at the same time. Leave the rest at zero. The typical rating column is what the calculator uses for that row – change the quantity rather than the rating, unless you know the real figure, in which case use the last row to add that load with its own kW.
| Load | Typical rating | Quantity | Connected kW |
|---|---|---|---|
| Lighting and general sockets, per 100 m2 | 2.0 kW | 0.0 | |
| Air conditioning, split or window unit | 2.5 kW | 0.0 | |
| Air conditioning, packaged or central, per 10 kW cooling | 3.5 kW | 0.0 | |
| Water pump, up to 3 kW | 2.2 kW | 0.0 | |
| Water pump or process pump, 5 to 15 kW | 11.0 kW | 0.0 | |
| Refrigeration, walk-in or cold room | 3.0 kW | 0.0 | |
| Kitchen, per commercial kitchen | 15.0 kW | 0.0 | |
| Laundry, per line | 25.0 kW | 0.0 | |
| Lift, per lift | 15.0 kW | 0.0 | |
| Air compressor | 15.0 kW | 0.0 | |
| Welding set | 20.0 kW | 0.0 | |
| Telecom or server rack, per rack | 5.0 kW | 0.0 | |
| DC rectifier or power plant, per 100 A at 48 V | 5.0 kW | 0.0 | |
| Machine tool or small motor, up to 7.5 kW | 5.5 kW | 0.0 | |
| Crusher, mill or large process motor | 75.0 kW | 0.0 | |
| Other load, enter the running kW per unit | 0.0 |
Step 2
The four numbers that move the answer
Two of these are engineering judgement rather than measurement, and they are the reason two quotations for the same building can differ by a whole class. The defaults are the ones we use when nothing else is known.
| Item | Value | Why it matters |
|---|---|---|
| The starting current of the largest motor, not the running load, usually decides the rating. | ||
| A direct-on-line motor draws six times its running current for the first seconds. | ||
| Not every connected load is on at the same moment, and a set sized for the connected total is oversized. | ||
| Room for the load you have not been told about, and for a set that will not always be new. |
Result
Recommended rating
| Step | Value |
|---|---|
| Connected load | 0.0 kW |
| After the demand factor | 0.0 kW |
| Starting allowance for the largest motor | 0.0 kW |
| Peak load to be carried | 0.0 kW |
| With the margin, at 0.8 power factor | 0.0 kVA |
| Recommended standard rating | 0 kVA |
The recommendation is rounded up to a standard rating, because generators are built in steps rather than to order. If the figure lands just under 200 kVA, the answer quoted will normally be 200 kVA rather than 190, and that is deliberate: the small step buys back the derating that a 38 °C day takes away.
Questions
Questions about the calculator
Why does the calculator ask for a demand factor?
Because a set sized for the sum of every connected load is a set that will run at a fraction of its rating for most of its life. A hotel laundry, kitchen and pool plant rarely run at full load together; a plant machine tools are intermittent by nature. The demand factor is the engineering judgement that turns a connected list into a running load, and it is why two engineers can price the same site differently.
Why is the largest motor treated separately?
Because a motor starting across the line draws about six times its running current for the first few seconds, and that transient – not the steady load – is usually what decides the rating. A set that carries the running load comfortably can still stall or drop its voltage when a large motor starts, so the starting allowance is added on top of the design load rather than averaged into it.
Does altitude or temperature change the answer?
Yes, and the calculator does not apply either. A diesel engine loses output as ambient temperature rises above about 40 °C and as altitude rises above about 1000 m, and the derating is applied to the set you buy rather than to the load you have. A site at altitude in a hot plant room can need the next class up for the same load: tell us the site conditions and the quotation applies it.
What is the difference between kVA and kW here?
kW is real power and kVA is apparent power; they are related by the power factor, which for a generator set is normally 0.8. A 500 kVA set delivers 400 kW at 0.8 power factor. The calculator works in kW because loads are rated in kW, and converts to kVA at 0.8 at the end, which is why the recommended rating is always larger than the peak load in kW.
Related
Related pages
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Diesel generator sets 20 kVA to 2000 kVA
How each range is used and what decides the rating.
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How to size a diesel generator
The longer version of the arithmetic on this page, with worked examples.
-
Prime and standby ratings
Why the same machine carries two different numbers.
-
Silent sets
What an acoustic canopy adds to the footprint and the cost.
-
Philippines
60 Hz, 220/440 V and the conditions that change the build.
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Request a quote
Send the load list and the site conditions, and we size the set.
Have the load list but not the rating?
Send the list on WhatsApp or through the form and we will come back with the rating, the configuration and the price.