Sizing diesel generators is a common problem for construction, water pumping and houses (especially now electric vehicles are entering the market), but sizing the units is a problem.

Too large a generator and you not only use too much fuel but running below about 30-50% for sustained periods “Low-Load” – causes engine problems that aren’t covered under warranty, leading to significant costs. Too small a unit will be insufficient to run the items you wish to power, meaning the unit will as the load increase slow down and potentially stall.

Typically when considering diesel generators people choose an off the shelf solution. In some cases this can be fine, however, in many cases, a better custom made designed package would give a significantly better result, especially lower fuel consumption during operation and lower voltage drops when the load is applied.

The load on a diesel generator is actually divided into two distinct parts. The kW, or Active power and the kVAr, the reactive power. Vector summed these two values add up to the kVA. The kW is concerned only with the engine power.

The kVAr is concerned only with the alternator electrical loading. The relationship between these two is referred to as the power factor. Most industrial generators are rated at 0.8 power factor – but this is the generator’s rating, it is the load that determines the power factor. If the power factor differs from 0.8, you would potentially benefit from having a more custom generator solution to your power needs.

Key Considerations

Unlike the mains utility, which for residential applications and smaller industrial applications has unlimited scope to support the starting of motors and other electrically disruptive equipment, a diesel generator must be properly sized to ensure it meets the required demands of the application.

Firstly it’s important to understand the rating of a generator that you might purchase. Generators are usually sold in kVA or sometimes kW. These are not the same and the relationship between the two is expressed as the power factor, which is typically 0.8, so kVA x pf = kW. So if you have a 100kVA generator, at a 0.8 power factor, it will produce 80kW of electrical power.

In simple terms, the kW portion of the generator is the real work done. This powers your lights, heaters and moves the air from your fans. The difference between the kVA and kW value is “wasted” power, that’s called kVAr. It flows around the circuit but does nothing useful.


Broadly these can be split into two categories – resistive and inductive. Inductive loads cause “real” and  “wasted” part whereas resistive loads cause only the  “real” part.

Resistive Sizing

If you only have resistive loads, then the sizing is easy. You can safely add the ratings of all the items you have and work out how many kW you need. The only thing you really need to do is check the load acceptance of the generator. All generators have a load acceptance which limits the single step load to prevent the engine from slowing down more than 10%. Typically this single step it might be 60% (you would find this on the engine or generator specification sheet), so if you want to apply your full load in one attempt then you need a generator where 60% of its rating represents the load you have just calculated. If you can turn everything on gradually, you won’t need it to be as big, which saves you money.

Inductive Sizing

If you have inductive loads it gets more complicated. You need to be able to calculate how much “real” power ( that’s kW) you need and how much “wasted” (that’s the kVAr) power you will have.

When you have the kW and kVA values, you can use the kW as above. Then with the kVA you need to do a similar calculation to the alternator. With the alternator taking this kVAr (the difference between the kVA and kW), what happens is the voltage drops. This voltage drop will cause light flicker and potentially some equipment to turn off. 30% is considered the maximum, but for many controls, its too much and 15% is a much more reasonable figure again you need to look at the generator or alternator specification sheet.

Speed drop on Alternator Voltage

One final consideration – as the engine speed falls, the alternator voltage also drops a little further, so you need to take this into account as well – back to the alternator specification sheet!


While for small home applications where the outcome is not critical, you can perhaps size your own, it is highly recommended to get a professional to size it for you in most situations.