Virtually all household in the nation has become a generating station in terms of electricity. Some generate electricity by the use of PV (Photo-voltaic) panels popularly called solar panels while the most common means of generating electricity you see around is by generator engine set (popularly called GEN in this part of the world).

Our focus for today is solely based on generators.

Although, there are different types of generating set, the most commonly used in households and small industries today are petrol engine generators, gas engine generators and diesel engine generators.

Are they properly sized? NO!!! They are either over-sized or under-sized.

This post is a guide to sizing your generator which does not take the place of consulting a certified personnel such as ours, but provides you enough information required for a good purchase or help you check if your generator is over-loaded or under-loaded.

Examples of Power Engine Generating Set


All generators are rated in terms of voltage, power, frequency, power factor, efficiency and so on, to say the least.

You would not want to buy a generator whose capacity is smaller than your requirements (in terms of loads to be powered) or buy a very large capacity generator and at the end of the day you spend more on fueling the generator with a lot of the energy wasted which is not in any way economically.

Aside from the above, having an under-sized generator is detrimental not just to your generator but to the equipment being powered but an over-sized generator is detrimental to your generator only, based on short life span and maintenance intervals.

So you could say, having more power is better than less.

However, for domestic use, the following rating should be considered as a minimum but not limited, in choosing the right Generator engine set;

Voltage: The rated voltage of your gen should align with your country power supply. For a country like ours, Nigeria, 230 V is the supply voltage to consumers. Therefore, your generator rated voltage should be 230 V with a tolerance of plus or minus 5%. That is, a range of 220-240 V from the output of your generator.

Frequency: Also, the required frequency differs from nation to nation but for Nigeria, it is 50 Hz and the allowable tolerance is plus or minus 2% (49-51 Hz). Other countries, like US, works on 60 Hz. Therefore, rated frequency of the generating set should be 50 Hz for people based in Nigeria. This rating should be clearly marked on the generator tag.

Power: Majorly, generators are rated in kVA (which is called ‘apparent power’) and not kW (called active power or real power). The power rating differs from one generator to the other based on capacity. Your load requirements will determine your choice of the power rating as we shall soon see.


In order to size your generator properly, to suit your needs and wants, there are few things to consider, though not limited to the following but for the purpose of this post, will be sufficient for a good purchase;

  1. Know your load requirements-in order to accomplish this, you need to make a list of all the loads that will be powered by your generator.  For example, six (6) lighting bulbs, two (2) ceiling fans, one (1) plasma TV etc.
  2. Calculate the total power requirement-After making a list of your load requirements, check to confirm the power rating of your equipment. You can find the rating on the name tag of the equipment or at the back, imprinted on the equipment. Then, make a summation of all the power rating. Let’s assume the lighting bulb is a 60W bulb, the fan is 80W and the TV is 250W. Your calculation should somewhat look like this:

Lighting bulbs = 6 x 60 W

   = 360 W

Ceiling fans = 2 x 80 W

             = 160 W

Plasma TV = 250 W

∴ Total power required = 360W + 160 W + 250 W

   = 770 W.

There might be a need to add more loads in the future, so it’s better to cater for such loads while purchasing a new generator engine set by making an allowance.

Allowance for spare = 20%

            = 1.2 x 770 W

∴   Power required = 924 W

Over time, the efficiency of the generator reduces based on usage. You may have one time experienced that your generator’s capacity has depreciated and the loads powered by your generator at inception can no longer be powered by the same generator. In order to avoid this, there is a need to consider an ageing factor for the generator.

In this regard, a 15% ageing factor is considered.


Total capacity of generator required = 1.15 x 924 W

     = 1063 W

Since, generators are rated in kVA and not kW, we will have to convert our answer to kVA.

Let’s assume a power factor of 0.8


Total kVA = Power rating (kW) / (power factor x 1000)

   = 1063 W / (0.8 x 1000)

   = 1.328 kVA

1.328 kVA is the generator size you need but I can assure you that you won’t get this particular size. The standard practice is to choose the next standard rating of the generator you want to purchase.

In this case, a 1.5 kVA should be your desired generator size. With this you can be rest assured that your generator is sufficient to power your load requirements.

It is to be noted that the above calculations did not consider and did not cover starting current of inductive loads such as motors for your pumping machine and so on.