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Wattage Calculator for Generator


After buying a generator, it is very easy to come across the power rating of the said generator during an inspection. For most people, they wouldn’t bother with the mathematical aspect of the power rating as long as the generator they bought is of good quality and performs to their satisfaction. But for the curious individuals, the promise of expected performance is not enough to satisfy them unless they do their own calculations to verify that indeed what they bought works as promised.

The easiest way is usually through the wattage calculator for a generator that can be found online where all you need to do is put in the values given by your generator. The other is by manually calculating it yourself.

What is wattage?

It is derived from the unit watts, which is a unit of power, which is the amount of energy required to do 1 joule of work per second. The simplest and direct way to calculate the wattage of a generator is through the well-known formula:

Power (P) = Voltage (V) x Current (I)

However, this formula works only in a system that is 100% efficient, which is only theoretical and impossible to achieve in today’s engineering. Due to this, there is the introduction of the power factor.

Power factor (pf)

This is the measure of how effective a system can convert volt-amperes to watts. This effectiveness is a dimensionless number that ranges between 0 and 1. If a system’s power factor is closer to 1, then the better the ability for this system to convert volt-amperes to power will be while a system’s power factor is closer to 0, then the poor the ability for the system to convert the volt-amperes to power will be.

Knowing the power factor of your generator will be very important if you want to know the real power consumption of the generator because this is what will be directly converted into the cost of operating the generator.


In theory, finding the wattage of your generator will be:

Power (P) = Voltage (V) x Current (I),


P is the power in watts

V is the electrical pressure in volts

I is the current

This formula works only for a system that is 100% efficient, which is referred to as real or actual power. But because we are dealing with a system that is not 100% efficient, then we will have to find the apparent power. The correct formula will be:

Watts (W) = Amps (A) x Volts (V) x Power Factor (pf)

How to calculate

While having the correct formula is a big step in determining the apparent power, there is one thing you need to take note before you start; the compliance label. The compliance label most often than not tend to show the maximum power instead of the actual power.

To compensate for this, you need to use a power meter. This is because of two reasons:

To get the actual power

To get the power factor

Once you have the correct measurement of these units, then you can calculate the wattage.


You buy a generator, and the compliance label is listed as 240V, and the amperes is 0.5A. In theory, your generator should be generating the following power:

Power (P) = Voltage (V) x Current (I)

P = 240V x 0.5 A

P = 120W

But using the power meter, you get a different measurement. A voltage of 230V and a current of 0.5A, but you also get the power factor as 0.6. The power, in this case, will be:

Watts (W) = Amps (A) x Volts (V) x Power Factor (pf)

W = 0.5A x 230V x 0.6

W = 69W

Understanding the difference between true power and apparent power is very important if you want to get the correct measurement of the amount of power your generator produces. This also important in calculating actual costs for operating your generator.