SECONDHAND GENERATORS

How to work out the size of Generator required

Generators come in a vast range of sizes; from small hand held generators used to power small devices, to huge generators to provide back up power for hospitals. Generally they are given a power rating in kVA (Kilovolt-amperes), Kw (Kilowatts) or BHP (Brake Horsepower). Which is very confusing to most people.

The first thing to understand is that all three measurements are expressing the same thing, the amount of power your generator can provide. You can use simple calculations to convert between all three and more usefully into the number of Amps your generator can supply at a given voltage.

Volts, Amps and Watts

Before I start with the maths it's best to understand a little about the common terms used to describe electricity.

A neat analogy to help understand these terms is a system of water pipe. The voltage is equivalent to the water pressure, the current (amps) is equivalent to the flow rate. Watts is the amount of water which, can be calculated by multiplying the water pressure (V) with flow rate (A).
For example most household electrical equipment uses 230v. If you know that a devise uses like a kettle uses 9 amps then you can calculate that 230V x 9A = 2070w or 2.07Kw.

Watts = Volts x Amps
Amps = Watts / Volts

To convert kVA to Amps find out how many volts your equipment uses (normally 230v for mains power). You cannot convert kVA to amps unless you know the volts.

Divide the number of kVA by the number of volts. For example, 10 kVA divided by 230 volts equals 0.043. This is the number of kiloamps your appliance uses.

Multiply your number by 1000 to find the number of amps. For example, 0.043 multiplied by 1000 equals 43 amps. To convert Kw to Hp 1kw = 1.34 hp

How much electricity will I need?

You will have a decision to make are you going to buy a generator which can provide enough power for every device you would like run all at once or are you going to accept that you will only use one power hungry activity at a time.

Most devices use more power when they start then reduce when the have began working. Sometimes you can schedule activities so power consumption is kept to a minimum. Also by choosing alternative fuels such as gas for heating or cooking this can reduce peak power.

This list is not definitive but will give an indication of how much power may be required.

Lighting 10x 15 watt energy saving light bulbs (equivalent of the old 70 watt) = 150 watts

Simply total up how many watts you think you require then divide by 1000 to give the size of generator in Kva.

Generators don’t like to be run at full power for long, so I would suggest a minimum of 25% spare capacity to 50% depending on how long the generator was going to be operational.