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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

ApplianceMinimumMaximum
25" Colour TV 150W 150W
Clothes Dryer 2000W 4000W
Desktop Computer 50W 150W
Dishwasher 1200W 1500W
Electric Blanket 200W 200W
Electric Kettle 2000W 2000W
Electric Mower 1500W 1500W
Electric Shaver 15W 20W
Fridge / Freezer 200W 500W
Hair Blowdryer 700W 1200W
Iron 1000W 1000W
Laptop Computer 20W 50W
Lawnmower 1000W 1400W
Microwave 600W 1500W
Oven 2150W 3500W
Power Shower 240W 240W
Toaster 800W 1500W
Vacuum Cleaner 200W 700W
Washing Machine 500W 3500W
Wedding DJ Sound 2000w 6000W
Wedding DJ Lights 2000w 6000w
Power Tools 1000w 3000w
Industrial Coffee Machine 2000w 3500w
Fan Heater 1000w 2200w

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.