# If you are going to analyse your gas and electricity consumption, you will need regular (preferably weekly) data in kilowatt hours (kWh).

• Utility bills provide quarterly data - usually quarterly and in kilowatt hours - but some bills may only give estimates which will skew any analysis.
• Meter reading - preferably weekly - but not all meters are the same.

The following notes may not apply to all meters - other types may exist

Electricity meters: These usually read directly in units of kilowatt-hours (kWh) so no conversion is necessary.

Gas meters:  These don’t usually read energy directly, but measure the volume of gas going through. Most of them use units of cubic metres (m3) although older ones are in units of one hundred cubic feet.

To convert from volume of gas to energy you need to know two things (which are usually shown on the utility company bill) which are:

1. Calorific value. This tells you how many Joules are generated by burning one unit of gas. In the UK the current value is around 38.7 million joules per cubic metre. It varies from time to time so ideally you copy down the relevant value from a utility bill. Failing that use the value given here, it won’t lead to a big error.

2. Volume correction factor. This is a number very close to 1 which allows for the fact that the density of the gas in the pipes varies with the seasons (due to temperature changes?)

Analysing a gas bill - see below  - assumed to be in cubic metres:

Let’s say we want to find how much energy was used in the period from 01/07/2010 to 22/07/2011 (approx. one year). It’s good from the point of view of accuracy that these are not estimated readings.

• To find the number of kWh consumed over a period between the two meter readings, subtract the earlier reading from the later one, this gives the number of cubic metres used (5213-5018=195).
• To convert to mega joules (1 million joules) multiply by 38.7 (we’ll assume the volume correction factor is exactly 1, it won’t make a significant error). So the amount of gas energy supplied is 195x38.7 = 7546.5 megajoules.
• To convert megajoules to kWh, you need to know that a watt is 1 joule per second; a kilowatt is 1000joules per second and a kilowatt hour is 3600x1000 joules (= 3.6 megajoules).
• To convert from megajoules to kilowatt hours just divide by 3.6. So the final answer for the energy consumption between 01/07/2010 to 22/07/2011 is 7546.5/3.6 = 2096 kWh.

Units - gas and electricity data