Domestic or business?
What type of meter do you have?
Number of units used:
Your gas usage in kWh:
When checking your energy bills, or doing a energy price comparison, you need to know how much energy you use.
In the UK, units of energy, whether gas or electricity, are priced in kilowatts hours (kWh).
Electricity meters measure units of electricity consumed in kilowatt hours (a measure of electrical energy). This is great because it means that you can simply pop the readings from your meter, or your energy bill / statement, directly into a energy comparison website like TheEnergyShop.com to check your best deals (or into a electricity cost calculator if you are checking the accuracy of your electricity bills).
With gas however it is not so simple. That is because gas meters measure the volume of gas delivered to your meter (and not the energy content directly). This volume then needs to be converted into the equivalent kilowatt hours in order that a kWh price per unit can be applied to the energy used for the purpose of generating the bill (or quote).
A further complication is that not all meters measure volume in the same units.
When it comes to comparing gas prices and measuring gas volumes, there are basically 2 types of gas meter in the UK - imperial and metric.
Metric gas meters were introduced from 1995 and all new gas meters are metric. They measure gas volume in cubic meters (m 3). An example of a metric gas meter is shown below. Metric gas meters will display the units M3 next to the reading on the meter.
Luckily, these are being phased out and, with the move to smart metering, should all be gone by 2022. However, a significant number are still in existence. These meters measures volume in cubic feet (ft 3). They have the words "cubic feet" or the letters ft 3 shown on the front of the meter, and will typically have revolving dials showing the reading.
Once you've identified the type of meter you have and taken a reading, follow the steps below to convert from gas units into kilowatt hours (kWh).
In this part we will take you through the conversion in stages detailing why each step is necessary. If you are not too fussed about the detail, then just scroll to the bottom of the page where we have simplified it significantly.
The detail of how the gas calculations need to be done are detailed in a piece of legislation called the The Gas (Calculation of Thermal Energy) Regulations 1996. If you really want to, you can read it here.
Take a meter reading.
Not required if you are taking a reading from a gas bill.
Subtract the new meter reading from the previous reading to work out the volume of gas used.
This step only applies to imperial meters.
Convert from cubic feet to cubic meters.
Cubic feet and cubic meter are both measures of volume. However, since we will ultimately be expressing units in metric form we need to get the units into metric form.
1 cubic foot = 0.0283 cubic meters. So, either multiply by 0.0283 OR divide by 35.315.
As we know from GSCE Physics (or is it Chemistry?) (Boyle's Law) the volume of a gas is dependent upon pressure and temperature and these vary from location to location. Therefore the volume of gas measured at the meter is adjusted by multiplying by a correction factor to take account of the temperature and atmospheric conditions at a particular site. That correction factor is typically 1.02264 unless your property has unusual atmospheric conditions. The correction factor used will be shown on your gas bill.
Now that we have a corrected volume in metric terms we can begin the conversion from volume to thermal energy. The first step used is to convert the volume into a calorific value.
Calorific Value (CV) is a measure of the heating power of the gas and depends upon the composition of the gas (all gas is not the same). The CV refers to the amount of energy released when a given volume of gas is completely combusted under specified conditions. The CV of the gas is measured at standard conditions of temperature and pressure and is usually quoted in Megajoules per cubic metre (MJ/m 3). In the National Grid the CV ranges from 37.5 to 43.0 MJ/ 3. Not all gas has the same CV so National Grid measures it across 110 different locations across the UK and passes it to energy suppliers for billing purposes. For the purposes of this calculation an average value of 40 will give you an accurate result but check your bill to see if a different number has been used.
If you'd like to learn more about calorific values you can find information here.
The final step is to take thermal energy measured in Mega joules per cubic metre (MJ/m 3) and convert it into kilowatts per hour.
1 Watt is a derived unit of power defined as 1 Joule per second.
1 W = 1 J/s
1 Ws = 1 J
1 kWh = 1 * 1000 * 60 * 60 J
1 kWh = 3.6 MJ
So the final step is to divide by 3.6 to convert MJ into kWh
This is the easy bit without the detailed explanation.
The multiplication/division steps can be combined them into a single step. This simplification assumes that the volume correction factors and calorific values for your property are constant at 1.02264 and 40 respectively. This reduces the calculation as follows.