Gas calculation

British Gas could clarify their explanation of gas used as follows:

"Imperial units used" are 100 cubic feet units = 2.83165 cubic metres of gas which expands by a factor of 1.0226400 when the pressure at which it is measured drops to atmospheric pressure, at which 1 cubic metre has a mass of 0.74 kg.

The Heat of combustion [not"calorific value"] of natural gas is 53140 kJ/kg (kilojoules per kilogram) or 53140 multiplied by 0.74 = 39324 kJ/m3 (kilojoules per cubic metre).

3.6MJ = 1kWh

1W = 1J/s (1 watt = 1 joule per second). There are 3600 seconds in 1 hour.

1 kWh = 1000 W for 3600 seconds = 3600 kJ (kilojoules)

So, 1 imperial unit of gas (*100) (100 ft3/unit)

= 100 ft3 (/35.315) (ft3/m3) [change to exact] 1” = 25.4mm, 1ft = 304.8mm = 0.3048 m exactly

= 2.83166 m3 (*1.02264) (expansion as gas pressure in pipe drops to standard measuring pressure in meter)

= 2.89577 m3 (*39323.7) (kJ/m3)

= 113872 kJ (/3600) (kJ/kWh)

= 31.6312 kWh

* Natural gas is mainly methane with small quantities of ethane, nitrogen, propane, buthane & carbon dioxide.

1 kg of methane gas = 0.5076 therms = 50760 Btu = 14.58 kWh = 53.45 MJ = 1.403 m³

1 unit of electricity = 1 kWh (kilowatt hour) = 3600 kJ (kilojoule) = energy required to heat 10 litres of water from 14°C to 100°C. i.e. 1 kW will heat 1 litre of water from 14°C to 100°C in 6 minutes, assuming no losses. A 3 kW kettle will boil a litre of water in slightly more than 2 minutes.