Comparing Gas and Electricity

Traditionally gas has been cheaper than electricity but with the price of gas rising this has changed.

Actually comparing value for money is quite complicated so I though I would explain how to do the various calculations.

Cubic Metres to Megajoules

Gas is measured at the meter in cubic metres but you pay by the megajoule (MJ) To convert cubic metres of gas to megajoules you have to multiply by

  • The Heating value (*Typically around 38.7)


  • A Pressure Factor (*Typically around 1.01)

*check a recent gas bill for the actual values

The value you have calculated is the Megajoules input into your heating system. The actual output can be quite a lot less.

Gas Efficiency

To find the heat output from your heating unit you need to find the actual efficiency of the unit, which can very considerably.

No gas heater has 100% efficiency as some heat is always lost up the flue.

A basic gas heater (3 Star) may have an efficiency of around 60% while some of the 6 star units have an efficiency closer to 95%.

Once you know the efficiency you can just multiply the Megajoule input by the efficiency to get the Megajoule output/Cubic Metre

Megajoules to Kilowatt Hours

1 MJ = 0.278 kilowatt-hours (kw-hr)

Electrical Efficiency

There are two different ways of using electricity to heat your new home.

Direct, or conventional, heating or using the power to run a Heat Pump, or Reverse cycle air conditioner.

The method you choose can make a dramatic difference to the heating efficiency.

Conventional Electrical Heating

With conventional Electric heating, such as panel heaters or fan heaters things are much more straightforward.

Every kw into your house means one kw output (100% efficiency)

That doesn’t mean that overall electricity is 100% efficient, just the inefficiencies occur before the power goes through the meter.

Heat Pumps and Reverse Cycle Air Conditioning

An advantage of Heat Pumps/Reverse Cycle Air conditioning is they have a positive efficiency, they can put out more kw of Heat than the electricity that goes in.

They do this by extracting the heat out of the outside air.

Pick a good unit and you might find them 200% or even 300% efficient for heating!