Efficiency can be seen as the amount of heat we manage to get into the building for each unit of heat liberated by burning the fuel. With a coal fire it is fairly simple - burn enough coal to liberate one kilowatt of heat, and you get one kilowatt of heat. But wait a minute! Some of that heat went up the chimney. In fact about half of it went up the chimney. That's with a good stove!.
What if we switched on our electric fire. Why, then all the electricity is converted into heat in the fire, that is 100% efficient. But think again! Coal was burnt that in a power station to generate the electricity. Or maybe it was oil or gas, but it is the same idea. How much coal to generate our one kilowatt of electricity? The answer is that 2kWh has to be liberated by the coal in order to get 1kWh at the electric fire. That is if the generation and distribution losses are 50%. This is a good figure. Many older power stations operate well below that figure, but with modern gas turbine generators, a generation + distribution loss of 50% is achievable.
We say it requires 2kWh of primary energy to provide 1kWh of heat in the home.
Which Heating Method will use LEAST Primary Fuel?
For the same heating requirement of 15kW the boiler consumes gas. The gas, when burning, delivers 17.5kW of heat. It uses 17.5kW of primary energy. That is energy obtained from burning fuel. The remainder (2.5kW) goes up the flue. That is for an 85% efficient boiler, which is fairly typical. An ordinary boiler may have an efficiency of 85%, while a super high efficiency condensing boiler may have a claimed efficiency of 95%.
The heat pump (HP) will typically have a COP of 3. This is saying that for every kWh of electrical input, the unit delivers 3kWh of useful heat. Where did the other 2kWh come from? We gather them from our heat source Of course, there is still the kWh lost in the cooling tower at the power station, and in the distribution:
So, if we use a heat pump to deliver the same 15kW of heat into the building how much primary fuel (coal, oil or gas) would need be burned in the power station? Taking the heat pump COP of 3, for our 15kW of useful heat, we need 5kW electricity. We gather the other 10 from our heat source. Generation and distribution the 5kW of electricity requires 10kW of primary fuel combustion. So there you have it. Only 10kW of primary fuel used to generate 15kW of useful heat, instead of 17.5kW with a gas boiler or 30kW for direct electric heating. Using a super efficient condensing boiler may reduce that 17.5 to 16, but that is still a lot more than 10! Find out about Heating COP