To calculate the cost of running a heat pump, you need to work out your household heat demand using your current annual gas consumption and boiler efficiency.
Potential heating and hot water costs using a gas boiler
A new A-rated gas boiler must be a minimum of 92% efficient; however, studies have shown that the actual in-use performance is generally lower. Older boilers are also less efficient
The UK government energy department (DESNZ) did a live study and found the average gas boiler to be around 82.5% efficient, so that's what we've used
Current gas consumption is 11,700kWh (see FAQ above) but as only 82.5% of that is being turned into heat, we calculate actual heat demand at 11,700 x 0.825 = 9,653kWh.