Using an air conditioner for heating
A reversible air conditioner in heating mode operates as an air-to-air heat pump. The calculator’s “Heat pump / AC” option covers this use. Enter the heating SCOP, not a cooling SEER or EER rating. A cooling-only air conditioner is not covered by this heating model.
For a seasonal comparison, divide the electricity price by heating SCOP to get the cost per useful kWh. At 0.30/kWh and SCOP 4, that is 0.075 per kWh of useful heat. Gas priced at 0.09/kWh with 90% boiler efficiency costs 0.10 per useful kWh in the same example.
Compare the same rooms and the same heat
An AC unit warming one living room should be compared with the gas required to heat that room, not automatically the whole-house gas bill. The calculator holds useful heat constant across the selected sources so the comparison is explicit.
If you change which rooms you heat, comfort levels or operating hours, use a corresponding heat-demand estimate. Heating one room instead of an entire home can reduce demand, but the calculator does not attribute that saving to the efficiency of the appliance.
Cold weather and defrost
For a particular outdoor condition, select “COP scenario” and enter a value from a suitable manufacturer performance table or measurement. Check whether the value accounts for defrost, fans and other electrical inputs. Do not add the same loss twice.
This calculator does not guess a universal COP from outdoor temperature. Two devices at the same temperature can perform differently. Its annual result with a fixed COP is a sensitivity calculation, not a forecast of how often each weather condition occurs.
Hot water and other costs
Many air-to-air systems heat rooms but do not supply domestic hot water. Keep that separate when comparing with a boiler that provides both. You may also retain a gas standing charge even if AC takes over most room heating.
Running-cost differences exclude installation, maintenance unless explicitly added, and the value you place on noise, airflow or summer cooling. Use the calculation to compare energy use, then assess whether the system fits the rooms and comfort you want.
Background: Energy Saving Trust: air-to-air heat pumps.