How to calculate your heating costs
To compare different fuels, you need to turn their consumption and price into a common unit: the price of a kWh of usable heat. There are two short steps.
Step 1: Find the usable heat
Usable heat is the energy that actually reaches your home once the appliance's efficiency is taken into account.
usable heat (kWh) = fuel consumption × calorific value × efficiency
For a heat pump, you multiply the electricity consumption by the coefficient of performance (COP) instead.
usable heat (kWh) = electricity consumption (kWh) × COP
Step 2: Divide the cost by the heat
price of 1 kWh of heat = yearly payment ÷ usable heat
You find the yearly payment on your bills, or calculate it as consumption × price per unit.
Worked example: gas
Over a year you use 1,500 m³ of gas and pay €1,650. The calculator assumes a calorific value of 10.5 kWh/m³ and a condensing boiler efficiency of 92%, which means 9.66 kWh of usable heat from 1 m³.
- Usable heat: 1,500 × 9.66 = 14,490 kWh
- Price of 1 kWh of heat: 1,650 ÷ 14,490 ≈ €0.114
Worked example: the same heat from a heat pump
To get the same 14,490 kWh of heat with a heat pump at a COP of 4.5 and electricity at €0.20 per kWh:
- Electricity needed: 14,490 ÷ 4.5 = 3,220 kWh
- Yearly cost: 3,220 × 0.20 = €644
- Price of 1 kWh of heat: 644 ÷ 14,490 ≈ €0.044
The difference compared with gas is €1,006 a year (1,650 − 644).
What to watch out for
- Use the real prices from your bills, including fixed fees, if you want to compare the total cost.
- Take consumption for a full year. Shorter periods distort the result because of the seasons.
- If your gas bill also covers cooking and hot water, heating is only part of it.
The numbers in the tables and examples are based on the calculator's default values (rough average prices for Slovakia). Real costs differ with your prices, equipment and house.
Frequently asked questions
Why isn't comparing prices per unit enough?
Because the units differ (tons, m³, kWh) and appliances turn energy into heat with different efficiencies. The common denominator is the price of a kWh of usable heat.
What is the COP of a heat pump?
The coefficient of performance (COP) says how many kWh of heat the pump delivers from 1 kWh of electricity. A value of 4.5 means 4.5 kWh of heat from 1 kWh of electricity. The seasonal value (SCOP) is usually lower than the value under ideal conditions.
How do I find out how much heat my house needs?
The easiest way is to convert last year's fuel consumption into kWh of heat using the formula above. A designer or an energy adviser can make an exact estimate of your house's heat loss.