Hong Life CalculatorsUnderstanding the heating fuel cost comparison — concept and principle
Fuels are sold in different units: gas by the kWh, therm or cubic metre, heating oil and LPG by the litre, electricity by the kWh. This calculator puts them on the same basis, the heat your home needs per year in kWh, and shows the yearly cost of supplying that heat with a gas boiler, an oil boiler, an LPG boiler, a heat pump or direct electric heating.
How it is calculated
- Gas boiler = heat ÷ 0.9 × gas price per kWh
- Oil boiler = heat ÷ 0.9 ÷ 10.35 kWh/L × oil price per litre
- LPG boiler = heat ÷ 0.9 ÷ 7.08 kWh/L × LPG price per litre
- Heat pump = heat ÷ SCOP × electricity price
- Electric resistance = heat × electricity price
Heat needed is useful heat delivered to the home per year. The boilers are assumed to be 90% efficient. The energy content of heating oil is taken as about 10.35 kWh per litre and LPG as about 7.08 kWh per litre (gross calorific values). SCOP is the heat pump's seasonal coefficient of performance.
The principle
A boiler cannot turn all of a fuel's energy into heat in the radiators; some leaves with the flue gases. Dividing useful heat by efficiency gives the fuel energy you must buy, and dividing that by the energy per litre gives litres. A heat pump does not burn anything: it uses electricity to move heat from outdoor air or the ground, so it delivers several kWh of heat per kWh of electricity. SCOP is that ratio averaged over a heating season. Direct electric heating has a ratio of 1.
Worked example
- Inputs: 12,000 kWh heat; gas $0.08/kWh; oil $1.00/L; LPG $0.80/L; electricity $0.25/kWh; SCOP 3.2.
- Gas: 12,000 ÷ 0.9 = 13,333 kWh × $0.08 = $1,066.67.
- Oil: 13,333 ÷ 10.35 = 1,288.24 L × $1.00 = $1,288.24.
- LPG: 13,333 ÷ 7.08 = 1,883.24 L × $0.80 = $1,506.59.
- Heat pump: 12,000 ÷ 3.2 = 3,750 kWh × $0.25 = $937.50.
- Electric resistance: 12,000 × $0.25 = $3,000.00.
| Heat needed per year (kWh) | Gas boiler (90%) |
|---|---|
| 3,000 | $266.67 |
| 6,000 | $533.33 |
| 9,000 | $800.00 |
| 12,000 | $1,066.67 |
| 15,000 | $1,333.33 |
| 18,000 | $1,600.00 |
| 24,000 | $2,133.33 |
How to read the result
The cheapest option depends on the ratio of prices. A heat pump beats a 90% gas boiler when electricity price ÷ SCOP is lower than gas price ÷ 0.9; in this example $0.078 against $0.089 per kWh of heat. Running costs are only part of the decision: installation cost, standing charges, maintenance and the home's insulation matter too. Estimate heat demand as past fuel use × the old system's efficiency.
Common mistakes
- Entering fuel bought instead of heat needed, which double-counts boiler losses.
- Mixing gross and net calorific values between fuels.
- Using a heat pump's best-case COP instead of its seasonal SCOP.
Frequently asked questions
How do I convert a gas price per therm or m³ to per kWh?
One therm is 29.3071 kWh. For m³, use the calorific value on your bill, often roughly 10–11 kWh per m³.
What SCOP is realistic?
It depends on climate, flow temperature and installation quality; lower flow temperatures give higher SCOP.
Why is LPG dearer per litre of heat than oil?
A litre of LPG holds less energy than a litre of heating oil.
Sources: EN 14825 — seasonal performance of heat pumps (SCOP) · UK Government, Digest of UK Energy Statistics (DUKES) — fuel calorific values