Heating cost comparison
Enter the prices you actually pay and the efficiency of your system: you get the cost of one kWh of useful heat, a ranking from cheapest to dearest and what a whole season costs.
The prices already filled in are only a starting example: replace them with the ones on your own bill or quote, because that is what really decides the ranking.
The systems to compare
Use the all-in price, not the raw commodity rate: take the total on the bill and divide it by the gas you used, so standing charges, distribution, taxes and VAT are all inside it. The 9.59 kWh per m³ figure is the net calorific value, which is what boiler efficiencies are measured against.
Price of bulk LPG delivered into a tank: 10 or 15 kg cylinders cost far more per kilo. One litre of LPG weighs about 0.54 kg, which is why the kWh per litre is less than half the kWh per kilo.
Price of heating oil delivered to the door, taxes included.
Price of a 15 kg bag or of one kilo: if you buy a pallet, check whether delivery is included. ENplus A1 certified pellets stay below 10% moisture and leave little ash.
Firewood is often sold by volume: a stacked cubic metre weighs anywhere from 350 to 500 kg depending on the species, so weigh a couple of baskets if you want to know what you are really paying per kilo.
Efficiency is 100%: every watt that goes in comes out as heat. Use the final price on the bill, standing charges and taxes included, not just the energy rate.
The seasonal COP (SCOP) is how many kWh of heat you get per kWh of electricity you pay for: 3.5 means 3.5 kWh of heat from 1 kWh of power. It drops with high-temperature radiators and rises with underfloor heating.
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How the cost of one kWh of useful heat is worked out
The formula never changes: price of the unit ÷ (kWh inside that unit × efficiency). A 15 kg bag of pellets at €6.00 holds 15 × 4.8 = 72 kWh; in a stove running at 85% only 61.2 kWh actually reach the room, so €6.00 ÷ 61.2 = €0.098 per kWh of heat. Gas at €1.15 per cubic metre in a condensing boiler (9.59 × 0.98 = 9.4 useful kWh) works out at €0.122 per kWh. That single number is what makes prices comparable when they are quoted in units that have nothing in common: euros per bag, per 100 kg, per cubic metre, per kWh.
Gas: use the all-in price, and mind net versus gross calorific value
A gas bill is never just the commodity: it carries standing charges, distribution and metering, system costs and taxes. The honest way to find your own price is to divide everything you paid over a year by the gas you burned in the same period. There is also a technical trap worth about 10%: gas is metered and billed against the gross calorific value (around 10.7 kWh/m³, or 11.1 in the UK where bills already come in kWh), while boiler efficiencies are measured against the net value (9.59 kWh/m³). Mixing a gross figure with a net-based efficiency makes gas look roughly 11% cheaper than it is, so this tool works on the net value.
Why a condensing boiler can be more than 100% efficient
The net calorific value deliberately ignores the latent heat carried away by the water vapour in the flue gases. A condensing boiler condenses that vapour and claws part of it back, so it delivers more heat than the net value says it should: datasheets routinely quote 104-108%. In real life, with hot radiators and return water above 55 °C, very little condensing happens and the seasonal figure falls, which is why 98% is the cautious default here. Raise it for underfloor heating, lower it for an old high-temperature system.
Pellets, firewood and the moisture trap
Pellets are sold by the bag or the pallet, firewood by weight or by volume. The default calorific values (4.8 kWh/kg for pellets, 4.0 kWh/kg for firewood at 20% moisture) only hold for genuinely dry fuel: freshly cut wood at 50% moisture drops below 2.5 kWh/kg, because part of the heat is spent boiling off the water, so at the same price per kilo it costs almost twice as much and tars up the flue as a bonus. When wood is sold by the stacked cubic metre, weigh a basket and convert to kilos: a stacked cubic metre ranges from 350 to 500 kg depending on the species.
Heat pumps, and what this comparison deliberately leaves out
For a heat pump the divisor is not an efficiency but the COP: 3.5 means 3.5 kWh of heat for every kWh of electricity. What matters is the seasonal figure (SCOP), because the instantaneous COP falls on the coldest days and stays low all winter if the heat has to go into high-temperature radiators. And remember what is not in these numbers: buying and installing the appliance, annual servicing, the flue, the space to store pellets or logs, the time spent feeding a stove, and any grants or tax credits. The floor-area estimate is a deliberately rough shortcut — use a year of real bills whenever you have them.