Home Heating Cost, Explained
After reading this you will be able to estimate what heating your home costs, predict how much a night setback saves, and compare gas, oil, electric and heat-pump running costs at your own prices.
What this tool answers
A house loses heat through its walls, roof, windows and ventilation. Your heating system spends energy replacing that loss to hold the indoor temperature steady. The cost depends on three things: how leaky the house is, how big the gap is between inside and outside, and how much you pay per unit of energy.
Here is the hook. A 100 m² house of average insulation, held at 21 °C while it is 5 °C outside, loses heat at a rate of about 3.52 kW. Over a full day that is 84.5 kWh. On gas at 0.10 per kWh, roughly 8.45 per day, or about 253 in a 30-day month. Turn the thermostat down 3 °C for 8 hours each night and you shave a few percent off that. The rest of this article shows exactly where those numbers come from.
When to use it, and when not
Use the simulator to compare heating systems at your own prices, to see whether a setback is worth the morning chill, and to sanity-check a quote for a heat pump against your current gas bill. It is a steady-state model, so it is at its best for average conditions and for month-scale bills.
Do not expect it to reproduce a single day's meter reading. Real weather swings, solar gain through south windows, cooking, and the heat given off by bodies and appliances all move the number. The model also excludes hot water and appliances entirely: it is the space-heating share only. If your gas bill includes a big hot-water share, subtract that before comparing.
Calibrate once against a real bill. Pick a past winter month, read the space-heating kWh from your statement, and adjust the insulation preset until the tool's monthly kWh roughly matches. Then trust its comparisons.
The formula and the intuition behind it
The whole model rests on one idea: heat flows out at a rate proportional to the temperature difference. Bundle every loss path (walls, roof, glazing, air changes) into a single heat-loss coefficient U measured in watts per kelvin. Then the instantaneous heat demand is:
Here \dot{Q} is the heat power in watts, U is the whole-house loss coefficient in W/K, and T_{in} - T_{out} is the indoor-outdoor temperature difference in kelvin (the same size as a degree Celsius difference). The presets set U per square metre of floor: 3.5 W/K/m² for poor, 2.2 for average, 1.2 for good, 0.4 for passive-house. Multiply by floor area to get the whole-house figure.
Energy is power times time. Over a day of 24 hours at constant conditions:
Divide by 1000 to turn watt-hours into kilowatt-hours. The cost is that energy divided by the system efficiency, times the price per kWh. A gas boiler at 90% efficiency needs E / 0.9 kWh of gas. A heat pump with a coefficient of performance (COP) of 3.5 delivers 3.5 kWh of heat per kWh of electricity, so it needs E / 3.5 kWh.
Worked example reproducing the demo
The default 100 m² house on gas
Load the demo: area 100 m², average insulation, indoor 21 °C, average outdoor 5 °C, cold snap −5 °C, gas boiler, electricity 0.25/kWh, fuel 0.10/kWh, setback 3 °C for 8 hours.
- Heat-loss coefficient: U = 2.2 \times 100 = 220 W/K.
- Average-day demand: \dot{Q} = 220 \times (21 - 5) = 3520 W, that is 3.52 kW.
- Daily heat energy: 3.52 \times 24 = 84.5 kWh.
- Gas needed at 90% efficiency: 84.5 / 0.9 = 93.9 kWh.
- Daily cost: 93.9 \times 0.10 = 9.39. Monthly (30 days): about 282.
- Cold-snap demand: 220 \times (21 - (-5)) = 5720 W. The gap grew from 16 K to 26 K, so the bill scales by 26/16 = 1.625. That is 137 kWh of heat per day.
Notice the bill tracks the temperature difference, not the outdoor temperature alone. Dropping from 5 °C to −5 °C is a 10-degree change outside but a 62% jump in demand, because the indoor-outdoor gap is what drives the loss.
Why the night setback saves less than you would guess
The naive calculation: during the setback you hold the house 3 °C cooler, so the gap during those hours shrinks. At 5 °C outside the daytime gap is 16 K; setback to 18 °C indoor makes it 13 K, an 18.75% cut for those hours. Over 8 of 24 hours that is a naive whole-day saving of 0.1875 \times 8/24 \approx 6.25\%.
Reality is gentler. The house does not snap to the lower temperature: thermal mass keeps it coasting down, still losing heat at close to the old rate for a while. Then the morning reheat pulls the mass back up and claws some saving back. The tool applies a factor of 0.7 to the naive figure, so the modelled saving here is about 4.4% of the daily bill, roughly 12 per month on the gas example.
The setback always saves energy, never costs it. The old "it takes more to reheat than you save" claim is a myth: a cooler house loses less heat while cool, and reheating only restores the energy the mass gave up, never more. The damping factor reduces the saving, it does not reverse its sign.
Comparing heating systems, and the heat-pump twist
The daily heat demand is the same whatever supplies it. What differs is efficiency and fuel price. For the average day (84.5 kWh of heat):
| System | Efficiency / COP | Input energy (kWh) | Price/kWh | Daily cost |
|---|---|---|---|---|
| Gas boiler | 0.90 | 93.9 | 0.10 | 9.39 |
| Oil boiler | 0.85 | 99.4 | 0.10 | 9.94 |
| Electric resistance | 1.00 | 84.5 | 0.25 | 21.1 |
| Air-source heat pump | 3.8 | 22.2 | 0.25 | 5.56 |
| Ground-source heat pump | 4.5 | 18.8 | 0.25 | 4.69 |
Electric resistance is always the most expensive per kWh of heat, because it buys 1 kWh of heat for 1 kWh of the priciest fuel. The heat pumps win here despite electricity costing 2.5 times as much as gas, because they move 3.8 to 4.5 kWh of heat per kWh of electricity.
The twist is that a heat pump's COP falls as outdoor air gets colder, since it has to lift heat across a bigger gap. A typical air-source curve runs from about 4.2 at +10 °C down to 1.9 at −10 °C. On the −5 °C cold snap the COP is near 2.3, so the ASHP's advantage over gas narrows sharply just when the bill is largest.
Reading the results without fooling yourself
Two numbers matter most. The monthly cost tells you the running expense at your prices. The gap between systems tells you whether a switch pays back its install cost. If gas and ASHP are 30 apart per month, that is 360 a year against a five-figure install, so payback is slow unless fuel prices move.
Watch the cold-snap column separately. Sizing a heat pump for the average day leaves it short on the coldest days, when many units fall back on an electric booster at COP 1.0. That backup energy is priced like resistance heating and can dominate a cold week's bill.
Common mistakes
Comparing a full gas bill against a space-heating-only estimate. Your gas statement includes hot water and cooking. Subtract those (often 15 to 30 kWh/day for hot water) before you match the tool's number, or you will conclude the model is wrong when it is you who added apples to oranges.
Other traps: using the coldest night as your "average outdoor temp" (use the seasonal mean, and the cold-snap field for extremes); forgetting that the boiler efficiency of 90% and the COP are doing all the work in the cost ranking; and expecting a bigger setback saving than the physics allows. Doubling the setback depth roughly doubles the saving only until the house cannot coast that far in the setback hours.
Related tools
Heating physics and vehicle physics share the same idea that a temperature difference drives loss. If you drive electric, the EV Range & Charging Simulator shows how cabin heating in cold weather eats into range, the mobile cousin of the house losing heat to a cold night.
Frequently asked questions
Does turning the heat down at night really save money?
Yes. A cooler house loses heat more slowly, and the morning reheat only replaces the energy the building gave up, never more. The demo case saves about 4.4% of the daily bill with a 3 °C, 8-hour setback, roughly 12 per month on gas.
Why is my electric heating so much more expensive than gas?
Resistance heating turns 1 kWh of electricity into 1 kWh of heat, and electricity typically costs 2 to 3 times as much per kWh as gas. In the demo that is 21.1 per day electric against 9.39 gas for the same heat.
Is a heat pump always cheaper to run?
Usually, but not guaranteed. Its cost is electricity price divided by COP. If electricity is very expensive relative to gas, or the weather is very cold (COP near 2), a gas boiler can win. Use the widget to find your crossover.
Why does the tool ignore hot water and appliances?
To keep the space-heating comparison clean. Hot water and cooking depend on household habits, not on the heat-loss physics, and mixing them in would blur the system comparison. Add them separately if you want a total bill.
How accurate is the steady-state model?
Good to within 10 to 20% for monthly bills if you calibrate the insulation preset against a past bill. It ignores solar gain, wind, and daily weather swings, so single-day figures will scatter around it.
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