What does it actually cost to heat this building?
Describe the building as it really is, pick a winter's day, then move the thermostat and watch the bill move with it. Nothing here is looked up in a table — every figure is solved from a heat-loss model as you change the inputs.
Thermostat
21.0°C
To heat it for this day
£3.72
£0.26 more a day for each degree warmer.
A typical year
£700
about £58 a month
One degree warmer
+£73
a year, every year
Heating on 06:30–08:30 and 16:30–22:30— holding 16 °C the rest of the time
Heat needed
51kWh
Delivered into the building over the day.
Gas bought
58kWh
At 88% efficiency.
Heat loss
179W/K
Watts lost for every degree between inside and out.
Size needed
4.1kW
To hold 21 °C when it's -2 °C outside.
The building
Roughly right beats precisely wrong.
Property
Storeys
Neighbours
Glazing
Cavities filled, 270 mm in the loft, floor insulated. A solid retrofit.
Draughts
Heating & tariff
A modern combi or system boiler, running at 88% seasonal efficiency.
On before work and again after it. Off in between.
A typical setback. Stops the building getting properly cold, so there's less to make up in the morning.
Typical UK unit rates. Put your own in from a recent bill.
The day
One winter's day, hour by hour.
Grey with brighter spells. Most of a British winter looks like this.
Cost against thermostat setting
Tap a bar to move the thermostat
The whole day resimulated at every setting from 15 to 24 °C.
Through the year
£700 a year
The same building run against each month’s real weather — not the day picked on the left. That works out at £58 a month if you pay by direct debit, though the bills themselves land mostly between October and March.
Fuel over the year: 10,937 kWh of gas
Heating taken as off for 3 summer months
The day, hour by hour
Indoor and outdoor temperature, with the heat going in along the bottom. Shaded bands are the hours the heating is allowed to run.
Coldest it gets indoors: 16.7 °C
Peak output called for: 20.0 kW of 20 kW
Where the heat escapes
Every part of the building, as a share of the 179 W/K it loses for each degree of difference between inside and out. This is what to fix first.
- Walls
- 27%48 W/K
- Draughts & ventilation
- 27%48 W/K
- Windows
- 26%46 W/K
- Floor
- 11%19 W/K
- Roof
- 5%9 W/K
- Doors
- 5%8 W/K
Assumes a broadly rectangular building of medium-weight masonry with 2.4 m ceilings and windows of about 1.6 m² each — two external doors for a house, one for a flat, and two of a flat's four walls facing outside. Hot water, cooking and lighting are not included — this is the heating alone.
What would actually help
Each of these is the whole year modelled again with that one change made, ranked by what it saves. Change the building above and the list changes with it.
- 01−£173a year
Insulate to “new build standard”
Built to current regulations, or taken back to brick and done properly.
A major retrofitWould bring the year to £527
- 02−£103a year
Draught-proof it to “well sealed”
Strips round doors and windows, a chimney balloon, sealing where pipes come through. The cheapest saving in the building, almost always.
A weekend, or a day's labourWould bring the year to £597
- 03−£92a year
Replace the double glazing with triple or A-rated glazing
Expensive per pound saved compared with insulation, but it fixes the cold draught off the glass that makes a room feel colder than it is.
A big-ticket jobWould bring the year to £608
- 04−£73a year
Turn the thermostat down to 20.0 °C
The one change that costs nothing and takes effect this evening. Most people can't tell one degree apart from a jumper.
Free, right nowWould bring the year to £627
Savings are modelled, not quoted, and they don’t stack neatly — doing the first one makes the rest worth a little less, because there’s less heat left to save.
How the numbers are worked out
No lookup tables and no rules of thumb. The building is solved from its own geometry every time you move a slider.
01
Heat loss, element by element
Wall, roof, floor and window areas come from the floor space and storeys you set. Each is multiplied by a U-value for how it's built, and added up into a single figure for how many watts the building leaks per degree of difference between inside and out.
02
Air as well as fabric
Draughts are the loss people forget. Air changes per hour are turned into watts per degree using the volume of the building, and pushed up on windy days — the same gaps leak far more air in a gale.
03
A day, not an average
Outdoor temperature follows a real daily curve, coldest around six in the morning and mildest mid-afternoon. Sunshine through the windows and heat from people, cooking and appliances are both counted as free heat.
04
Only what actually faces outside
A wall shared with a heated neighbour loses you nothing, so it isn't counted. That's what separates a detached house from a mid-terrace, and why a middle-floor flat — no roof, no ground floor, two party walls — is the cheapest thing in the street to heat.
05
Two thermal masses
The air warms in minutes; the masonry takes hours. Modelling them separately is what makes the overnight cool-down and the slow morning warm-up come out right — and why a heat pump struggles to recover from a setback.
06
Heat turned into money
Heat delivered is divided by boiler efficiency, or by a heat pump's coefficient of performance — which is recalculated at every step, because a heat pump gets worse as the air outside gets colder. Then it's multiplied by your unit rate.
07
Then a whole year of it
The same building is run again against each month's own weather, temperatures and daylight, which is where the monthly and annual figures come from. June to August the heating is taken as switched off for the season, because that's what people do.
08
Suggestions that are measured, not guessed
Every recommendation is the entire year modelled again with that one change made, and ranked by the difference. Nothing is a rule of thumb, and anything that doesn't pay for itself simply isn't listed.