Why does a conservatory cost so much to heat?
Work out what a room actually loses in heat, the heater size it needs, and what holding it at a comfortable temperature costs per hour, per month and across a winter.
Using the Austrian average of 32.72c/kWh — Eurostat, the second half of 2025.
Almost entirely glass, on three sides, with a roof and a floor both facing the cold. The arithmetic is brutal and it is worth seeing why.
Common cases
What drives the cost
A conservatory is the clearest demonstration of the physics on this page, because almost every surface faces outside. Three glazed walls, a glazed or thin polycarbonate roof, and a floor laid over cold ground: put those numbers in above and the heat loss typically lands three to four times a normal room of the same floor area. Old polycarbonate roofing is often the single worst element in the building, sometimes above 3 W/m²K, which is worse than a Victorian solid wall.
This is why conservatories tend to be unusable at both ends of the year — freezing in January and unbearable in July, for the same reason in reverse. The interventions that actually work are structural rather than behavioural: replacing a polycarbonate roof with an insulated one transforms the room, and thermal blinds help considerably. Short of that, the honest answer is to treat it as a spring and autumn room, heat it only while you are in it, and keep the door to the house shut so it is not quietly draining the rooms behind it.
How this is worked out
A room loses heat through every surface facing somewhere colder, and through the air that leaks out and has to be warmed again. Each surface loses in proportion to its area, how well it insulates, and how much colder it is outside. Add those together and you get a heat loss coefficient — watts per degree — which belongs to the room alone and does not change with the weather. Multiply it by the temperature difference you want to hold and you have the heat the room needs. For a plug-in electric heater that is exactly the electricity it draws, because a resistive heater turns all of its electricity into heat. Nothing about this is brand-specific: a £15 fan heater and a £300 designer panel of the same wattage cost precisely the same to run.
What this assumes
- U-values for walls, roofs and windows are the published defaults from BRE's RdSAP 10 specification (February 2024) and consultation paper CONSP:16. Those are in-situ measured figures — the solid-brick default was cut from 2.1 to 1.7 W/m²K after fieldwork found real walls beat the theoretical calculation. The five options above use, in order: walls 1.7, 1.5, 0.7, 0.30 and 0.18; windows 4.8, 2.8, 2.0, 2.0 and 1.4; roofs 0.68, 0.40, 0.16, 0.16 and 0.11.
- Floor U-values (0.7, 0.7, 0.45, 0.22, 0.13) and air change rates (1.2, 1.0, 0.7, 0.55, 0.4 per hour) are representative rather than measured. A real floor depends on the ratio of exposed perimeter to area, and real air leakage depends on chimneys, flues and how sheltered the building is — neither is knowable from the questions above, and both are the largest source of uncertainty here.
- The room is assumed square. A long thin room of the same floor area has more external wall and so loses more: 2 × 8 m has a 20 m perimeter against 16 m for 4 × 4 m, and this would under-read it by roughly a quarter.
- Only surfaces facing unheated space are counted. Walls to a heated neighbour, and ceilings under heated rooms, are treated as losing nothing — close enough in practice, though a wall to a neighbour who never puts the heating on is not.
- Nothing is credited for heat you get free. People, cooking, lighting, appliances and winter sun through a south-facing window all add real warmth — often 200 to 400 W in an occupied living room — so a real bill usually lands below the figure here.
- The seasonal figure multiplies the daily cost by the number of days a year your country needs heating at all, at the average outdoor temperature for those days. It is a fair estimate of a whole winter, not a prediction of any particular one.
- The rate filled in for you is an average that spreads fixed charges across a year's usage, so it sits above what one extra unit really costs. Entering the unit rate from your own bill gives a closer answer.
Questions
Why is my conservatory so cold in winter?
Because glass loses heat several times faster than an insulated wall, and a conservatory is mostly glass on three sides plus the roof. Heat leaves as fast as a heater can supply it, so the room never really gets ahead. It is not that the heating is inadequate — it is that the room has very little ability to hold what it is given.
Does an insulated conservatory roof pay for itself?
On comfort, almost immediately: the roof is usually the largest and worst-performing surface, and replacing it is what turns a conservatory into a room used all year. On pure energy saving the sum is slower, because most people were not heating it much anyway. The honest way to judge it is as buying usable floor space rather than as an energy measure.
Should I leave the door to the conservatory open?
No. An open door makes the conservatory part of the heated house, and it is the worst-insulated part by a wide margin. Building regulations in several countries require conservatories to be separated from the house by a door for exactly this reason. Keep it shut and heat the conservatory deliberately when you want to use it.
Are some electric heaters cheaper to run than others?
No. Every plug-in electric heater is 100% efficient at the point of use — fan, oil-filled, convector, panel, halogen and infrared alike. A 2 kW heater of any type uses 2 kWh an hour and costs the same. What differs is how the heat feels and how quickly it arrives: infrared warms objects and people directly so it can feel effective sooner, and an oil-filled radiator keeps releasing heat after it switches off. Neither changes the meter reading. Anything advertised as a cheap-to-run electric heater is either a heat pump or a claim worth ignoring.
Is it cheaper to leave the heating on low all day?
No, and the physics above is why. A room loses heat in proportion to how much warmer it is than outside, so a room kept warm all day spends every one of those hours losing heat. Letting it cool means a smaller temperature difference for most of the day and therefore less total loss. Reheating costs something, but less than the standing loss it avoids. The exception is a home so damp that letting it go cold invites condensation, which is a different problem with a different fix.
Why is my conservatory so expensive to heat?
Because almost all of it faces outside. Glass loses heat several times faster than an insulated wall, a conservatory usually has three glazed walls plus a glazed or thin roof, and its floor sits over cold ground. Put a conservatory's numbers in above and the heat loss often comes out three or four times a normal room of the same size. The honest answer is usually to heat it only when it is in use, and to accept it as a spring and autumn room.
Should I heat one room instead of the whole house?
Often yes, if the rest of the house genuinely stays unheated and you accept it being cold. Heating one room costs roughly what this calculator says; heating a whole house costs the same arithmetic repeated for every room, plus the corridors between them. The catch is that unheated rooms get cold surfaces, and cold surfaces attract condensation and then mould. If you go down this route, ventilate the cold rooms rather than sealing them up.
How much does turning the thermostat down by one degree save?
Roughly 7 to 10% of the heating bill for that room, and you can see it in the maths: cost is proportional to the gap between inside and outside. Holding 20 °C against 8 °C is a 12-degree gap; holding 19 °C is 11 degrees, which is 8% less. The saving is bigger in a mild climate, because one degree is a larger share of a smaller gap — which is also why the same advice is worth more in Portugal than in Finland.
Is an air-to-air heat pump worth it for one room?
On running cost, usually yes and by a lot. A heat pump does not make heat, it moves it, so it delivers three to four units of heat per unit of electricity — set efficiency to 350 above and the bill falls by about two thirds. Against that sits the installation cost, which is thousands rather than tens, so the sum turns on how many hours a year you actually heat that room. A room heated all winter pays it back; a spare bedroom heated at Christmas does not.
Related tools
- How much does it cost to run an appliance? Work out what any appliance costs to run per use, per month and per year on your own electricity rate — including what it draws on standby.
- Why do my windows get wet, and will mould grow? Work out the dew point in a room, whether a cold window or wall will run with water, and the humidity you need to stay below to keep it dry.
Sources
- BRE — RdSAP 10 specification (roof, floor and window U-value tables) — checked 2026-08-27
- BRE CONSP:16 — review of default wall U-values for existing buildings — checked 2026-08-27
- BRE — in-situ measurements of wall U-values in English housing — checked 2026-08-27
- ERA5 reanalysis via the Open-Meteo archive — heating climate, 2016–2025 — checked 2026-08-27
- Eurostat — heating degree days by country (cross-check on the climate figures) — checked 2026-08-27
Rates from Eurostat for the second half of 2025. Last reviewed 27 August 2026.