Heating and energy: heat loss, radiators, boilers and running costs

Every heating decision starts from the heat a home loses on a cold day. An averagely insulated 100 m² house needs about 7.0 kW, so the heat pump calculator suggests a 7 kW pump, and a 4 x 3.5 m living room needs a radiator of about 1,848 W from a boiler. Cut the heat loss first, with loft insulation, lagging and draught-proofing, and every one of those numbers goes down.

This page gathers our UK calculators and guides on heating and energy: room heat loss, radiator, boiler and heat pump sizes, loft insulation, pipe lagging, draughts and what each appliance costs to run. It covers planning and quantities only: gas and electrical work must be done by a registered gas engineer or electrician.

Heat pump size by floor area and insulation

The heat a home needs on a cold day, from the heated floor area and the watts per m² the heat pump calculator uses for each level of insulation. The pump size is the smallest common size that covers the load.

A heat pump installer should do a room-by-room heat loss survey before you buy; this table is for a first budget.

Room heat loss and radiator sizes

A room loses heat through its walls, windows, floor and ceiling and in the air that leaks out. The radiator calculator turns that into the output to look for: a 4 x 3.5 m living room with a 2.4 m ceiling needs about 1,848 W at ΔT50, and the same room as a bedroom 1,617 W, because bedrooms are kept cooler. On a low-temperature heat pump the living room needs a radiator labelled about 3,590 W, which is why a heat pump often means bigger radiators.

Boiler size

A combi boiler is sized for hot water, not heating. An averagely insulated 100 m² house needs about 7 kW of heat, but heating a shower or tap straight from the mains needs about 20.9 kW, so the boiler calculator suggests a 24 kW combi. A system boiler with a hot water cylinder can be sized closer to the heating load.

Heat pumps and their running cost

The same 100 m² house uses about 12,000 kWh of heat a year. At a typical efficiency of 3 that takes 4,000 kWh of electricity, about £1,040 a year at the calculator's default prices, against about £859 for a gas boiler. Put in your own tariff: the answer swings on the price of electricity against gas and on how well the system is designed.

Loft insulation and pipe lagging

Topping a 40 m² loft up from 100 mm to the usual 270 mm means adding 170 mm, which is 9 rolls of 100 mm mineral wool with 5% for cuts; starting from nothing it takes 14. Lag pipes in the loft and other cold spaces before the first frost: 12 m of 15 mm pipe and 6 m of 22 mm pipe take 14 and 7 one-metre lengths of foam with 10% spare.

Draught-proofing

Air that leaks out has to be replaced and heated again. In the heat loss calculator's example room, one air change an hour accounts for 266 W of a 390 W total; the same room at two air changes an hour, which is draughty, loses 656 W. Draught strips on doors and windows and a letterbox brush are cheap, but keep trickle vents and extractor fans working, because a home still needs fresh air.

What it costs to run

Running cost is the kilowatts times the hours times your unit rate. A 2 kW heater costs about £0.54 an hour, or £64.80 a month at 4 hours a day, at the electricity cost calculator's default unit rate. Low-power things cost pence: the guides below have the figures for heated airers, electric blankets, Christmas lights and LED bulbs.

Every calculator for this topic

Guides

• How Much Do LED Bulbs Save? UK Halogen to LED SavingsOne 50 W halogen swapped for a 5 W LED saves £13.30 a year at 3 hours a day and 27p per kWh. 20 spotlights save £266.09 and pay back in 2.7 months.
• Heated Airer, Electric Blanket and Dehumidifier Running Costs (UK)At 27p per kWh a 250 W heated airer costs about 41p to dry a load, and a double electric blanket about 81p a month. Charts by watts.
• How Much Does It Cost to Run Christmas Lights? (UK, per Hour)Running costs for Christmas lights in the UK: LED tree lights cost about 29p for the season at 27p per kWh, filament sets £2.33.
• PIR vs Mineral Wool Insulation: Which Should You Use? (UK)PIR board is about half the thickness for the same U-value; mineral wool is non-combustible and better for sound. Thickness charts for each target.
• Electric Underfloor Heating Cost: Running Cost and Size Chart (UK)A 4 × 3 m room with electric underfloor heating needs about 1,440 W and costs £70 a month at 6 hours a day and 27p. Charts by room size and price.
• What Size Heat Pump Do I Need? UK kW Chart and Running CostA 100 m² home needs about a 7 kW heat pump and spends £1,040 a year heating against £859 for gas. Size and running cost charts by home, SCOP and price.
• How Many Loft Boards Do I Need? Area, Thickness and Screws (UK)A 12 m² loft needs 10 boards of 2400 × 600 mm with 10% extra. Charts of boards and screws by area, and the thickness for your joists.
• How Much Pipe Lagging Do I Need? 15 mm and 22 mm Foam (UK)12 m of 15 mm pipe and 6 m of 22 mm pipe take 21 pieces of 1 m lagging. Charts of the pieces needed for common runs, and where to lag first.
• How Thick Should Insulation Be? Thickness for a Target U-ValueTo go from U 2.0 to 0.16 you need 127 mm of PIR board. Charts of insulation thickness for each target U-value in Approved Document L.
• How to Calculate Room Heat Loss: Watts, U-Values and ExamplesA room 4 × 3.5 m loses about 390 W at 21 °C inside and −3 °C outside. The U-value formula, worked examples and how to size a radiator.
• What Size Boiler Do I Need? kW by Home Size (UK)Boiler size charts by floor area and hot water flow, and why a combi must meet the hot water demand as well as the heating load.
• Cost to Run Household Appliances: UK Table per kWhWhat common household appliances cost to run per hour and per month, from a kettle to a tumble dryer, and how to work it out at your own rate.
• Loft Insulation: How Many Rolls Do I Need to Reach 270 mm?How many rolls of loft insulation you need to top up to 270 mm, with a chart by loft size and existing depth, how to lay the layers, and what to watch out for.
• What Size Radiator Do I Need? Watts and BTU by Room (UK)A UK chart of watts and BTU for common rooms, how glazing and walls change the answer, and the larger radiators a heat pump needs.

Other guides that use these calculators: Wrapping Paper Size Chart (UK): Sheet to Cut for 11 Boxes, in cm · How Many Floor Joists Do I Need? Spacing and Strutting (UK) · How Many Solar Panels Do I Need? UK Output, Savings and Payback · How Much Does It Cost to Charge an Electric Car? UK Cost Chart · Laminate, Vinyl (LVT) or Engineered Wood Flooring: Which? · What Size Wood Burner Do I Need? Room Size to kW Chart (UK) · How Many Roof Tiles Do I Need? Per m² and for a Gable Roof (UK) · Watts to Amps UK Chart: 13 Amp Plug, Kettle, Shower, MCB Sizes

Answers for common sizes

Already know the size? These pages give the worked answer for one common size each, with the inputs filled in so you can adjust them.

Project planners

Planning the whole job rather than one material? These step-by-step planners put the calculators above in the order you will need them.

Frequently asked questions

Estimates only. Quantities depend on your materials, site and supplier, so check pack sizes and coverage on the label. Check the Building Regulations and any planning rules for anything structural or close to a boundary. How we calculate.