Building Heat Loss Calculator
Calculate a building heat loss coefficient from element areas and U-values, then the peak heating load and annual heat demand.
💼 Are you a real estate agent or broker? Send this calculator to your buyers on your own branded listing page — every enquiry comes straight to your WhatsApp. Create free agent page →Heat loss is the number behind almost every energy decision in a building. It sizes the heating system, it determines the annual fuel bill, and the change in it is what any insulation upgrade is actually buying.
The calculation is a sum of areas multiplied by U-values, plus ventilation. A U-value is the heat lost per square metre per degree of temperature difference, so a lower number is better — modern walls are around 0.2 and uninsulated solid walls above 1.5.
The calculation
Heat loss coefficient = the sum of each element area × its U-value, plus ventilation losses, giving watts lost per degree of temperature difference. Peak load = coefficient × design temperature difference. Annual demand = coefficient × degree days × 24 ÷ 1000, which converts to kilowatt hours.
Worked example
Walls of 120 at 1.5 lose 180 W/K, a 60 roof at 0.4 loses 24, a 60 floor at 0.7 loses 42 and 20 of glazing at 2.8 loses 56 — 302 W/K of fabric loss. Adding 50 W/K of ventilation gives 352 W/K. At a 23 K design difference the peak load is 8.1 kW, and across 2,500 degree days the annual heat demand is about 21,120 kWh.
Where to spend on insulation
The elements losing the most are where money works hardest. Here the walls lose 180 W/K on their own — more than everything else combined — so wall insulation dominates any retrofit, while replacing the already-decent roof insulation would barely register.
Turn a calculation into an enquiry
Buyers who run these numbers are ready to talk. Give every listing its own page with your photo, phone and WhatsApp on it — leads land straight on your phone.
Create your free property pageFrequently asked questions
What is a U-value?
The heat lost through a square metre of an element for each degree of temperature difference, in watts. Lower is better: modern insulated walls are around 0.18 to 0.30, while uninsulated solid walls are frequently above 1.5.
Where do degree days come from?
They are published for most regions and measure how far and for how long outdoor temperature falls below a base level across a year. Using the figure for your own location is essential, since climates vary by a factor of several.
Does this size a heat pump correctly?
It gives a defensible peak load, which is the starting point, but a heat pump also needs flow temperature, emitter sizing and hot water demand considered. Use this to sanity-check a quote, not to replace a proper design.