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Heat Loss Calculator

Add up the conductive heat loss through walls, windows, doors and roof from their area and R- or U-value.

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Heat pump savings calculatorWhat a heat pump costs to run against your current boiler, from your own fuel bill: the heat you actually need, the electricity a given SCOP would draw, the yearly difference and the payback. Plus the one number that decides it — the electricity price above which the heat pump stops being cheaper.Appliance Energy Cost CalculatorTurn any appliance's wattage and daily use into a daily, monthly and yearly running cost in your currency.Home Energy Audit CalculatorEstimate your home's annual energy use, cost, intensity and HVAC share from its shell and major appliances.HVAC Sizing CalculatorEstimate the heating and cooling load of a room in BTU/hr, kW and tons from its area, climate and insulation.Insulation CalculatorFind the recommended R-value for your climate and component, and how much insulation you still need to add.Light Bulb Savings CalculatorCompare LED, incandescent and CFL bulbs on yearly energy plus replacement cost, and see your LED savings.Power Factor CalculatorCompute power factor, apparent and reactive power, and PF-correction capacitance.Heat transfer rate calculatorThe rate of heat flow by any of the three mechanisms: conduction through a wall (Q = k·A·ΔT/d), convection from a surface (Q = h·A·ΔT), or radiation between bodies (Q = ε·σ·A·(T₁⁴ − T₂⁴), with temperatures in kelvin). Pick a mode; only its fields matter.

Open Heat Loss Calculator and you get an answer straight away, with no account to create. Its place is under Energy & power; Heat pump savings calculator and Appliance Energy Cost Calculator answer the questions closest to this one.

How to use it

  1. Open the tool — no signup or install needed.
  2. Enter your input or adjust the available options.
  3. Get your result instantly, then copy or download it.

Frequently asked questions

What does Heat Loss Calculator do?

Add up the conductive heat loss through walls, windows, doors and roof from their area and R- or U-value.

When would I actually use this?

Putting a number on a bill: what an appliance costs to run, whether a solar array covers the load, and how much heating a room actually needs.

What is the most common mistake?

Confusing power with energy. A kilowatt is a rate and a kilowatt-hour is a quantity; a 2 kW heater on for half an hour uses 1 kWh, and only the second figure appears on a bill.

How is Heat Loss Calculator different from Heat pump savings calculator?

They sit next to each other but answer different questions: Heat pump savings calculator is the one to open when you need it to what a heat pump costs to run against your current boiler, from your own fuel bill: the heat you actually need, the electricity a given SCOP would draw, the yearly difference and the payback. Plus the one number that decides it — the electricity price above which the heat pump stops being cheaper. Pick whichever matches what you're starting from — both are free.

Is there a tool for the next step?

Appliance Energy Cost Calculator is the closest one after this: Turn any appliance's wattage and daily use into a daily, monthly and yearly running cost in your currency.

What else is worth having open alongside it?

Home Energy Audit Calculator and HVAC Sizing Calculator — they come up in the same task often enough to be worth a second tab.

Where do the figures come from?

The arithmetic is exact for the rating and the tariff you enter. Nameplate wattage is a maximum, not an average, and solar output depends on latitude, orientation and shading that no formula can guess.

Further reading

All guides →
ComparisonInsulate or Replace the Heating: Which Repays First, and Why the Answer InvertsPut both on one axis — euros paid per kilowatt-hour a year of permanent saving — and the comparison becomes decidable. The same loft job costs 0.83 euros per kilowatt-hour a year in a 1965 house and 5.00 in a 2005 one, a factor of six. That, and not the age of the building by itself, is what inverts the verdict.ComparisonHeat Pump or Gas Boiler: the SCOP Your Local Prices DemandBefore any grant, the price of electricity divided by the price of gas sets a seasonal efficiency the heat pump must beat just to break even on running cost. In late 2025 that number was 1.53 in Portugal and 2.79 in Germany — and on old radiators a German heat pump can cost more to run than the boiler it replaced.ExplainerR-Values, U-Values, and What an Insulation Number HidesR and U are reciprocals, and the two markets that read this site use different ones in different units — so an R-value quoted without its system means nothing. Worse, the number on the batt is not the number your wall achieves, because the studs are a parallel path.How-toHow to Estimate the Running Costs of a HomeAdd up energy, water, heating, and maintenance with simple rules of thumb so you know what a home really costs to run each month.ExplainerLux and Lumens: The Difference, and How to Convert Between ThemLumens are what a lamp emits; lux is what lands on a surface. You cannot convert one to the other without an area, or without a distance and a beam angle. Anything that offers you a bare conversion factor is selling you a fiction.ExplainerWhere Household Water Actually GoesPeople picture taps and showers, and metered end-use studies say the biggest single line is somewhere else. Here is the ranked budget from named national agencies, plus the arithmetic that changes behaviour: the dripping tap, the running tap, and the shower flow rate at which a bath becomes the cheaper option.