Windows & Doors
Heat Loss Through Glazing Calculator
Watts and annual kWh lost through glass
Updated July 12, 2026 · Live
What this tool does
Works out the rate of heat loss through glazing in watts and the annual energy in kWh, from the glazing area, U-value and the indoor–outdoor temperature difference.
Enter the glazing area, U-value, temperature difference and heating hours; the calculator returns the heat-loss rate in watts and the annual energy in kWh.
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Formula Used
Works out the rate of heat loss through glazing in watts and the annual energy in kWh, from the glazing area, U-value and the indoor–outdoor temperature difference. It runs entirely in your browser and shows the working, so you can check every step against your own figures.
How the heat loss through glazing calculator works
The instantaneous heat-loss rate is the U-value times the area times the temperature difference — the standard fabric heat-loss equation. Multiplying that rate by the effective heating hours over a year and dividing by 1000 gives the annual energy lost in kilowatt-hours. Better glazing lowers the U-value, which cuts both figures in direct proportion.
What you enter
Enter the glazing area, U-value, temperature difference and heating hours; the calculator returns the heat-loss rate in watts and the annual energy in kWh. Every field is editable, and the result recalculates the moment you change a value.
Reading the result
The watts figure is the steady rate on a design-cold day; the annual kWh is the running total you actually pay for. Halving the U-value halves both — which is the case for upgrading single to double, or double to triple, glazing.
Safe by design
This tool sizes glazing, cost and thermal performance — not the structural opening. Sizing a lintel or a load-bearing opening is a structural engineer’s calculation, and this page never attempts it.
Assumptions and limits
This models conduction through the glass only. It ignores solar gain (which offsets some loss on sunny days), draughts, and the fact that the temperature difference varies through the season. The heating-hours figure is your own estimate of equivalent full-load hours.
Using it with the rest of BuildMetricLab
This calculator is one step in planning the job. Pair it with the related tools in this category and across BuildMetricLab to go from a single figure to a full costed plan — a sizing figure feeds a cost estimate, a cost estimate feeds a project budget, and each result carries straight into the next tool. Every BuildMetricLab tool runs entirely in your browser — no sign-up, nothing sent to a server, and the formula is shown on the page so the maths can be audited.
Sources & methodology
Works out the rate of heat loss through glazing in watts and the annual energy in kWh, from the glazing area, U-value and the indoor–outdoor temperature difference. Every result is calculated from the values you enter, all inputs are editable, and no time-sensitive prices or rates are embedded.
Frequently asked questions
What temperature difference should I use?
A design difference of around 15–20°C represents a cold day. For an annual estimate the tool uses effective heating hours to average out milder periods.
What are heating hours?
Equivalent full-load hours are a way to turn a design heat-loss rate into an annual figure. Around 2,000–3,000 is common for a heated home; use your own.
Does solar gain reduce this?
Yes — on sunny days glazing gains heat as well as losing it. This tool models conduction loss only, so treat it as the gross figure.
How much does better glazing save?
Loss is proportional to U-value, so halving the U-value (for example by upgrading to a better-performing unit) roughly halves the conduction loss, all else equal.
Spotted something off?
Calculations or display — let us know.