Heat Loss Calculator
About This Heat Loss Calculator
This heat loss calculator estimates how much heat escapes through a building surface such as a wall, window, roof, or floor. By entering the surface area, U-value (thermal transmittance), and the temperature difference between inside and outside, you can see how quickly energy is lost. With optional inputs for heating hours and energy price, the tool also converts that heat flow into daily energy use and heating cost.
Use it when you want to:
- Compare the heat loss from different components (e.g., a wall vs. a window).
- Estimate potential savings from better insulation or new windows.
- Get a rough idea of how heat loss contributes to your heating bill.
- Understand how climate (temperature difference) affects energy demand.
How the Heat Loss Formula Works
The calculator is based on a steady-state conduction model through a flat surface. The core equation is:
Q = A × U × ΔT
- Q = heat loss rate (watts, W)
- A = surface area (square metres, m²)
- U = thermal transmittance (W/m²·K)
- ΔT = temperature difference between indoors and outdoors (kelvin or °C)
Because a difference of 1 kelvin is equal to a difference of 1 °C, the formula works the same whether you express the temperature difference in kelvin or degrees Celsius.
In more formal notation, the same relationship can be written using MathML:
Interpreting the units:
- A has units of m².
- U has units of W/m²·K.
- ΔT has units of K or °C.
When you multiply these, the m² and K cancel out, leaving watts (W), which is a measure of power – the rate at which energy is being lost at that moment.
From Watts to kWh and Heating Cost
The raw heat loss rate Q tells you how many watts of heat are flowing through the surface continuously under the assumed temperature difference. To connect this to your heating bill, it is helpful to convert watts to kilowatt-hours (kWh) over time.
The calculator does this in two steps:
- Compute daily energy use: Energy (kWh/day) = Q (W) × hours per day ÷ 1000.
- Compute daily cost: Cost per day = Energy (kWh/day) × energy price ($/kWh).
So if your heat loss is 500 W and you heat for 24 hours per day, you use:
500 W × 24 h ÷ 1000 = 12 kWh/day
If your tariff is $0.12/kWh, the associated cost is:
12 kWh/day × $0.12/kWh = $1.44 per day
Typical U-Values for Common Building Elements
U-value indicates how easily heat flows through a building component. Lower U-values mean better insulation and less heat loss. The table below shows approximate ranges for typical constructions. Actual values vary by design, thickness, and materials, so treat these as rough guides only.
| Building element | Typical U-value range (W/m²·K) | Insulation performance |
|---|---|---|
| Uninsulated solid brick wall (older building) | 1.5 – 2.0 | Poor insulation, high heat loss |
| Older double-glazed window | 2.5 – 3.0 | Relatively high heat loss |
| Modern double-glazed window | 1.2 – 1.6 | Moderate insulation |
| Modern triple-glazed window | 0.7 – 1.0 | Good insulation |
| Well-insulated timber-frame wall | 0.15 – 0.25 | Very good insulation |
| Insulated roof / attic | 0.10 – 0.20 | Very good insulation |
When entering a U-value in the calculator, you can use product data from manufacturers, building regulations, or energy certificates. If you do not know the exact value, choose a value from the range that best matches your construction and age of building.
Factors Affecting U-Value
Several design choices influence the U-value of a building element:
- Wall construction: Solid masonry walls without insulation have high U-values, while cavity walls or timber-frame walls filled with insulation have much lower values.
- Window glazing type: Single glazing has the highest U-values. Double and triple glazing, gas fills (argon, krypton), and low-emissivity coatings reduce U-values.
- Roof and attic insulation: Adding mineral wool, cellulose, or rigid foam boards over ceilings or under roofs significantly lowers U-values.
- Thermal bridges: Structural elements that penetrate insulation (e.g., concrete beams, balcony slabs, metal studs) locally increase heat flow and raise the effective U-value.
- Air films and finishes: Internal linings, external cladding, air layers, and surface resistances also influence the overall U-value, although their impact is usually smaller than the main insulation layer.
Choosing a Temperature Difference (ΔT)
The temperature difference ΔT is typically taken as indoor temperature minus outdoor temperature. For example, 20 °C indoors and 0 °C outdoors gives ΔT = 20 °C.
Here are some indicative values:
- Mild winter day: indoor 20 °C, outdoor 10 °C → ΔT ≈ 10 °C
- Cool winter day: indoor 20 °C, outdoor 0 °C → ΔT ≈ 20 °C
- Cold winter day: indoor 21 °C, outdoor −5 °C → ΔT ≈ 26 °C
- Very cold winter day: indoor 21 °C, outdoor −15 °C → ΔT ≈ 36 °C
For heating design, engineers often use a relatively large ΔT that reflects a worst-case winter condition. For quick cost estimates, you might use a more moderate value that better reflects your typical winter temperatures.
Worked Example
Consider a window with the following properties:
- Area A = 10 m²
- U-value U = 1.5 W/m²·K
- Temperature difference ΔT = 25 °C (e.g., 20 °C inside, −5 °C outside)
- Heating hours per day = 24
- Energy price = $0.20/kWh
Step 1 – Heat loss rate:
Q = A × U × ΔT = 10 × 1.5 × 25 = 375 W
This means that, under these conditions, the window loses 375 joules of heat every second.
Step 2 – Daily energy use:
Energy = 375 W × 24 h ÷ 1000 ≈ 9.0 kWh/day
Step 3 – Daily cost:
Cost = 9.0 kWh/day × $0.20/kWh = $1.80 per day
If you replaced this window with a better unit with a U-value of 0.9 W/m²·K, keeping everything else the same, the new heat loss rate would be:
Q = 10 × 0.9 × 25 = 225 W
Daily energy use would drop to 225 W × 24 h ÷ 1000 ≈ 5.4 kWh/day, costing about $1.08 per day at the same tariff. The improvement saves roughly 3.6 kWh/day, or about $0.72 per day in this simplified example.
Interpreting the Results
When you run the calculator, you will typically see:
- Heat loss (W): the instantaneous rate at which heat leaves the chosen surface.
- Daily energy use (kWh/day): how much energy your heating system must supply per day to compensate for this heat loss, assuming constant conditions and heating hours.
- Estimated cost per day: an approximate contribution of that surface to your daily heating bill.
To put watt values into context, many small electric space heaters are rated around 1000–2000 W. If a single poorly insulated window loses 400 W in winter, that is equivalent to running roughly a quarter to half of a small heater continuously, just to offset the losses through that one component.
Remember that your total building heat loss is the sum of all surfaces (walls, roof, floor, windows, doors) plus ventilation and infiltration losses. To approximate whole-building conduction losses, you can repeat the calculation for each surface and add the results.
Comparison: High vs. Low U-Value Surfaces
The table below compares two example surfaces with the same area and temperature difference but different U-values, so you can see how insulation quality changes energy use and cost.
| Scenario | Assumptions | Heat loss (W) | Daily energy (kWh/day) | Daily cost at $0.15/kWh |
|---|---|---|---|---|
| Poorly insulated wall | 20 m², U = 1.5 W/m²·K, ΔT = 20 °C, 24 h/day | 600 W | 14.4 kWh/day | $2.16/day |
| Well-insulated wall | 20 m², U = 0.20 W/m²·K, ΔT = 20 °C, 24 h/day | 80 W | 1.9 kWh/day | $0.29/day |
In this simplified comparison, upgrading the wall from a U-value of 1.5 to 0.20 cuts the conduction losses by more than 85%, reducing both energy use and heating costs significantly over a full heating season.
Model Assumptions and Limitations
This calculator is designed as a simple, transparent tool. To keep it easy to use, it relies on several assumptions:
- Steady-state conditions: It assumes temperatures and heat flows are constant over time. In reality, outdoor temperature, wind, and solar radiation vary throughout the day.
- Uniform U-value: It treats each surface as having a single U-value. Actual constructions may have thermal bridges, frames, or junctions with higher local heat flow.
- Conduction-dominated losses: The calculation focuses on conduction through solid elements. It does not explicitly account for ventilation, air leakage (infiltration), or complex radiant effects.
- No solar or internal gains: Sunlight, occupants, appliances, and equipment that add heat to the building are not included. These can offset part of the heating demand.
- Single-surface analysis: Each run applies to one surface at a time. Whole-building estimates require summing results for multiple components and adding ventilation losses using other methods.
Because of these simplifications, results should be viewed as estimates for comparison and planning. For code compliance, detailed retrofit design, or high-stakes decisions, consult a building energy professional or use a more comprehensive energy modelling tool.
Using the Calculator for Planning
Here are some practical ways to make use of the results:
- Identify high-loss components: Try typical U-values for your walls, roof, and windows with realistic areas. Components with the highest heat loss are often the best candidates for upgrades.
- Prioritise insulation measures: Compare the reduction in kWh/day for different upgrades (e.g., wall insulation vs. new windows) to understand which offers the greatest impact.
- Estimate seasonal savings: Multiply the daily savings in kWh by the number of heating days per year to get an approximate annual impact, then multiply by your tariff to estimate cost savings.
- Check sensitivity to climate: Try smaller and larger ΔT values to see how a colder or milder climate would change the heat loss and heating demand.
Remember that comfort, condensation risk, and building durability are also important considerations when planning energy efficiency upgrades. This calculator focuses only on the heat loss and cost side of the picture.
Next Steps
After you have explored how different U-values, areas, and temperature differences affect heat loss, you may want to sum results from multiple surfaces to get a rough total for your building envelope. Combining this with information about ventilation and infiltration will give a more complete view of your heating needs.
For more detailed guidance, consider consulting local building regulations, manufacturer data sheets for windows and insulation products, or professional energy audits that can account for three-dimensional heat flow, air leakage, and dynamic weather conditions.
Used thoughtfully, this simple calculator can highlight where improvements to insulation and glazing will have the biggest effect, and how small improvements in insulation can compound into substantial savings over a full heating season.
| Surface area | |
|---|---|
| U-value | |
| Temperature difference | |
| Instantaneous heat transfer | |
| Daily energy | |
| Daily cost |
Heat loss is a major factor in determining how much energy a building consumes. In the winter, warm air inside naturally seeks to escape through walls, windows, and ceilings. In the summer, the heat tries to creep in, forcing air conditioners to work harder. The rate at which heat flows through a material is governed by its thermal conductivity, and when builders design structures, they combine layers of insulation, drywall, and exterior cladding to achieve a specified thermal transmittance, commonly referred to as the U-value. A low U-value means the material is a good insulator, while a high U-value indicates poor resistance to heat flow.
This calculator is a straightforward way to estimate steady-state heat loss using that U-value. By specifying the area of a surface and the temperature difference across it, you get a quick approximation of the energy required to maintain comfort. For example, suppose a wall has an area of 20 square meters and a U-value of 0.4 W/m²·K. If the indoor temperature is 20°C while it’s 0°C outside, the difference is 20 degrees. Multiplying 20 by 0.4 by 20 yields 160 watts. That means every hour, 160 watt-hours of energy are needed to offset the heat leaving through that section of wall alone.
Understanding heat loss at this level helps homeowners and engineers plan energy efficiency improvements. Replacing old windows with double-pane or triple-pane units lowers the U-value dramatically, translating to reduced heating costs. Likewise, additional insulation in the attic or walls reduces the amount of energy required to maintain a comfortable indoor temperature. By calculating heat loss before and after making upgrades, you can determine the potential savings in fuel or electricity.
The beauty of this model is its simplicity. Though real-world heat flow involves factors like convection, radiation, and air leakage, the A×U×ΔT equation provides a reliable baseline. Builders often use it during the design phase to comply with energy codes, which specify maximum U-values for roofs, walls, and floors. Even if you aren’t a professional, understanding the metric lets you evaluate the efficiency claims of windows, doors, and insulation products on the market.
When you use this calculator, keep in mind that U-values are the inverse of R-values, another common metric in building science. While U-value measures how much heat flows through a material, R-value measures how much the material resists heat flow. A higher R-value corresponds to a lower U-value, indicating greater thermal resistance. This tool expects the U-value as input, but if you know the R-value instead, you can quickly convert it with U = 1 / R.
Another practical consideration is that this calculation assumes a uniform temperature difference across the surface. In a home, the interior may be heated unevenly, or there may be thermal bridges—areas where studs, beams, or other materials conduct heat faster than the surrounding insulation. The model doesn’t account for these intricacies, but it’s still very useful for comparing different materials or approximating energy requirements when precision isn’t critical.
For more accurate energy modeling, engineers often turn to detailed simulation software, which breaks down a building into many components and accounts for ventilation, infiltration, solar gain, and occupant behavior. Those tools can predict seasonal energy consumption with high precision, but they require significant data about the building’s geometry and materials. In contrast, this calculator aims to be lightweight and fast, ideal for quick estimates or educational purposes. If you’re planning a renovation or simply want to understand how much difference new insulation could make, try adjusting the U-value and temperature difference to see how the heat loss changes.
Heat loss isn’t just a concern for houses. Industrial facilities, warehouses, and commercial buildings all spend huge amounts of money on heating and cooling. Even small improvements in insulation or air sealing can lead to significant savings over time. Energy auditors often use handheld sensors to measure surface temperatures, then calculate heat loss to pinpoint problem areas. By comparing the predicted energy use with actual utility bills, they can determine whether additional improvements are cost-effective. This approach is becoming increasingly popular as more jurisdictions enforce building energy standards to combat climate change.
Although this calculator focuses on conduction through building envelopes, the same principles apply to a variety of other systems. Refrigerated transport trucks need insulation to minimize heat gain from the environment. Cold storage facilities must calculate heat loss to size their cooling equipment. Even pipes carrying hot water can benefit from insulation to reduce energy waste. In each case, the underlying formula is virtually identical—multiply the area by the U-value and temperature difference to find the heat transfer rate.
Whether you’re a homeowner curious about your heating bill or an engineer sizing insulation for a new project, understanding heat loss helps you make informed decisions. Thermal comfort and energy efficiency are intertwined, and with rising fuel costs, a well-insulated building pays dividends year after year. This calculator gives you a practical starting point for exploring how improvements in your building envelope could reduce energy consumption. Try experimenting with different U-values and areas to see how your heat loss changes. Each small step toward reducing energy waste contributes to a more sustainable and cost-effective future.
The new daily energy and cost outputs make it easy to translate thermal performance into budget terms. After seeing the wattage of a wall, you can instantly gauge how much that surface adds to your utility bill. Planning a renovation becomes less abstract when you know that upgrading a 50 m² wall could save several dollars each day during cold snaps.
Seasonal projections extend this insight further. If you heat for six months of the year, multiplying the daily cost by roughly 180 days reveals the annual burden of a specific surface. Combining results from multiple surfaces—walls, windows, and roofs—helps prioritize which upgrades deliver the largest financial payoff.
Because energy prices and climates vary, the calculator encourages experimentation. Adjust the energy cost field to reflect current utility rates or future scenarios where prices rise. Likewise, modifying the heating hours allows you to model setback thermostats or intermittent heating schedules. By capturing these nuances, the tool supports more accurate budgeting and smarter efficiency investments.