About Energy Efficiency Calculator
The energy efficiency calculator analyses a whole building envelope to estimate its thermal performance, heating and cooling loads, annual energy use, running cost, and carbon footprint. It is used by energy auditors, green building consultants, and designers to compare options and prioritise upgrades.
Define the envelope (walls, roof, floor, windows, doors), choose a climate zone, and enter energy prices; the tool computes the area-weighted U-value, the transmission, ventilation, and infiltration losses, the solar and internal gains, and the resulting annual consumption, cost, and CO2 with ranked improvement recommendations.
How It Works
- Compute the area-weighted envelope U-value and total UA from each element U-value and area.
- Calculate heating and cooling loads from transmission, ventilation, and infiltration plus internal and solar gains.
- Model monthly energy use by applying load factors and dividing by the HVAC system efficiency (COP).
- Convert annual energy into cost and carbon, and rank improvement measures by payback period.
Worked Example
Transmission loss through a 100 m2 wall at U = 0.3 W/m2K with a 20 C indoor and 0 C outdoor (deltaT = 20 K) is Q = 0.3 * 100 * 20 = 600 W, which feeds the building heating load.
Formulas
- Transmission heat loss
Q = U * A * (Ti - Te)- Ventilation heat loss
Q = m_dot * cp * (Ti - Te)- Delivered energy
E = Load * hours * f / COP
Standards & References
- ASHRAE 90.1
- EN ISO 13790
- ISO 6946
Frequently Asked Questions
What does the energy efficiency calculator estimate?
It estimates the envelope U-value, peak heating and cooling loads, annual energy consumption, operating cost, and carbon footprint, plus a ranked list of improvement measures with payback periods.
How does the COP affect energy use?
Delivered energy equals the thermal load divided by the system efficiency (COP), so a heat pump at COP 3.5 uses roughly a third of the energy a direct-electric system would for the same load.
How is the carbon footprint calculated?
Annual delivered energy by fuel is multiplied by the relevant emission factor to give operational CO2, which the tool reports in tonnes per year alongside the running cost.
Is the energy efficiency calculator free to use?
Yes, it runs entirely in your browser at no cost and stores none of your input data.