How the calculator works
The calculator estimates the annual heat demand for space heating using a simplified model derived from standard EN ISO 13790. It is intended as a first orientation and for comparing measures against each other - not as a substitute for an energy performance certificate or design documentation. All inputs are processed entirely in your browser and are never sent to the server.
The core equation
Heat losses through each element of the envelope are calculated as the product of thermal transmittance, area and the temperature deficit:
Q = U × A × HDD × 24 / 1000 (kWh per year)
QNH = Qtrans + Qvent − η × (Qsolar + Qinternal)
Here U is the thermal transmittance of the element (W/m²K), A its area, and HDD the heating degree days. For Ljubljana we use 3,100 K·day, based on ARSO data. Usable solar and internal gains are subtracted from the transmission and ventilation losses.
Building geometry
Since you enter only the heated floor area, the model derives a simplified two-storey geometry from it: the footprint is half the heated area, the side length is the square root of the footprint, storey height is 2.6 m, and the window area is 17 % of the heated area. Roof area is the footprint increased by 15 % to account for pitch. Because of this simplification, results are less accurate for atypical layouts such as very elongated or single-storey buildings.
Default U-values by building age
If you do not know the actual values, the model assumes typical figures for the construction period - for example a wall at 1.6 W/m²K before 1970 and 0.28 W/m²K after 2005, and windows at 3.0 and 1.3 W/m²K respectively. After a measure is applied, the values of the renovated element are used: facade 0.22 W/m²K (16 cm ETICS), roof 0.18 W/m²K (30 cm mineral wool) and windows 0.9 W/m²K (triple glazing).
Ventilation
Ventilation losses are calculated as Qvent = 0.34 × V × n × HDD × 24 / 1000, where 0.34 Wh/(m³·K) is the volumetric heat capacity of air and n is an air change rate of 0.5 h⁻¹, corresponding to the residential standard. Where a heat recovery system is selected, we apply 85 % net recovery - 90 % at the heat exchanger, reduced by fan electricity consumption.
Energy classes
Classification follows the Slovenian energy performance certificate scale, based on annual heat demand for heating (QNH): class A1 up to 10 kWh/m²a, A2 up to 15, B1 up to 25, B2 up to 35, C up to 60, D up to 105, E up to 150, F up to 210, and G above 210 kWh/m²a.
Energy prices and emission factors
Cost calculations use the following defaults: electricity €0.21/kWh, natural gas €0.10/kWh, and exported solar electricity €0.10/kWh. Emission factors are 0.23 kg CO₂/kWh for electricity, 0.20 for natural gas and 0.26 for heating oil. Primary energy factors are 2.5 for electricity, 1.1 for gas and heating oil, and 0.1 for wood biomass. Existing appliance efficiencies are assumed at 80 % for an older gas boiler, 78 % for oil, 100 % for electric heating and 75 % for wood.
Investment costs
Default prices are estimates for the Slovenian market in 2026: heat pump €7,500 (7 kW) or €10,000 (14 kW), windows €400/m², facade €55/m², roof insulation €50/m², mechanical ventilation with heat recovery €8,500, and solar €8,000 (5 kWp) or €12,600 (10 kWp). Payback is calculated as the ratio of investment to annual saving, without accounting for subsidies, inflation or discounting - with Eko sklad incentives applied, the actual payback will be shorter.
Known limitations
- The model does not treat thermal bridges individually; they are captured only indirectly through the default U-values.
- It does not account for actual occupant behaviour, which can change consumption by tens of percent.
- It does not account for shading, microlocation, the building's real airtightness or the condition of the heating distribution system.
- Energy and equipment prices change; the values given are a snapshot as of the last update.
For an accurate assessment of a specific building, consult an authorised energy adviser or an energy performance certificate assessor. Free, contractor-independent advice is available from the ENSVET network under the auspices of Eko sklad.