Building Energy Demand Calculation

The energy demand of a building follows from an energy balance: heat losses through transmission and ventilation are offset against solar and internal heat gains. Whatever is not covered by gains must be supplied as heating demand; in summer a cooling demand arises analogously. The demand is calculated either steady-state as a monthly balance (DIN V 18599) or dynamically in an hourly building simulation. Which method is appropriate depends on the goal — statutory proof, plant sizing or comfort analysis.

What counts as a building’s energy demand?

Several demand quantities are usually distinguished:

• Heating demand $Q_H$ — the heat that must be supplied over the year to maintain the setpoint temperature.
• Cooling demand $Q_C$ — the heat to be removed to limit summer indoor temperatures.
• Useful energy demand for domestic hot water and, where relevant, lighting.
• Final energy demand — useful energy plus the losses and efficiency factors of the building services.

For the heating and cooling load (the peak power rather than the annual energy), temporal resolution is additionally decisive — it determines the sizing of the generators.

Two routes: steady-state and dynamic

The steady-state calculation to DIN V 18599 balances the energy demand month by month and is the mandatory method for statutory proof. It is robust and fast, but represents dynamic effects such as thermal mass, peak loads and indoor-temperature behaviour only approximately.

The dynamic building simulation solves the energy balance hourly (8760 hours of a climate year) and accounts for thermal mass, real weather and usage profiles and the control of the building services. It yields not only annual totals but also peak loads, indoor temperatures and comfort metrics. The distinction between the two methods is covered in detail in steady-state vs. dynamic.

What goes into the calculation

Regardless of the method, the same physical quantities enter the balance:

• Transmission losses through walls, windows, roof and floor slab (U-values, thermal bridges).
• Ventilation losses from infiltration and mechanical ventilation (with heat recovery).
• Solar gains through glazed areas, depending on orientation and shading.
• Internal gains from occupants, equipment and lighting according to the usage profile.
• Thermal mass of the components, which shifts gains in time and dampens peaks.

The quality of the result stands or falls with the input data — especially the usage and climate profiles.

Standards

The key references are DIN V 18599 (energy assessment, monthly balance) for statutory proof, DIN 4108-2 for summer heat protection and VDI 6007 as the basis for dynamic room models. For reliable peak loads and comfort statements, dynamic simulation is superior to the simplified methods.

Conclusion

A building’s energy demand always follows the same energy balance — the difference lies in the method. The steady-state monthly balance suffices for statutory proof; for peak loads, summer heat protection and comfort, there is no way around dynamic building simulation.