In large-scale systems for domestic hot water supply with and without heating support, the Proportionate Energy Savings, according to DIN CEN/TS 12977-2, are presented in the project report.

Proportionate Energy Savings = ($Q_{\text{conv}}$ - $Q_{\text{aux}}$) / $Q_{\text{conv}}$

$Q_{\text{conv}}$ represents the energy consumption of a conventional reference system. (Term in the standard: “Gross energy requirement of the reference system”)

$Q_{\text{aux}}$ is the conventional energy consumption of the simulated solar system, i.e., the energy supplied to the system by auxiliary heating. The standard states: “$Q_{\text{aux}}$ is the gross additional energy requirement of the solar heating system to meet the required heating demand.” The same boiler efficiency as in the conventional system is assumed.

When calculating $Q_{\text{conv}}$, circulation losses are taken into account. Considering circulation in the simulation (Parameters > Hot Water Consumption > Circulation present), $Q_{\text{aux}}$ becomes larger because the auxiliary heating supplies more energy to the system than it would without circulation.

The Proportionate Energy Savings are equal to 1 or 100% when the additional energy requirement $Q_{\text{aux}}$ is equal to 0. In this scenario, all energy was provided by the solar system, and auxiliary heating was inactive throughout.

The Proportionate Energy Savings turn negative when the additional energy requirement $Q_{\text{aux}}$ exceeds $Q_{\text{conv}}$.

Additionally, the following holds:

$ Q_{\text{conv}} = \frac{Q_{\text{conv.net}}}{\eta_{\text{conv}}}, $ where
$ \eta_{\text{conv}} = $ efficiency of the reference system,
$ Q_{\text{conv.net}} = $ net energy requirement in [Wh].
$ Q_{\text{conv.net}} = Q_{\text{Hzg}} + Q_{\text{ww}} + Q_{\text{sp.conv}}, $ and
$ Q_{\text{Hzg}} = $ energy requirement for heating,
$ Q_{\text{ww}} = $ energy requirement for hot water,
$ Q_{\text{Sp conv}} = $ energy requirement for the storage tank $= 0.16 \times \sqrt{\text{Storage volume}} \times \Delta T \times \text{Operating hours} $.

The temperature difference ΔT is derived from the difference between the storage tank temperature and the ambient temperature of the tank, typically amounting to 30 K.