The collector loop efficiency is defined as follows:

Collector loop efficiency = $\frac{\text{Energy delivered by the collector loop through the heat exchanger}}{\text{Energy incident on the collector surface (reference area)}}$

The system efficiency is defined as follows:

System efficiency = $\frac{\text{Energy delivered by the solar system}}{\text{Energy incident on the collector surface (reference surface)}}$

The energy released by the solar system consists of the energy transferred from the solar storage tank to the standby tank. In some systems, like the single storage tank model, the system efficiency cannot be determined, as there is no distinction between the solar and standby tanks. The storage losses are, therefore, considered as part of reheating.

The solar fraction is defined as follows:

Solar Fraction = $\frac{\text{Energy supplied to the storage tank by the solar system}}{\text{Sum of energies supplied to the storage tank (solar system + auxiliary heating)}}$

For a solar system with a bivalent storage tank for domestic hot water heating and heating support:

For a solar system with a combined storage tank, the calculation is as follows:

Energy Delivery for Domestic Hot Water Heating

The energy delivery for domestic hot water heating is the energy needed to heat the cold water to the temperature of the drawn domestic hot water. Losses from the storage tank or circulation are not considered in this context.

Utilized Fuel

The utilized fuel is the amount of fuel needed to reheat the standby tank to the target temperature. This includes the heat losses from the storage tank and the boiler efficiency.