## 2 Method
### Evaluation of Single Action
The evaluation of renewable energy-producing systems requires further development. It is proposed that the cost parameter for investments in renewable energy production is with the same unit as the CEP of investments in energy efficiency. This is done with the aim of obtaining grounds for direct comparison of cost-effectiveness between energy-producing systems and energy-efficiency improvements. The evaluation cannot be completely the same, as investments in energy efficiency yield energy savings, while investments in renewables result in the generation of energy. Therefore, they are contrasting by definition and require different considerations for estimating their benefits and costs. Operation and maintenance cost throughout the lifetime of an energy-producing system can add up to a considerable amount, compared to the cost of purchasing and installing the system [40]. Therefore, operation and maintenance cost are included in the cost-effective evaluation. Furthermore, different types of systems run on different fuel, which is with distinct cost. Thereby, fuel costs are estimated on an annual basis considering the fuel type and the amount necessary to cover the demand of the building. For heating systems other than district heating connection and electricity, it is necessary to use a calorific value of the fuel for calculating the required total amount. Examples of such fuels are natural gas, coal, wood, and its by-products. The analysis in this paper considers only natural gas with a calorific value of 11.36 kWh heating per m\({}^{3}\) of gas. Still, CEP for renewable energy systems is also calculated using Equation (1), and the annual investment includes the aforementioned cost types, while the energy is produced instead of saved. In a similar manner, when energy storage systems are evaluated, the stored amount of energy is used instead of the produced energy. The analysis in this paper accounts for energy storage only when it is an inseparable part of a system and not a separate energy-saving measure. Therefore, energy storage options are not discussed at the individual level in this paper. To obtain an estimate for the investment cost of an energy-producing system, it is also necessary to know the capacity (or size) of the system. In the present work, the necessary capacity is determined by the reference calculation during the project definition stage. In principle, a system dimensioned according to the demand of the building prior to energy renovation would be able to cover the building's energy demand without implementing further energy-efficiency measures. By implementing energy-efficiency measures to the building, a reduction of the energy demand is achieved. As a consequence, it may be possible to reduce the required system size and with that its investment cost. In cases where there is no necessity for the implementation of energy-efficiency actions, but the heating system needs replacement, it would be appropriate to dimension the new system in accordance to the reference demand. All considered systems in this paper are based on the reference demand with the aim to obtain a relative comparison between the options. The specific system dimensioning and resulting costs can be taken into consideration in the next stage when all renovation actions are selected. The two previous paragraphs relate to systems that deliver energy on demand throughout the year (e.g., district heating connection, heat pumps, boilers on varying fuel type). Those are an inseparable part of a residential building and for ease are further referred to as the main energy systems. While there are some cases where the only required renovation measure is changing the main system, those are quite few. Moreover, given that all the main systems in this paper are dimensioned with the same energy demand, evaluation of CEP for main systems becomes redundant, as it is dictated by implementation and fuel cost. Instead, all main systems are evaluated by LCC both individually and in combination with secondary (renewable) energy systems.