# Cost-Benefit Analysis of Small-Scale Rooftop PV Systems: The Case of Dragotin, Croatia
## 3 Results and Discussion
### Payback Time  
The investment for a 3.6 kW PV power plant totals 64243.72. The PV system is connected to the electricity grid via a home installation without incentives. From the network 1456.92 kWh of energy is taken and 2754.14 kWh of energy is taken from the PV system. Into the network is injected 1804.32 kWh from the PV system. The price of electricity with VAT 13% [40] for a higher tariff is 0.139 \(\upepsilon\)/kWh, and for a lower tariff 0.076 \(\upepsilon\)/kWh. The annual cost of energy in case without a PV system (acc. Table 8) is: \begin{table}
\begin{tabular}{c c c} \hline
**Electricity** & **With PV System** & **No PV System** \\ \hline Higher tariff (kWh) & 1017.16 & 3710.92 \\ Lower tariff (kWh) & 439.75 & 500.13 \\ Total & 1456.91 & 4211.05 \\ To grid (kWh) & 1804.32 & \(-\) \\ \hline \end{tabular}
\end{table} 
Table 8: Data on energy consumption and production on an annual basis. \begin{table}
\begin{tabular}{c c c} \hline
**Net Metering Scheme** & **Grid Injection (kWh)** & **Grid Consumption (kWh)** \\ \hline
1 h & 1804 & 1457 \\
1 month & 1048 & 554 \\
1 year & 347 & 0 \\ \hline \end{tabular}
\end{table} 
Table 9: Energy injection to grid and energy consumption from grid for different net metering schemes. Figure 12: Electricity prices for household consumers, second half 2020 (EUR/kWh) [41]. Figure 13: Share of taxes and levies paid by household consumers for electricity, second half 2019 [41]. Profit for surplus electricity delivered to the grid: \(\xi\)46.60 Annual system maintenance \(\xi\)20. Total annual profit: \(378.60+46.60-20=\xi\)405.20 Return on investment time is calculated as the simple payback period (_SPP_):  
\[SPP=\frac{\text{Cost of investment ($\xi$)}}{\text{Annual profit ($\xi$/year)}} \tag{7}\]  
_SPP_= 4243.72/405.20=10.47 years (no incentive)  
In case of exercising the right to incentives based on the Law on the Fund for Environmental Protection and Energy Efficiency and the Statute of the Fund for Family Homes, it is possible to exercise the right to incentives up to 40% for the investment amount not exceeding \(\xi\)5006. The simple payback time with incentives is:  
\[SPP_{\text{i}}=\frac{(100-\text{incentives ($\%$)})}{100}\cdot\frac{\text{Cost of investment}}{\text{Annual profit}} \tag{8}\]  
\(SPP_{\text{i}}=(100-40)/100\times 4243.72/405.20=6.3\) years  
The main way to increase the cost-effectiveness of PV systems is to increase self-consumption. There are two ways to improve your own consumption, namely load management and energy storage. These methods can be applied separately or in combination. There are several definitions of load management, and common to all is the improvement of the energy system on the consumption side [42]. In this research, the term is used to time-shift the load in the household, for example, the use of washing machines, heating, ventilation, and air conditioning, from periods with lower energy production from PV systems to periods with higher production from PV systems. In this way, the periods of interaction with the distribution grid can be reduced. Timeshift of the load can be achieved manually, where people turn on electrical appliances when there is excess PV production, or automatically, which requires microprocessor control, and for more advanced systems, ambient temperature, and insolation weather forecasts [43; 44]. As an additional measure to improve the consumption profile, air conditioning can be installed, which would use the excess electricity for cooling in the summer, while it would be used for space heating in spring and autumn during colder days (see Figure 14). In this way, the cost of heating would be somewhat reduced.