## 4 Results
### Energy Storage System Requirements
The energy capacity requirement for ESS decreased with decreasing length of the ISP time when using the full one year observation period of 2015, but the power capacity requirement did not. The effect of the observation period length on the energy capacity requirement was studied more closely to particularize these findings. The required energy capacities are shown in Figure 4, when the observation period is one day and one month before the observation point. The highest energy capacity requirements for 15 and 60 min ISP times and observation period of one month are quite close to the values based on the data of one year shown in Table 1, which confirms that measurement data for this time period provides reliable overall results. However, variations along the year are considerable and the seasonal changes are clearly visible, i.e., energy capacity requirement during winter is less than during summer, as expected. The overall energy capacity requirement also decreases slightly with shortening observation period and only few occasions cause high requirements while using one day observation period. This implies that much lower ESS energy capacities can be used in practice than indicated in Table 1 to meet the electricity market requirements for most of the year. If some occasional power delivery failures to the electricity market are allowed or could be foreseen beforehand to make corrective market action, considerable savings in sizing ESS can be achieved. Figure 4: Required ESS energy capacities for (**a**) 60 min and (**b**) 15 min ISP times shown at the end of the observation periods of 1 day and 1 month for the 0.1 MW PV plant in 2015. The probability density distributions of the ESS energy capacity requirements for the studied ISP times using observation periods of the ISP time (15 min or 60 min), one day and one month are shown in Figure 5. The distributions for both ISP times are qualitatively similar, only the energy scales are different. Most of the time, the ESS energy capacity requirement is lower than the required highest energy capacity and, in practice, only half of the required maximum capacity is enough for over 99.5% of time of the year. Therefore, the required energy storage capacity could be smaller if the peak periods, i.e., periods of plentiful power fluctuations, could be recognized beforehand. Then, a tiny portion of the energy yield could be sacrificed to meet the electricity marked terms with a smaller ESS. The distributions of one day and one month observation periods in Figure 5 show how the occasions requiring high ESS energy capacity are distributed among days and months of the year. Roughly half of the days and months cause the high requirements taking place around the summertime (Figure 4). The occasions that cause high ESS energy capacity requirements take place very seldom and quite randomly around the year. Fluctuations taking place in the wintertime cause most of the low requirements as expected. Because the maximum energy capacity requirements are set by power fluctuations occurring during the summer season, results are also applicable to similar climatic conditions, i.e., most of Europe [15]. Probability density distributions of ESS power capacity requirements for the studied PV generators and ISP times are show in Figure 6a. Distributions seem to be almost equal for the different PV generator sizes, only a clear difference in magnitude exists between the 15 and 60 min ISP times, which is in line with previous observations. As was demonstrated already in Figure 5 for the ESS energy capacity requirement, only few occasions cause high ESS power capacity requirements, but they are distributed around the year. Therefore, the results regarding power capacity requirements are applicable to PV systems located in corresponding climatic conditions, i.e., most of Europe [15]. The probability density distributions of the ESS power capacity requirement, PV generator output power and the grid feeding power of the 10 MW PV generator are shown in Figure 6b for 60 min ISP time and observation period.