# Determining Signatures for Energy Mix Produced by Photovoltaic Systems and Wind Turbines
## 4 Discussion
A well-balanced energy grid powered by renewable energy sources is the best solution to power factories producing low-emission hydrogen for various applications [78]. The signatures of energy produced in the energy mix also allow for determining the need for additional balancing in the form of energy storage systems, water turbines, and microturbines [79]. The article uses the clustering method, known for over 20 years. However, it was used in a practical way to determine the signatures of the photovoltaic-wind energy mix, which constitutes the originality of the work. Rarely found in the literature is the presentation of the entire computational algorithm related to the acquisition of measurement data, their processing using innovative tools, and an expert validation of the obtained results indicating their practical importance in balancing energy produced by various renewable sources. It is worth emphasizing that in the preliminary studies, the authors perform traditional statistical analysis of time series of generated power. The histograms and box plots performed provide the necessary knowledge about the analyzed processes. Learning the characteristics of each energy source is necessary for the effective construction of energy mixes. In this way, the authors prove that it is possible to create energy mixes with specific signatures intended for supplying dedicated green energy receivers. The article does not use general power modeling methods for individual energy sources, but an analysis of specific cases based on real measurement data. The analysis concerns the exceptionally favorable conditions for energy production from photovoltaic systems and wind turbines on a nationwide scale, which takes place in the Lublin region in Poland. The research results support investment processes in the construction of new generating capacities in a specific region of a European country. The signatures of the generated power in the photovoltaic-wind energy mix were made for one location, i.e., for the Lublin region in Poland. Additionally, the research covers a period of only one month. The algorithm presented in the article has a universal application and can be used for photovoltaic-wind mixes all over the world. The signatures obtained from different combinations of two or more power generation sources precisely characterize the entire energy mix. The type of clusters and their size assessed on the basis of relative frequency provide important information on the complementarity and substitutability of some sources by others. Analysis of the frequency of the power generation process in individual time periods provides information on the need to balance the energy mix. Heat maps are very useful in this respect. For each energy mix, it is advisable to make signatures for all months. Based on such comprehensive studies, it will be possible to assess the efficiency of the energy mix, the possibility of its balancing, or the need to introduce changes. The energy mix signatures can be used to compare the properties of investments in two different locations. For this purpose, the authors recommend conducting clustering with labeling (supervised). Determining signatures of generated power in the photovoltaic-wind energy mix using clustering has its advantages and disadvantages compared to other methods. Compared to probabilistic analysis using the Metalog probability distribution family, it allows determining the complementarity and substitutability of the generated power of one source by another. The Metalog probability distribution family allows calculating the probability of generating individual power levels by the tested energy mix [32]. Clustering does not have such a possibility. Signatures of generated power by the energy mix and presentation of their results in the form of heat maps allow locating in time periods requiring the accumulation of overproduction of power and periods of its deficit when power must be taken from the power grid or energy storage systems.