(3) Multiple correlation coefficients

Ryx1x<sup>2</sup> = 0.966 (strong, positive, direct relationship) characterizes the density (tightness) of the relationship between the biogas output from different agricultural crops and the proportion of dry substance in crops, and the potential for methane production from different agricultural crops.

Checking the closeness of the general relationship (influence) of independent variables on the dependent variable using the coefficient of determination obtained the following results:

(1) Multiple

Dyx1x<sup>2</sup> = 99.24% means that the variation in biogas yield from different types of agricultural crops by 99.24% depends on two factors: x<sup>1</sup> (share of dry substance in crops) and x<sup>2</sup> (potentially possible production of methane from agricultural crops), and 0.76% due to the impact of unaccounted random factors.

(2) Partial coefficients of determination

dyx1 = 25.44% means that the variation in biogas yield from different types of agricultural crops by 25.44% depends on the factor x<sup>1</sup> (share of dry substance in agricultural crops), provided that the factor x<sup>2</sup> (potentially possible production of methane from agricultural crops) is eliminated;

dyx2 = 73.80% means that the variation of biogas yield from different types of agricultural crops by 73.80% depends on factor x<sup>2</sup> (potentially possible production of methane from agricultural crops), provided that factor x<sup>1</sup> (share of dry substance in agricultural crops) is eliminated.

Fisher's (F) and Student's (t) criteria were used to assess the reliability of correlation characteristics.

As a result of checking the multiple correlation coefficient for materiality using Fisher's F-criteria, its following value was obtained F<sup>R</sup> <sup>2</sup> = 854.3, which is greater than its tabular value Ftabular = F0.05 (2;15) = 3.68. This means that we reject the null hypothesis and accept the alternative (H0: R<sup>2</sup> <sup>6</sup><sup>=</sup> 0)—the multiple correlation coefficient is significant.

As a result of estimating the significance of partial regression coefficients using Student's t-criteria, the following results were obtained: ta1 = 17.83, which is greater than its tabular value ttabular = t0.05 (2;21) = 2.13. This means that we reject the null hypothesis and accept the alternative (H0: ta16=0)—partial regression coefficient ta1 is significant. ta2 = 47.28, which is greater than its tabular value ttabular = t0.05 (2;21) = 2.13. This means that we reject the null hypothesis and accept the alternative (H0: ta26=0)—partial regression coefficient ta2 is significant.

To estimate the relationship between the effective sign and its factors, a multiple correlation index of 0.966 was calculated. This means that the link between biogas output from different agricultural crops and the proportion of dry substance in crops, and the potential for methane production from different agricultural crops, is strong and functional, as η = 0.966 → 0.

Therefore, we consider the results of the study as an argument in favor of further development of biogas production in Ukraine from various types of organic biomass produced by the agricultural sector in order to strengthen energy security. This issue is especially relevant in modern military cataclysms, threats to the energy security of many countries, and the accession of the Ukrainian energy system to the European one. The results of the study are also relevant for other countries where they have issues in the transition of bioenergy.

**Author Contributions:** Conceptualization, N.T. and T.M.; methodology, N.T. and V.N.; validation, N.G., O.T. and O.N.; formal analysis, T.M. and V.N.; writing—original draft preparation, N.T. and N.G.; writing—review and editing, T.M. and V.N.; visualization, O.T. and O.N.; supervision, N.G. and O.T. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Data Availability Statement:** The data presented in this study are available on request from the corresponding author.

**Conflicts of Interest:** The authors declare no conflict of interest.
