From GridLAB-D Wiki
The probability density function for the beta distribution is
<math>\varphi(x;a,b) = \frac{\Gamma(a+b)}{\Gamma(a)\Gamma(b)}x^{a-1}(1-x)^{b-1}</math>
where <math>a</math> and <math>b</math> are the parameters of the distribution and <math>\Gamma(z)</math> is the gamma function.