An experimental solar array consists of two independent panels, $S_1$ and $S_2$, that generate power during daylight. The probability $S_1$ is functional is $p_1$, and the probability $S_2$ is functional is $p_2$. If the total power output is the sum of the power of functional panels, and we observe that the total power output is strictly greater than zero, what is the probability that both panels are functional?