Extra variable systems, such as wind and solar photovoltaics, are typically applied Any time they are offered, in large part simply because their gas – sunlight and wind – is free.P R = V two × I = V R R + R C × V R + R C = R R + R C × V two R + R C = R R + R C P V displaystyle P_ R =V_ 2 situations I=V frac R R+R_ C situations frac V R+R_ C