We show that energy conservation arguments can be used to extract useful, simple and robust information from bolometric light-curves of different types of supernovae. This bypasses the difficulty and uncertainty in radiation transfer calculations. In particular, the average column density of type Ia ejecta can be derived and for many is inconsistent with Chandrasekhar explosions, the Ni56 contribution to type II supernovae can be separated and turns out to play a major role in the type IIL - IIP distinction, and some slow declining super-luminous supernovae are shown to be very likely powered by Ni56.