Based on the tank energy balance, net gain into the solar storage tank can be inferred from the time derivative of the average tank temperature. Using temperature at the tank wall under insulation as a surrogate for fluid temperature, sensor mounting is simple and the inferred gains are useful for solar water heater (SWH) diagnostics. Positive daytime gain is compared to solar gain computed from site-specific parameters for an assumed clear day. The solar storage tank loss coefficient is inferred from temperature decay at night and is compared to the value computed from tank description. Larger draws are evident as sharp drops in storage temperature. Analyses are embodied in a tool validated against directly measured gains. Inoperative single-family SWHs are easily detected, signaling the need for repair. Detection of system control issues and shading are exemplified. In large multi-family systems, frequent day-time draws will bias the comparison to expected solar gain.