1. The problem statement, all variables and given/known data A hot water heater is operated by using solar power. If the solar collector has an area of 6 m^2, and the power per unit area delivered by sunlight is 1000 W/m^2, how long in hours will it take to increase the temperature of 1 m^3 of water from 20C to 60C? (1 cal=4.186 J) 2. Relevant equations I'm not really sure... I know that: power=(sigma)AeT^4 sigma = Stefan -Boltzman constant = 5.6696x10^(-8) W/m^2*K^4 A = surface area e = emissivity T = surface temp. but we weren't given emissivity so i dont think I use this... maybe mc(Tf-Ti)/P = (change in time) ?? I'm really clueless on what equation to use, please help!!