So if we start with question a) to calculate P = Q/t we can use that Q=m*c*ΔT so for for the 10 liters of water in the hot reservoir we get P=10*4186*25.8/1616=668 W. However it says the answer for a) is 667 W. Any idea why the difference? For question b) we have Vf = Pout / Pin where Pout = m*c*dT/dt dT/dt = a*t^-1 so Vf = m*c*a*t^-1 / Pin = 10*4186*0.0163 / 158 = 4.3 Here the number is correct but I wonder about something about the units. The variable a is said to have unit °C/s so when deriving with respect to time we should get °C/s^2 ? But in order for the answer to be correct the unit should rather be K/s in order for COP to be unitless? Please help how can this be cleared up?