Question

The water in the water heater is heated at 80 ° C overnight and its average temperature must not fall below 55 ° C. How much water should be in the reservoir to store 25 kWh of thermal energy? The reservoir's own thermal capacity should be insignificant.

Answer 1

1kWh = 3.6*10⁶J

then 25kWh = 25*3.6*10⁶ J = 9*10⁷J

c(water) = 4200 J/(kg* ⁰C)

Q = cmΔT

-9*10⁷J = 4200 J/(kg* ⁰C) *m* (55⁰C - 80⁰C)

m = 857 kg

Answer: 857 kg