a 200 L electric water heater uses 15 kW of electric power. If the herter is 90% efficient, how long does it take to heat the water in the tank from 16 C to 61 C?

Answer 1

Please try to put your class subject in the subj column. "College" is totally meaningless.

time*heatrate*efficiency=200*Cwater*(61-16)
solve for time. Your heat rate is 15kJ/sec, Cwater= 4.8KJ/Kg, and the units on the 200 is kg

check the specific heat of water.