7. A passive solar house that is losing heat to the

outdoors at an average rate of 45,000 kJ/h is

maintained at 19°C at all times during a winter night

for 10 h. The house is to be heated by 50 glass

containers each containing 20 L of water that is

heated to 80°C during the day by absorbing solar

energy. A thermostat-controlled 15-kW back-up

electric resistance heater turns on whenever

necessary to keep the house at 22C. [MO2]

a. How long did the electric heating system run

that night?

b. How long would the electric heater run that

night if the house incorporated no solar

heating?