(2008? Jiamusi) In Jixi City, where we live, the water of solar water heater can't reach the required temperature because of the short sunshine time in winter.

v = 100 l = 100× 10-3 m3，

Water quality:

m =ρv = 1× 103kg/m3× 100× 10-3 m3 = 100kg，

Heat absorbed by water:

Q suction =cm△t

=4.2× 103J/(kg？ ℃)× 100kg×(50℃-20℃)

= 1.26× 107j；

(2) If all electric energy is converted into heat energy, that is, Q absorption =W electricity,

The water heater is working normally.

∴P=P amount = 1500W,

∫P = W electricity t,

∴ Heating time:

T=W electricity p =1.26x107j1500w = 8400s =140min;

(3)∵W total = pt′= 1500 W×3×3600s = 1.62× 107j；

∴ Efficiency of water heater:

η=Q absorption w total =1.26×107j1.62×107j ≈ 77.8%;

(3) The electric energy consumed by the electric water heater is converted into internal energy, which is absorbed by the water, thus increasing the temperature of the water; Solar water heaters convert solar energy into internal energy, saving energy (electricity).

Answer: (1) The solar water heater is full. When the water temperature is heated from 20℃ to 50℃, it needs to absorb1.26×107j;

(2) The heater needs to work normally140min; ;

(3) The efficiency of the water heater is about 77.8%;

(4) Save energy.