v = 100 l = 100× 10-3 m3，

According to ρ=mV, the mass of water: m = ρ v =/kloc-0 /×103kg/m3×100-3m3 =100kg,

Heat absorbed by water:

Q suction =cm△t

=4.2× 103J/(kg？ ℃)× 100kg×(50℃-20℃)

= 1.26× 107j；

(2)P = Wt consumes W = Pt =1500 w× 3× 3600s =1.62×107j,

η=Q absorption w×100% =1.26×107j1.62×107j100% ≈ 77.8%;

(3) Dapeng's method of calculating efficiency is that the electric water heater works normally by default. If the electric water heater does not work under the rated voltage, the calculation method is unreasonable; Tucki's method does not explain that the use of electrical appliances always works alone, otherwise the method is unreasonable;

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 boiling efficiency calculated by the rated power of the water heater is 77.8%;

(3) Dapeng's method of calculating efficiency is that the electric water heater works normally by default. If the electric water heater does not work under the rated voltage, the calculation method is unreasonable; Tucki's method does not explain that the use of electrical appliances always works alone, otherwise the method is unreasonable.