A family uses 8 kW of power.

(a) Direct solar energy is incident on the horizontal surface at an average rate of 200 W per square meter. If 20% of this energy can be converted to useful electrical energy, how large an area is needed to supply 8 kW?

(b) Compare this area to that of the roof of a typical house.

Solar energy received per square metre = 200 W 
Efficiency of conversion from solar to electricity energy = 20 %
Area required to generate the desired electricity = A 

We have
8 × 10³ = 20% × (A × 200)
             = (20 /100) × A × 200 
 
∴        A = 8 × 10³ / 40  =  200 m²
(b) 
The area of a solar plate required to generate 8 kW of electricity is almost equivalent to the area of the roof of a building having dimensions 14 m × 14 m. (≈ √200).