P4. A single-story retail store wishes to supply its lighting requirements with batteries charged by photovoltaic cells. The PV cells will be mounted on the horizontal rooftop. The time averaged lighting requirement is 10W/m2; the annual average solar irradiance is 150 W/m²; the PV efficiency is 10%; the battery charging - discharging efficiency is 80%. What percentage of the roof area will the photocells occupy?