I'm Lost...

Question

The photovoltaic array of a solar-powered calculator measures 30 mm by 10 mm and its efficiency in room lighting is 4%. What is the minimum light intensity, in watts per square meter, in which it will maintain a power output of one-tenth of a milliwatt?

Answer 1

The power output of an array is
output = efficiency * intensity * area
This makes sense because intensity is just the power of light per unit area, so you need to multiply it by the energy-absorbing area (area of the array) and by the efficiency of the array. We can solve this equation for intensity:
intensity = output/(efficiency * area)
Because we want our answer in watts per meter, we need to convert output to watts and dimensions to meters.
output = 0.1mW = 0.0001W
dimesions: 30mm by 10mm or 0.03m by 0.01m
Finally, we plug in and solve:
area = 0.03m * 0.01m = 0.0003m²
intensity = 0.0001W/(0.04 * 0.0003m²) = 8.3333 W/m²