Incandescent lights, the Sun and all other light sources produce what scientists call a “spectrum” -- a spread of light wavelengths including long infrared waves, visible light, short ultraviolet waves and X-rays. Every source has a distinctive spectral pattern; for example, the Sun generates copious amounts of ultraviolet whereas an incandescent bulb produces very little. A solar cell responds to light wavelengths in different ways, converting some wavelengths to electricity while ignoring others. The cell roughly matches the Sun’s spectrum; it processes visible light colors but cannot use the longest infrared waves. Because the spectrum of an incandescent light is close to the Sun’s, a solar cell has no problem running on its light.
In addition to its spectral qualities, solar energy on a sunny day amounts to about 1,000 watts per square meter on the Earth’s surface. However, a typical solar cell receives only a tiny fraction of this because its size is only a few square centimeters. A standard incandescent light bulb produces between 40 and 100 watts total and has most of the energy in the longest infrared wavelengths. If you hold a solar cell a few inches from a light bulb, it will receive a similar amount of light as it does from the Sun; although the Sun is more powerful by far, the close distance of the incandescent lamp makes up for its smaller output.
Time and Voltage
The energy received by a solar cell from an incandescent light diminishes rapidly with distance. The less light that falls on the solar cell, the weaker its output. This has the effect of reducing the cell’s voltage and current. When current is reduced, it takes longer to charge a battery. If the cell’s voltage is lower than a minimum threshold value, it becomes impossible to charge the battery; for example, a 12-volt battery needs 12.9 volts to charge it. As long as light shines strongly on a solar cell, voltage should not pose an issue.
Solar cells work better when cool than when hot; they become more efficient at converting light into electricity at lower temperatures. If you hold a solar cell close to a light bulb in order to capture more energy, you instead reduce the cell’s output because of the heat.