Solar cells transfer energy from the photons in sunlight to the electrons in the solar cell. The more photons of sunlight absorbed by the solar cell, the greater the electric current. That’s why the short-circuit current depends so strongly on the orientation of the solar cell. The maximum voltage, on the other hand, is fixed by the material the solar cell is made of. Solar cells also have an internal resistance, which reduces the voltage available at the terminals when current flows.Electric power is the product of the voltage across a device and the current through that device. Engineers use the theoretical power to characterize a solar cell. The power provided by the sun per unit area, known as solar intensity, is approximately 1,000 Watts per meter squared. This value is reduced by clouds, haze, and when the radiation from the sun has to travel a longer path through the atmosphere (such as at sunset or sunrise). However, it is a good approximation around midday with a clear sky.The solar cell has energy losses, so does not covert 100% of the solar power to electricity. Some of the light is reflected from the surface of the solar cell, and some of the light is blocked by the metal lines on top of the solar cell that conduct electricity through the cell. To make a solar cell more efficient, the manufacturers reduce reflected light and minimize cell shading by keeping the area of metal conductors small. Energy is also lost if the energy of the photon is higher than what the solar cell can accept.To determine how well a solar cell really works, it is important to measure the efficiency with which a solar cell converts the power of sunlight into electric power. There are additional losses when you attach a load to the solar cell. In this Snack, you measured the actual power delivered to a motor, and calculated how the efficiency changed when a load was attached.