The rover requires power to operate. Without power, it cannot move, use its science instruments, or communicate with Earth. The main source of power for each rover comes from a multi-panel solar array. They look almost like "wings," but their purpose is to provide energy, not fly.
When fully illuminated, the rover solar arrays generate about 140 watts of power for up to four hours per sol (a Martian day). The rover needs about 100 watts (equivalent to a standard light bulb in a home) to drive. Comparatively, the Sojourner rover´s solar arrays provided the 1997 Pathfinder mission with around 16 watts of power at noon on Mars. That´s equivalent to the power used by an oven light. This extra power will potentially enable the rovers to conduct more science.
The power system for the Mars Exploration Rover includes two rechargeable batteries that provide energy for the rover when the sun is not shining, especially at night. Over time, the batteries will degrade and will not be able to recharge to full power capacity. Also, by the end of the 90-sol mission, the capability of the solar arrays to generate power will likely be reduced to about 50 watts of power due to anticipated dust coverage on the solar arrays (as seen on Sojourner/Mars Pathfinder), as well as the change in season. Mars will drift farther from the sun as it continues on its yearly elliptical orbit, and because of the distance, the sun will not shine as brightly onto the solar arrays. Additionally, Mars is tilted on its axis just like Earth is, giving Mars seasonal changes. Later in the mission, the seasonal changes at the landing site and the lower position of the Sun in the sky at noon than in the beginning of the mission will mean less energy on the solar panels.