A spacecraft requires a large amount of energy to function. On-board avionics, cameras, and communication devices are only some of the materials that require electricity. We will discuss four major sources for energy in this section: batteries, solar panels, RTGs, and fuel cells. Each of these sources has its own advantages and disadvantages. Solar panels can use sunlight to generate electricity, but when a spacecraft does not receive sufficient light from the sun, it requires an alternate source of energy. Batteries, which carry stored energy, are reliable and serve as backup for solar panels when sunlight is temporarily blocked. However, they are used up quickly. Fuel cells, which convert chemical energy to electricity, are regenerable but very heavy. RTGs can supply energy for long periods of time, but they are expensive and also give off radiation.