As a power source， the sun offers some impressive advantages over typical battery cells： It generates virtually limitless energy， requires no recharging， and is expected to last for another five billion years or so. Today‘s more efficient， less expensive solar cells provide a practical means of converting the sun’s power into electricity to run our gadgets and gismos （see the appendix， From Photons to Electrons）。 The only clouds in this otherwise sunny picture are 。 . 。 well， clouds. Because they often obscure the sun， it‘s tough to design a system that can reliably provide power.

　　The output current for typical monocrystalline-silicon solar cells directly depends on the amount of incident sunlight （Figure 1）。 For example， a typical hobby-grade credit-card-sized silicon cell has an open-circuit voltage of 0.55V. Internal resistance causes a voltage drop as you draw current from the cell; but， as light energy drops below the level necessary to support the output load， the cell current-limits at an almost constant voltage. For a light level of one full sun （solar irradiance of 1kW/m2）， the cell provides a typical short-circuit current of 0.3A.