According to the California Science Center, energy can come from a variety of things, including lemons. The energy does not actually come from the lemon itself but from the chemicals that are found in the lemon.
Lemons contain acid, which is a solution that can conduct energy. Most foods that have the ability to conduct electricity are high in acid or potassium. Other citrus fruits that can conduct electricity include oranges, grapefruits and limes. The more fruit that is connected, the more electricity can be conducted or the stronger the home made battery will be. Potatoes and cucumbers are also foods that can conduct electricity. Potatoes can do so because of the potassium and cucumbers because of their high salt content.