Solar cells direct current when exposed to the sun, and the power of the electrical current produced depends on the construction of the cells and panels (one panel contains more cells), the angle at which the sun intrudes, and the temperature at which the cells are located. They are usually installed on the stern or aft above deck, and it is very important that the panels are turned so that they are exposed to the sun for most of the day and there is no shade. More complex installations include electric motors that keep the panels always perpendicular to the sun, but this is a much more expensive option. The best and most expensive solar cells have an efficiency of close to 25% and such panels are only used in laboratories. This means that solar cells do not have great efficiency. Cells used on ships have an efficiency of about 15%. Monocrystalline cells are most effective then polycrystalline and amorphous cells which have the least 2 efficiency. The electricity from the cells is stored in the batteries. On ships, electricity from solar panels is usually stored in lead-acid batteries. Electricity can also be stored in lithium batteries, which is more energy efficient but more cost effective. It is usually not possible to directly connect solar panels to the battery because they have a maximum voltage greater than the battery voltage. Charger regulators are used to charge the battery with energy from solar panels.