This device regulates rates of flow of electricity from the generation source (PV panels or wind generator) to the battery and the load. The controller keeps the battery fully charged without over-charging it.

When the load is drawing power, the controller allows the charge to flow from the generation source into the battery, the load, or both. When the controller senses that the battery is fully (or nearly fully) charged, it reduces or stops the flow of electricity from the generation source, or diverts it to an auxiliary or "shunt" load (most commonly an electric water heater). Many controllers will also sense when loads have taken too much energy from batteries and will stop the flow until sufficient charge is restored to the batteries. This last feature can greatly extend the battery's lifetime.

The cost of controllers generally depends on the ampere capacity at which your renewable system will operate and the monitoring features you want.