Batteries
That storage system is ordinarily in the form of a large array of batteries stored in a vented, secure location such as a basement or specially manufactured room. Though batteries are nonmandatory, in order to be completely 'off grid' some kind of storage system is needed and batteries are almost universally used.
There are two basic kinds of batteries used in most systems: lead acid and nickel cadmium.
Lead acid batteries are far and away the most common because of their much lower cost. They're very similar to lead acid car batteries with some intriguing differences discussed below. They have positive and negative terminals (to which leads are attached) and contain metal plates that connect to the terminals. The plates sit in a chemical solution, typically sulfuric acid.
Electricity is made available by a chemical reaction between the lead plates inside the housing and the sulphuric acid mix. When they chemically combine, they generate free electrons that flow through the terminals and down the attached leads to your home outlets. PV (photovoltaic) solar panel-generated electricity drives the chemical reaction backwards, (re)charging the batteries.
The equivalent principles apply in the case of nickel cadmium batteries, though there are some fascinating pragmatic differences. NiCad batteries are much costlier. But they garner that higher price by turning in more value. They last much longer and they can discharge more current before they need to be recharged.
Both types are what are known as deep-cycle batteries. Car batteries provide a lot of current for a short period of time, enough and long enough to start your car. Then the motor takes over to power the electrical devices. They are shallow cycle batteries.
But solar power system batteries have to deliver a smaller electric current over a much longer period of time, hence they are designed differently. During the night when the PV (photovoltaic) panels are not delivering electricity to run the home or charge the batteries, these deep cycle devices do their job.
Charge Controllers
Draining a battery completely (or overcharging it repeatedly), though, would shorten its lifetime drastically.
So, there's a device that's part of the scheme that keeps that from occurring. They're called charge controllers.
When the battery gets to a certain level, the controller cuts of any drain to prevent the stored charge going below a designed-in level. It also cuts off once the battery reaches a pre-set level of charge stored. Most are designed to allow lead acid batteries to discharge to no more than 50% of capacity. That ensures a long life.
PV (photovoltaic) control panel* will last 20 years, but even good batteries may last no more than a decade, often less. Replacing them increases the cost of the system, so extending their lifetime is key to keeping the overall investment as low as possible.
