PV panels

Also known as solar panels, photovoltaic (PV) panels are used to convert the energy from the sun into electricity. This electricity is then fed into your battery bank through a charge controller, where it is stored until needed. Panels come in many different shapes, sizes and voltages. Smaller off-grid systems often use either 12V or 24V panels, whereas larger systems using a special type of charge controller (MPPT) can use what are known as "grid-tie" panels. This doesn't mean you have to be connected to the grid - it is just that the panels cannot be used without an MPPT controller. For more information about PV panels and MPPT controllers, read more...

When you look at the data label on the back of a solar panel, you will see a number of different bits of data. These include VOC (open circuit voltage) MaxA (maximum current) and others. Typically, a 12V panel will have a VOC of about 20-22V. This means that on a bright day, if you measured the voltage of the panel when it was not connected to a battery or other load, it would be at around 20V. As soon as you connect the panel to a 12V battery then this voltage drops down to about 13-14V, depending on how full the battery is. A 24V panel will have a VOC of about 40-44V, but a "grid tie" panel has a VOC of around 30-35V.

Essentially what this means is that a grid tie panel operates at too high a voltage for a 12V system, and too low a voltage for a 24V system. This where your MPPT charge controller comes in. When "grid-tie" panels are connected in series (positive to negative in a line), the voltages add up and usually 3 panels are connected in series to give a VOC of about 100-110V. This is then connected to your MPPT (maximum power point tracking) charge controller which then finds the best voltage to keep the panels at, in order to generate the highest amount of current from them. The charge controller then steps down this high voltage current to the voltage of your battery bank, and charges the batteries with the maximum amount of current that the panels can generate. This principle also works with 12V and 24V panels but these types are more expensive per watt. This is why larger systems requiring many panels use the cheaper "grid-tie" panels, despite the extra cost of an MPPT type controller. It's all a matter of system sizing and economics!