Solar PV system types

There are two main types of solar PV systems, namely Grid-tied (or Grid-interactive) and Off-grid. Nearly all DIY systems will invariably be of the off-grid type.

Here, we explain the difference between the two types.

Commercial Grid-Tied (grid interactive) systems

Nearly all the solar PV systems sold by the renewable energy companies will be grid-tied (or grid interactive) systems. These systems feature a special inverter that converts the DC electricity from the PV panels into AC power, which is precisely synchronised with the sine wave of the grid. This generated AC electricity is then fed back into the mains wiring, which is connected to the electricity grid.

When energy consumption in the home is low or zero, most or all of the generated AC electricity will be fed back into the grid (which is when the electricity retailer will pay you for the generation). When the consumption in the home is equal to the capacity of the PV system, the net flow to the grid will be zero. But if the energy consumption is higher than what the PV system can supply, energy from the grid will be used to 'make up' for the difference - in this case, the inverter will 'suppliment' the mains power. And of course, at night, any electricity needed by applicances will be drawn from the grid.

Grid tied systems will only generate power when mains electricity is present. If there is a blackout during the day, the system is designed to shut down, preventing the system from supplying electricity back to a 'dead' grid. This is known as "anti-islanding", and is especially important, because sometimes linesmen need to work on power lines and poles, and it is extremely important that no electricity is present on the lines when they deliberately shut off the power in order to perform the work.

It also means that in a blackout, a grid-tied system will not be able to power any applicances or lighting in your home. Your grid-interactive PV system is useless on a sunny day without grid power.

Off-grid / Independant / remote solar PV systems

Off-grid systems are just that - they are not normally connected to a mains electricity supply. In most cases, they are installed in locations away from mains power, or in places where mains electricity is not available. They can also be installed in places where mains power is present and a desire exists to make use of renewable energy, with mains power acting as a 'backup'.

Unless electricity is only needed during the day, off-grid systems will include batteries for energy storage. Batteries can supply short-term, high power output to loads that the PV panels would be insufficient to supply. The batteries act as a resoviour. For lighting applications at night, batteries are essential to provide energy that was captured during daylight hours.

During the day, when the load is low or none, all the energy generated by the PV panels will be used to charge the batteries.