The advent of Lithium-Ion battery technology has finally made the logistics of having battery backup for your PV systems an economical possibility. Their storage capacity is far greater than the equivalent size of lead acid battery and there is no maintenance, such as topping up the electrolyte, as there can be with some lead acid batteries.
They can be installed at the same time as your new PV system or as a retrofit to an existing system. Battery backup can be used to increase self consumption only, grid failure mode only or a hybrid self consumption/grid failure mode.
The batteries are charged during the day when your PV system is generating more electricity than is currently being consumed. In a non battery system this surplus is exported back out to the grid however in a battery system this is ‘diverted’ to charge the battery until it is fully charged at which point any further surplus electricity is exported to the grid as before. This is then available for use in one of the three modes below. There is no guarantee that the battery will be fully charged at the end of the day (such as on an overcast day).
Self Consumption Mode
In this mode when your PV system isn’t generating then the electricity demand is first satisfied by using the stored power in the batteries rather than purchasing electricity from the grid. Once this power is completely discharged then demand is satisfied by purchasing from the grid network.
Grid Failure Mode
With this mode the battery is kept in a fully charged mode and is only discharged in the event of a power failure. When this happens the system disconnects the loads from the network and then starts generating power from the battery. This is not a seamless process and there will be a 5 to 10 seconds delay before the power is restored. If power is not restored before the battery is discharged then you will be in the same position as your neighbours! It is sometimes possible to rearrange the electrical circuits within a property to create a number of circuits that will be the only ones to receive power I.e. lighting circuits, power circuits for essentials like fridge, freezer and boilers. This will extend the length of time before the battery is fully discharged.
This mode is a combination of the two modes above with a proportion of the battery capacity always reserved for grid failures. In some systems this proportion can be altered to suit individual circumstances.