Can you add a battery to an existing solar system?

Battery energy storage systems are a great addition to any solar panel installation. They allow you to store the excess solar energy generated during the day, and use it later on – even at night. 

You may be wondering if it is possible to add battery energy storage to your existing system.

The short answer is yes it is possible. But the chances are some rewiring to the solar panels will be necessary. Also, if your existing inverter is not suitable, you will need to replace it with a new one.

Existing solar PV output

Different solar panels have different output voltages depending on how many solar cells make up the panel. Modern domestic rooftop solar panels have an output voltage between 30V - 50V. Panels can be wired together in series or in parallel to provide the correct voltage for the inverter. The inverter converts the DC voltage from the panels into an AC voltage that is suitable for use as standard mains electric power. 

When panels are wired together in series, their voltages add up, but the output current (amps) stays the same. When panels are wired together in parallel, the output voltage stays the same, but the current adds up (see diagrams).

A row of panels wired together in series are called a string. Note that even though the voltages and currents are different, the output power (W) of both these arrangements is the same.

To further increase power output, two identical strings of panels can be wired together as shown in this diagram:

Charge controllers

To charge the batteries, a unit called a charge controller needs to be added to the system. The DC output from the solar array goes into the charge controller instead of the inverter.  The charge controller converts it to an appropriate voltage to charge the battery bank. Charge controllers come in one of three different versions: 12V, 24V or 48V.

For high power, domestic rooftop solar installations, 48V charge controllers and battery banks are used. The smaller 12 / 24V charge controllers are usually used for smaller mobile solar systems (e.g., camping / sailing etc.)

If the output voltage of the existing solar array is not within the correct range for the charge controller, the panel wiring will need to be changed.

Suitability of existing system inverter

With a 48V battery bank and charge controller, your inverter will need to be able to accept a 48V DC input.  To find out if you can continue to use your existing inverter, you will need to look in the documentation that came with it. In the specifications, check the DC / PV input voltage range. If it cannot accept an input of 48V, it is not suitable, and you will need to replace it.

Batteries

The size of battery bank required will depend on your electricity usage. For typical daily domestic use, between 3kWh - 15kWh is appropriate. Speak to your installer for a detailed assessment.

There are two types of battery that are commonly used – lead acid and lithium ion.  Lead acid batteries have a life expectancy of about 2 years. This can vary, depending on how often they are completely charged and discharged. Lithium ion batteries will last 4-5 years under regular usage.

Lithium ion batteries are also more efficient, and are a lot lighter than lead acid, which means it is safe to install them in loft spaces for example.

Lead acid batteries also need regular maintenance to keep them operating at their best. Lithium ion batteries don’t need this maintenance.

Combined charge controller / inverters

You can buy all-in-one units that have both the charge controller and the inverter in one box. It is simpler to install these units, and they need less wiring. But if your inverter does not need replacing, it is cheaper to buy a charge controller only and keep your existing inverter.

Feed-in tariffs

Grid-tied solar installations are usually registered for feed-in tariff generation. When you add a battery energy storage system to your solar array, it will export a lot less solar electricity back to the grid. This is because most or all of it will be used to charge your battery bank instead. 

Your payments may go down because you will be exporting less electricity back to the grid.