Installers often encourage you to buy a battery to store electricity, but you should know that the environmental and financial aspects are rather different to those for solar panels. Treat the purchase of a storage battery as a separate decision with different considerations.
Whilst a battery does not save additional carbon in itself, it does mean that the carbon savings from the solar PV are maximised. At times of national peak electricity demand, the UK grid relies more heavily on electricity generated by fossil fuels. The more renewable electricity that can be stored during times of peak renewable energy generation, the less demand there is for grid electricity at peak usage times. As well as storing excess electricity that your solar panels generate during the day, you can store electricity at night from the grid, when demand is lower, and the electricity mix is greener.
These FAQs assume that your building is connected to the national electricity grid. If it is not, then your inability to import and export electricity should be taken into consideration as part of your decision-making.