Solar batteries: Your questions answered

stone barn with tesla powerwall on exterior wall with view of garden

With no room inside their converted barn, the homeowners chose a Tesla Powerwall so it could be installed on the exterior of their property (Image credit: Jeremy Phillips)

Solar batteries effectively allow you to make use of the electricity generated by your solar panels when you need it most. While they might sound like a niche purchase, they're actually growing in popularity of late, especially in light of fluctuating energy bills and climate uncertainty.

If you're investing in solar panels, it might seem like an obvious addition to your set-up, but there's certain elements you need to consider before you commit to solar batteries — including the price.

The battery is either charged up during the day by solar PV panels or can be charged up during off peak electricity times when the electricity is cheaper. The generation is either stored as Direct Current (DC) before it gets to the inverter or as Alternating Current (AC) after the inverter. If you have a system that pays you incentives for generation then you will have marginal losses if you choose a DC system. However, if you are installing a three phase electric supply and a three phase inverter then a DC battery system could be more beneficial.

"As a note of caution, it is reported that increasingly the permission of the grid operator is needed to enable battery systems to be connected to the grid and some suppliers are reporting around a 50% rejection rate," warns Tim Pullen. "A 4kW PV array can be connected to the grid without consent of the grid operator. If a 4kW battery system is installed there is a theoretical potential to export 8kW, which needs permission. Check this with your supplier, who may need to complete a G100 application form." The supplier will also be able to clarity how solar panels work alongside your chosen battery type.

Solar batteries can also work to store electricity bought from the grid without installing solar panels. You might be wondering why you need to do this, but it may be useful in the future when time-of-use tariffs become more widely available in the UK. This is where energy providers try to encourage its users to use more electricity at off-peak hours than peak hours with a cheaper price. Your solar battery can take store electricity from off-peak hours to use at peak hours in this instance.

"If we think about battery storage systems at all we probably think of lead-acid batteries, like we have in the car," detail Tim Pullen. "There are still a few of those about but the majority of the market are lithium-ion batteries. There is at least one manufacturer of salt-water batteries – The Aquion range from Bluesky Energy – which have obvious advantages in terms of disposal and sustainability but have not yet achieve the market dominance of lithium.

"The reason for choosing lithium is its longevity. A battery’s life is measured by the number of cycles – charges and discharges – it can endure before its capacity drops. Lead acid batteries are in the 2,000 to 3,000 range and lithium-ion are in the 6,000 to 10,000 range. What this means is that there is little real choice. If you are particularly concerned about sustainability then you will go for the saltwater battery, if not then you will go for the lithium-ion.

"Beyond this, there are two different types of batteries system: DC (Direct Current) systems and AC (Alternating Current) systems. DC systems connect to the PV array, which means an additional inverter is not needed. AC systems connect to the PV array via the electricity meter. In this case an additional inverter is needed to convert DC electricity produced by the PV array into AC electricity which is useable in the house. Unless there is a particular need for a DC system, AC would be the default option."

"Industry standards suggest roughly 1,500kWh/yr per person living in the house for electricity. If a photovoltaic solar system is the principal means of generating electricity, it;'s worth knowing that the industry set out that broadly 50% of the annual production will be in the three summer months and only 10% in the three winter months. So, do you want to store that summer surplus until winter – that will need a prodigiously big battery bank – or have a second technology, wind, say, that works better in winter?"

Charging an electric car at home might be a concern for those with solar panels, due to their requirements and battery sizes. "Home batteries have got better in the past few years but the average capacity of a home battery is around 14kWh, whereas a car battery could be anywhere from 30kWh to 100kWh," Tim Pullen comments. "Using home batteries to recharge your car would require a lot more batteries and PV than you'd normally install."

David is a renewables and ventilation installer, with over 35 years experience, and is a long-standing contributor to Homebuilding and Renovating magazine. He is a member of the Gas Safe Register, has a Masters degree in Sustainable Architecture, and is an authority in sustainable building and energy efficiency, with extensive knowledge in building fabrics, heat recovery ventilation, renewables, and also conventional heating systems. He is also a speaker at the Homebuilding & Renovating Show.

Passionate about healthy, efficient homes, he is director of Heat and Energy Ltd. He works with architects, builders, self builders and renovators, and designs and project manages the installation of ventilation and heating systems to achieve the most energy efficient and cost effective outcome for every home.