Solar batteries are a helpful add-on to a solar panel set-up, helping you store the electricity your home generates. Here's what you need to know
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.
Get to know solar batteries better in this expert guide, and discover whether they have the potential to offer a real return on your investment.
What are solar batteries?
Solar batteries are often referred to as a home battery — a means of storing electricity produced during the day so that it can be used at a different time.
"The point of a battery storage system is to store electricity produced by the solar PV (photovoltaic) array during the day for use in the evening," explains energy efficiency expert Tim Pullen. "If you live or work at home then the probability is that most of what is produced during the day can be used during the day and a battery system would be largely redundant.
"But if you do not spend all day at home then you need a system that will use as much electricity as possible in the house, then charge the batteries, and finally, export any excess to the grid."
Typical solar batteries do not work with solar thermal panels, however, hot water storage can be installed alongside this style of array, or thermal batteries can assist.
How do solar panel batteries work?
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.
A solar battery can be installed within a solar panel system after the inverter to store electricity generated. It then connects to household appliances. (Image credit: getty images)
How much do solar panel batteries cost?
Solar batteries vary in cost depending on their battery capacity and energy rating.
Home solar battery systems start from around £400 per kWh. The Tesla Powerwall 2 is a 14kWh system and you would expect to pay around £5,500 for one (with a £440 deposit upfront).
Other brands can vary in price up to around £800 per kWh but it is important to research each product to see how they work and whether they will be optimised for you.
The cost can vary depending on:
Battery life cycle
Storage capacity
The usable capacity
Chemical materials and how its made
Different manufacturers use different discharge management software that will affect the way the system works.
Luckily the cost of home battery systems is also due to drop as more electric vehicles go into use and some car manufacturing companies, such as Nissan, are recycling used car batteries into home systems.
These second life batteries will certainly be the way to make home batteries more affordable.
How much energy can I store with solar batteries?
Solar battery systems for the home come in many different capacities.
The capacity is measured in kilowatt hours (kWh). 1kWh is a 1000 Watts used for 1 hour. So, in theory, a 1kWh battery would run 20 LED bulbs for around 9 hours, or a typical modern fridge for around 6 hours continuous running.
Many battery systems have a fixed storage capacity starting at around 2kWh to 14kWh but there are also systems that allow you to add capacity by building extra battery modules on to the system to increase the energy storage.
Do I need solar panels for a solar battery?
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.
A solar battery doesn't take up much space and can be installed inside a garage or utility space. (Image credit: Solar Watt)
Is it worth getting a battery for my solar system?
As the cost of solar home batteries comes down and the amount of generation we use increases so will the return on investment increase.
It is worth checking how much export generation you have from your PV panels and then work out how much of that you could actually use if you had a home battery system. If your home has three-phase electricity, this may be when a battery is well worth the investment.
There are many reasons a solar battery is a good idea to supplement your solar PV set up:
The ability to store surplus electricity generated for future use, rather than being required to export it
Reduction in utility bills
Store solar electricity can be exported back to the grid, making a small amount of money for the owner
Ability to be independent from the electricity grid
However, there are also some limitations to consider:
High end solar batteries have large upfront costs, which will take time to offer a return on investment
Solar batteries have a relatively short lifespan, with some models as low as five years, and others maxing out at 15 years. This is less than half the lifespan of the average solar PV panel
Not all electricity suppliers will allow electricity to be exported back to the grid
Which batteries are best for solar panels?
The best home battery for solar storage will depend greatly on what you want to do with the energy.
Perhaps your car is the best storage battery as the energy goes directly to the car battery from the solar array and you will not need to redeploy it elsewhere.
It is almost more important to look at the energy management capabilities of the home battery system and make sure that it matches your requirements.
"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."
Tesla's Powerwallreduces your reliance on the grid by storing solar-generated electricity. In case of a power outage, the Powerwall kicks in, seamlessly powering your appliances for up to several days. (Image credit: Tesla)
How long will a solar battery last?
The life expectancy of a solar battery depends very much on how it is used. The storage capacity drops annually and it is expected that the battery will last somewhere between five and 15 years. Keep in mind that the average solar PV panel will last 20-30 years, so you may need to change these twice (or more) in the lifespan of your panels.
How can I optimise solar battery usage?
Installation Advice
A home battery system must be installed by a suitably-qualified and experienced installer. Different battery systems behave in different ways and they need to be wired and commissioned to best optimise their use and prolong their life expectancy.
The best way to optimise the use of a home battery system is to use the energy stored to run electrical appliances rather than turn it into hot water.
Also make sure that you do not run too many appliances at once as the demand will be higher than the maximum discharge rate.
By staggering the start up times of different appliances you can then aim to use 1 kWh for four hours which is better than 4kWh in hour hour because the home battery cannot usually discharge 4kWh.
Can you store solar batteries in a house or garage?
Generally, it is best to store a solar a battery in a well-insulated room in the house, away from sunlight or water. This could be a dedicated plant room in which your solar inverter or any other renewables are kept.
A Tesla Powerwall can be positioned outside if a room inside is not available or unsuitable.
A Tesla Powerwall has been installed on the outside of this stone barn. The slimline nature of the design means the renewable is barely noticeable. (Image credit: Jeremy Phillips)
How many batteries are needed to live off grid?
Solar battery systems are the primary solution for off grid homes. The battery systems are, however, set up differently as there is no electricity grid backup. There is more emphasis on appliance and demand management for a fully solar powered house.
"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?"
How will electric cars affect the future of solar batteries?
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."
Some car manufacturers, such as Nissan, have collaborated with battery suppliers to use the car batteries after they are no longer good enough for the cars.
When the capacity drops the batteries are then used to make home battery systems and this ‘second life’ battery will be significantly more cost effective when the turnover in cars increases.
"There is also an argument that a vehicle could double as the battery for a home," continues Tim. "During peak times the electric car battery could be used to provide power to the home, which would then be recharged later that night when the grid demand is lower. That is not currently an option in the UK, but could well be part of the future."
What are some solar battery providers?
Below are some of the most popular solar battery providers that you can look into before buying your battery for your eco house:
Energy provider Ovo Energy is currently undertaking a solar battery trial with a select group of its customers, for example, to test on a wider scale how solar batteries can be used to not only harness electricity from renewable sources, but better manage our homes usage and even potentially feed back into the grid at times when electricity usage is at its peak.
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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.
