What are solar batteries?
Solar batteries store excess energy generated by your solar to enable you to use the free electricity later such as when the sun sets or on rainy days. Having the ability to store this energy can make a massive difference to the efficiency, versatility and return on investment of your solar panel installation. Using a solar battery, you can also charge your solar battery at night taking advantage of cheaper off-peak electricity rates to be used when your electricity is more expensive. Another benefit is that you can set up a solar battery to power your home in the event of power cuts allowing you to experience no disruption. In most cases to enable this feature the installation of an earth rod would need to be completed.
How do solar batteries work?
In most cases during the day, your solar panel installation will be generating more electricity than you are using in your home. If you don’t have battery storage the only place this excess of electricity can go is to be exported to the grid for less money than it would cost to buy back that electricity. This is why it is essential to use as much of the free electricity that your solar panel installation generates in your home.
The solar battery does this by taking the excess solar electricity and converting it using a chemical reaction so it can be stored in the battery as chemical electrical potential energy. This process is then reversed at a time when the home requires the electricity. This could be at night, overcast days or even at times of sudden spikes in demand due to powerful electrical appliances in the home.
What are the different types of solar batteries?
There are four types of batteries that are used for home / solar battery storage. They are lead-acid, lithium-ion, nickel-cadmium and flow batteries. Their properties vary which means that most battery types suit different applications.
Lithium-Ion Batteries
Lithium-ion batteries are much newer technology than the other battery types. These types of batteries have a high energy density allowing more energy output from smaller batteries. This means that they are a great battery solution for portable technology such as phone and laptop batteries. This greater energy output for their size and weight also makes this technology a great solution for home battery storage because homes require large amounts of power with not much space to store large amounts of batteries. Litthium-ion batteries can store energy for long periods without losing power and can do lots of cycles without degrading their capacity. Most solar batteries are warrantied for 10 years or around 6000 cycles.
The main drawback of lithium-ion batteries is their higher cost. This is due to the higher-priced materials required to manufacture these batteries and the fact that they are still relatively new technology.
Lead Acid Batteries
Lead-acid batteries have been used for decades and are one of the most common types of batteries used in automotive, commercial, and industrial applications. They are cost-effective and reliable, but their energy density is low meaning that the amount of electricity that they can hold and output is lower requiring larger and heavier batteries. They are commonly used in off-grid solar installations where space is abundant because they are relatively inexpensive and reliable.
Lead-acid batteries are available in both flooded and sealed variants and are classified as either shallow cycles or deep cycles. The selection of a shallow or deep battery is dependent on the intended function of the battery and then the safe depth of discharge (DOD). The technology has continued to advance the lifespan and reliability of these batteries, so lead-acid continues to be a great option for many home battery storage solutions, especially in off-grid solutions.
Nickel-Cadmium Batteries
Nickel-cadmium batteries are common in power tools, portable electronics, airline and industrial applications due to their high durability and ability to function over a large range of temperatures. They have a high energy density allowing them to store large amounts of electricity for their size and weight and they require very little maintenance compared to other battery types making them useful for portable applications with limited space or where the weight is a concern.
One quite big drawback of Nickel-cadmium batteries is that cadmium is a highly toxic element that, if not disposed of correctly will have a significant negative impact on our environment.
Another downside of Nickle-cadmium batteries is that they suffer from a memory effect that can reduce the capacity of the battery if it is not fully discharged before recharging. For this reason, they are not very suitable for home battery storage when the charging can be dependent on solar production.
Flow Batteries
Flow batteries are becoming more common for large-scale applications such as battery banks for grid balancing where there are no size constraints due to the large size of these batteries and their 100% safe Depth of Discharge. They contain two chambers which have liquid containing electrolytes flowing between them. They have a low energy density, because of this the tanks that contain the electrolyte liquid must be large to store a significant amount of energy.
Flow batteries are not a suitable option for residential battery storage due to their large size and low battery density.
Do I have to have solar to get a battery?
It can still be beneficial to have a battery without solar panels at your home if you are on a flexible or variable energy tariff. If are on a flexible or variable tariff you can buy cheaper electricity at off-peak rates and then use it in your home or sell it back to the grid at peak times when the electricity rate is much higher. Off-peak hours vary by supplier, but the average is between 12.30 am and 4.30 am overnight. Between these times the Off-peak rate for electricity could be as low as 10p/kWh compared to 43.4p/kWh at peak times. Ofgem states the average UK household uses around 8kWh of electricity a day. This means that with 10kWh of home battery storage being charged overnight at the off-peak rate you could save yourself on average £3 a day, or £1100 a per.
Another benefit of installing a solar battery even if you don’t have solar is that it can stabilise your home electricity voltage and frequency giving you an improvement in the quality of your home electricity and the battery can be set up to provide backup power to your home in the event of a power cut.
One disadvantage of having a battery without solar is that to access some of the better export rates requires you to have solar.
It is much more effective to have solar panels paired with your battery storage so that the energy used to charge the battery is completely free clean energy generated by the solar panels and stored in the battery to be used whenever the sun goes down. If you don’t have a source of clean free electricity from solar your battery will have to charge from electricity supplied from the grid which is much less environmentally friendly because grid electricity will include electricity generated from fossil fuels.
This is why we recommend getting a home battery with solar panels. If you don’t have solar with your battery the imported electricity will still be coming from the grid, which does come at a price. When paired with solar you can fill up your battery with entirely free electricity meaning that any electricity you export to the grid will generate a much larger profit. Also, the fact that your house will be much greener with far fewer carbon emissions enables you to access the better tariffs for export such as Octopus intelligent flux which will also have a boost to the profitability of the battery system.
If you are wanting to use the backup feature on the battery for emergency power in the event of a powercut, this feature is also much more suitable when paired with solar because you can be topping up your battery during the day from the excess electricity generated from your solar even if the power cut continues for an extended amount of time. If you don’t have the solar the backup power will only last as long as the battery and then you will need to wait for grid power to be restored. In the United Kingdom power cuts are normally planned events that last in the region of 2-3 hours which should be short enough periods for a solar battery but in rural locations the length of time a power cut can last could be much longer which it is now essential to be able to top the battery up using the solar.
In conclusion, if you are unable to get solar panels due to requiring planning permission (normally because of your home being in a conservation area) or because of a lack of usable space for the panels then home battery storage will still have its uses.
Why would you need a solar battery?
As we have discussed in the article before the benefits of a home battery are multifaceted and can have a serious positive effect on your electricity bills. Here are the main benefits that you can expect from your home battery installation.
Energy Independence
When you install a solar battery, you are using more of the free electricity that your solar panels have generated in your home. This means that the amount of electricity that you need to buy from the grid is reduced increasing your grid independence.
Savings on Electricity Bills
Because the solar battery is reducing the amount of electricity that you will have to buy from the grid and also can be used to sell the electricity generated or stored at times when the electricity is at peak rates the savings can be substantial or even enable you to get paid.
Reduce your carbon footprint
The electricity that comes from the grid comes from a mixture of sources that include the combustion of fossil fuels. If you are using more of the free clean electricity generated by your solar panels then your household carbon emissions will be reduced.
The difference between AC coupled and DC coupled.
There are two types of batteries available to the consumer. They are called AC-coupled and DC-coupled batteries. The difference between the two battery types is where they are connected to the installation. DC-coupled batteries connect to the DC side of the inverter next to the input for the solar panels. AC-coupled batteries connect to the AC side of the inverter. The AC-coupled battery will normally be on its own dedicated circuit.
AC coupled
In a AC-coupled battery installation the solar panels will generate DC electricity that will flow to your inverter. Inside the inverter the DC electricity is then transformed into AC electricity which is the type of electricity that your home uses. If the electricity is not needed inside the home then it will be transformed back into DC electricity by the batteries built-in inverter to be stored as DC inside the battery. When the home requires that electricity later, the batteries inverter will change the stored electricity back to AC electricity again to be used by the home or to be exported to the grid.
One advantage of having a separate inverter built into the battery instead of relying on the solar inverter to convert the DC – AC means that you can use more current in the home without having to pull any power from the grid. When the battery is connected at the DC side of the solar inverter the output of both the solar and the battery are limited to the maximum output of the solar inverter. Because the AC-coupled battery can output AC power straight into the home at the same time as the solar inverter it means that the total output to the home is greater allowing you to power all your most powerful appliances without the need for any electricity importing from the grid.
One downside is that when electricity is converted from AC to DC or back there are small losses of power. This is because the conversion efficiency of the battery is not 100%. Modern batteries can be around 98% efficient so the amount of power that is lost is small but the energy when used from the battery could have been inverted a total of three times.
DC coupled battery
A DC system connects directly to the DC side of the solar inverter. In a DC-coupled system, Direct Current flows from your solar panels to the inverter. a charge controller that feeds into your battery system. This means that any electricity generated by your solar panels, will be only inverted once from DC to AC. Either at the point where the current flows from your battery to your home, or from your battery to the National Grid.
A DC-coupled battery installation can in most circumstances be more efficient than a AC-coupled battery because the electricity when used in the home only needs to be converted from DC – AC one time.
How much do they cost?
The cost of installing a home battery has dropped over the last few years. This is due to the batteries getting cheaper to purchase and the systems being optimised to enable faster and easier installations. The size and type of battery installed will affect the price to the customer too.
Lead acid batteries tend to be cheaper to install due to the fact they are relatively cheap to purchase. A single lead acid battery can cost between £100 to £400 or even more depending on the size and type of the battery required. In most cases, multiple lead acid batteries would be required to provide enough electricity to keep a household powered completely.
Generally, lead acid batteries will be cheaper to install than lithium-ion batteries due to the cheaper material costs but will need to be replaced more frequently. This could mean that over the lifetime of the battery installation lithium-ion batteries could be the cheaper option. The average cost of a residential lithium-ion battery system including installation can cost between £3999 to £10000 depending on the size and make of the battery.
The prices of nickel-cadmium and flow batteries can vary dramatically depending on the size of the installation. These batteries are not suitable for home battery storage solutions because of their size, price or their ability to discharge and charge many cycles without losing power or capacity.
What to look for when selecting a solar battery?
There are several factors to consider before choosing your battery system. Here are some of the factors to consider.
Battery Type
We have already covered the main battery types available but the type of battery is still crucial in selecting the most appropriate battery type for your home battery installation. Most domestic requirements for home batteries are the same. The normal requirements for a domestic battery system are compact, long life and safe. This is why most home battery storage options are lithium-ion.
Battery Life
The lifespan of a battery is normally stated in either years or cycles. This figure refers to the length of time the battery will perform. Referring to the manufacturer’s specifications for a battery can help determine how long it is likely to last. On average lead acid batteries can last anywhere from two to ten years depending on how they are used. Lithium-ion batteries typically last seven to fifteen years.
Depth of Discharge
The depth of discharge value refers to how much of the usable capacity of the battery is available before the battery will need recharging. The cells in batteries can be damaged when they are completely drained of charge. To prevent this damage from occurring and to prolong the life of the battery the manufacturer of the battery will set a limit to the amount of available power that the battery can discharge. This figure is normally around 80% for most common lithium-ion batteries but can be as high as 100%. If a battery states a depth of discharge of 100% then there is extra capacity in the battery but the manufacturer only states, the usable power so the depth of discharge is 100% of the stated capacity.
Efficiency
Solar batteries are not 100% efficient when converting electrical energy into chemical energy to be stored inside of the battery. When the battery converts the electrical energy into chemical energy some of the energy is lost in the process as heat. The amount of power that is lost in this process is very low and the benefit of having the battery far exceeds the power lost through this conversion. The efficiency rating of modern lithium-ion batteries is around 97% so a fraction of the energy is lost in this process.
How to choose the correct size of battery?
If you want to make the most of your solar panel system you will need a solar battery. When deciding on what is the correct size of battery capacity you must consider the amount of electricity that you are using and the amount of power that your solar panels can generate. As a simple rule of thumb when calculating the size of the battery you require you can double the inverter size to get a suitable battery size. For example, a 5kW inverter would be best suited to a battery capacity of around 10kWh. This is a very basic way of calculating the correct battery size for you and often more factors are taken into consideration such as the times that you are at home and the amount of electricity that you export to the grid. These are important because if you are at home all day you will be using a larger percentage of the electricity generated from your solar panels inside of your property and less will be exported to the grid. In this scenario, a slightly smaller battery is recommended because there is less excess power available to be stored in the battery so the battery utilisation will be less.
The correct battery should have enough capacity to cover your electricity demands so if you are unsure then it makes sense to go larger, especially if you want to power appliances such as electric ovens or charge an electric car. However, if you have too much capacity for your needs may mean that you are paying for battery capacity that you do not use adding additional and unnecessary costs. If you are unsure of the correct size of battery for you going smaller could be a sensible option because most modern home batteries are scalable allowing additional battery capacity to be added easily when required.