Batteries, or home energy storage units, can be installed to store surplus energy produced by your solar PV system, which can then be used in the home when your system is not generating energy, at night for example.
In doing so you can save money on your energy bills.
Batteries are often provided with a power rating in kiloWatts (kW) and those installed in the home are typically around 1kW to 7kW. This is the capability of the battery to provide power. The Energy Saving Trust reports that a typical 4kWh system would be able to power the kettle 37 times.
- your solar PV system generates electricity
- if this electricity is not used by appliances in the home, then surplus energy charges the battery
- once the battery is full, the surplus energy is exported back to the grid
- when your solar PV system is not generating electricity then the battery will discharge the stored electricity for you to use at no extra cost
- should you need extra electricity you can buy this from the grid.
A battery may be suitable for you if you have a solar PV system which is generating surplus energy which you are sending back to the grid. How much energy you will be able to store will depend on how much electricity is generated by the solar PV, how much energy you use in your home, and the size of the battery.
Batteries can be quite large and heavy. An average storage system will require approximately 1 – 2m² of wall space and there should be some space around the battery once installed for ventilation.
Batteries have a ‘nominal capacity’ (how much they can store) and a ‘useable capacity’ (how much they discharge or output). Usable capacity will be less than nominal capacity. Energy is lost during the charge-discharge process.
There are two different types of batteries normally used in homes:
- Lithium batteries which are the most common, usually more efficient and have a longer life expectancy, and
- Lead-acid batteries which are cheaper but also less efficient and heavier. They have a shorter life expectancy and are normally used in off grid systems.
There are also two ways in which batteries may be installed:
- A DC coupled install means the battery is installed on the same side of the inverter as the solar PV panels. The battery is charged directly from the panels and the energy is only converted from DC to AC when it is used.
- An AC coupled install means the battery is installed on the grid-side. The power generated by your panels will have already moved through the inverter and been measured by the generation meter before it reaches the battery.
You’re more likely to be offered an AC-coupled system if you’re looking to add a battery to an existing solar PV system (‘retrofitted’). You’re more likely to be offered a DC-coupled system if you’re installing a solar PV system and a battery from scratch.
Many DC-coupled systems will not operate in a power-cut and may affect your Feed-in- (FIT) income. No battery will work in a power cut if it does not hold charge.
The life expectancy of a battery is normally based on years or cycles (how many times it charges fully and discharges). Repeated incomplete charging can impact on the life of the battery. Batteries in homes typically last for about 10 years or 10,000 cycles, whichever comes first. Therefore, in the lifetime of your solar PV system you are likely to have to replace a battery at least once.
Some batteries can be connected to the grid so you can import energy from the grid, maybe to make sure your battery is fully charged before it discharges to maintain its life, or so you can import energy from the grid at its cheapest to use at peak times if you are on a time of use tariff.
A battery may also be partnered up with other technologies like a wind turbine or a hydro system, and could be used to charge an electric vehicle.
At present, a battery will typically cost between £4,000-£6,000, according to Energy Saving Trust.
How much energy and money you will save will depend on how much energy you use from the battery rather than the grid, and the cost you buy your electricity at. You should also factor in the cost of the battery, and the cost if you ever need to replace it. Claims of never having another electricity bill again are unlikely to be true.
If you have a solar PV system installed already and are thinking of installing a battery think about how this might affect your FIT or Smart Export Guarantee (SEG) payments, where you are paid for exporting generated energy back to the grid. DC coupled batteries may further affect your FIT. For more on the SEG and how it works with batteries see here.
Overall, it is unlikely a battery will pay for itself. However, prices are coming down so this may change in the future
Installers should install batteries in line with IET standards, and MCS also have a battery install standard.
By choosing an MCS certified installer you know they are required to adhere to certain standards of install, and will be a member of a consumer code, like Renewable Energy Consumer Code, who can help should anything go wrong.
Find MCS certified installers here.
