Battery Storage

Battery storage allows you to capture electricity when it’s cheap – or free – to use later in the day

Do I need battery storage? Read our 4-step guide:

To help you decide whether you want or need battery storage, we have written a 4-step guide:

1. Understand the technology: Find out how battery storage works
2. See if it makes sense for you: Read our 10 reasons to install battery storage in your home
3. Explore products: Compare and contrast products available in the UK
4. Find a competent, local installer: Get easy access to local installers and ask for three quotes

1. How Battery Storage Works

It’s all in the name. Battery storage allows you to store electricity in a battery. It’s as simple as that.

The question is: why on earth would you want to do that?

Why do battery storage products exist?

The answer lies in the fact that the value of electricity changes during the day.

For example, if you look at your electricity bill, you will often see a day rate and a night rate. The day rate might be say 14p and the night rate 7p.

Why is this? It’s economics – the law of supply and demand. There is greater demand for electricity during the day, so the price is higher. In the middle of the night when everyone is asleep, there isn’t much demand for electricity and the price is lower.

With battery storage, you can exploit the economics of energy pricing to your advantage, as follows:

With solar panels, you can charge your home battery for free. The rays of the sun don’t cost you anything. The sun will happily create free electricity via your solar panels and this solar electricity can be stored in your home battery. You can then let this zero cost electricity out of the battery at any time, e.g. to charge your electric car.

Or, you can charge your home battery with cheap night time electricity at 7p. During the day, you can then avoid the 14p day rate by discharging your battery with the electricity you bought at 7p. A handy saving.

The basics of battery storage

Your home battery will almost certainly be installed downstairs. Either on the floor or mounted on the wall, usually indoors. The Tesla Powerwall home battery is one of the few products that can also be mounted outdoors.

The battery is then connected to your fuse board. If you look at your fuse board, you will see a new trip switch, or MCB, labelled something like ‘Battery’ or ‘Battery Storage’.

Batteries connected directly to your fuse board are known as ‘AC-coupled’. This is the most common method nowadays.

Once the battery is connected up to the fuse board, it can do two things:

1. Receive electricity = charging, and
2. Deliver electric to the fuse board = discharging.

Charging can either be from electricity coming from your solar panel system, or from cheap rate electricity.

Discharging allows electricity to flow out of your battery into your home, either powering any devices turned on or charging your electric vehicle, via its charging point.

How does a battery know when to charge and when to discharge?

Battery storage is actually pretty dumb. It’s a lump of lithium-ion and has no idea what to do.

The solution is to connect the battery to a ‘brain’, sometimes known as a BMS (= Battery Management System) or a gateway.

The BMS is then in turn connected via clamps to:

1. The main electricity cable coming into your house from the grid, and
2. The electricity cables coming from your solar PV system.

With the clamps in place, the battery brain is able to monitor all the important electricity flows around the house. For example, it can see how much solar electricity is being generated. It can also tell if electricity is being imported from the grid.

Charging your battery with solar panels

In the day time, if there is more solar electricity than the house needs, the brain will sense this and divert the surplus solar electricity to charge the battery. The brain doesn’t let this excess solar electricity escape to the grid.

If, on the other hand, there is not enough solar electricity to charge all the devices turned on in the house, then the brain will tell the battery to discharge electricity to meet the shortfall.

As long as there are devices turned on in the home, calling for electricity, the battery will keep discharging until it is almost empty. It will then wait until there is surplus solar electricity again in order to recharge.

Charging your battery with cheap, off-peak electricity

It’s also possible to charge your home battery storage with off-peak electricity.

There are still Economy 7 tariffs available with a cheap night rate. There are also many other modern tariffs available, designed for customers with solar panels, electric cars, and batteries. Take a look at our Smart Meter Tariffs page.

These smart tariffs usually come with a smart meter. The smart meter keeps track of energy flows and the time of day, and allows the energy company to charge you different rates at different times of the day.

How much electricity can a battery store?

Battery storage varies enormously in size.

There are batteries available as small as 1.2 kWh and as big as 20 kWh. If you’ve no idea what ‘kWh’ stands for, please read our handy Energy Terminology guide.

Most home battery storage is in the range of 2.4 kWh and 13.5 kWh.

The size you need depends on several factors, including:

• How large your solar panel system is (if you have one).
• The amount of electricity you need in an average 24-hour period.
• How much money you have to spend.

When you get three companies in to quote for battery storage (see below), ask them to justify the size of battery they recommend.

Power Output

Not all batteries are created equal. Some can discharge power at a high rate, e.g. 5 kW. Others only at a low rate, e.g. 1 kW. This is basically about how quickly you can get that stored electricity out of your battery.

Let’s say you want to have a cup of tea. You have a fully charged battery and would like the stored battery energy to power the kettle.

A kettle is typically rated at 2.5 kW. If your battery has a power output (or discharge) rating of only 1 kW, it will not be able to boil the water. It will deliver 1kW to the kettle, but grid electricity will make up the 1.5 kW shortfall.

On the other hand, if your battery has an output rating of 5 kW, it will be able to power the kettle nicely (5 kW is greater than 2.5 kW). A perfectly green cup of tea. Well, we don’t mean necessarily green tea – it can still be black tea, builders’ tea, whatever you like.

Back-up Power in a Power-cut

It’s the day after a big storm. You wake up and turn the light on. Well, you press the light switch, but nothing happens. You’ve had a power-cut.

If you have a diesel generator, you turn it on, and electricity flows back into your house. You put up with the noise and diesel pollution for the comfort of being able to function normally.

There is another way: battery storage.

A home battery, if set up correctly, can provide back-up power in the event of a power-cut. If the battery is big enough, it can power the whole house for a certain amount of time (until the battery runs out). Often, however, the battery is wired into your ‘essential circuits’ for back-up power. This allows the battery to work for longer, powering the devices that you absolutely need, e.g. fridge/freezer, lights, some sockets, etc.

Not all batteries offer back-up power. Check the data-sheet, and ask the installer and manufacturer.

Also, having back-up functionality often requires a device called a ‘relay’ to be installed. This electronic device cuts your house off from the grid during a power-cut. It’s like an automatic switch. Being cut off from the grid in this way is known as ‘island mode’ or ‘off-grid mode’.

Once in island mode, the battery can function safely and power your home. If you weren’t cut off from the grid in this way, your battery could in theory spill electricity into the grid and injure someone working to restore power.

2. Ten Reasons to install Battery Storage

If you’ve read the section above, you will already have a feeling for what battery storage is and how it can help you.

1. Battery storage captures your surplus solar electricity that would otherwise be lost to the grid.
2. It allows you to use (stored) solar electricity when the sun isn’t shining or at night.
3. A battery can store cheap off-peak electricity and discharge it when prices are high.
4. Battery storage helps you charge your electric car with 100% renewable energy (when combined with solar).
5. If you have enough battery storage and solar panels, you can be almost completely independent of the grid.
6. When configured correctly, batteries can power your home, or part of your home, in a power-cut.
7. Some batteries are ‘smart grid ready’, meaning you might be able to earn money by exporting power to the grid.
8. Combined with solar panels, batteries help you reduce your carbon footprint.
9. Batteries and solar keep your electricity generation ‘local’ – if everyone did it, we’d need fewer, far-away power stations.
10. If you have the ‘magic four’ – electric car, charging point, solar panels, and battery storage – you are a full member of the modern energy revolution.

Are any of these points attractive to you? If you’re tempted, read on…

3. Explore Battery Storage Products

There are already many battery products available in the UK, with more arriving all the time.

The technology has been around for a few years now and certain manufacturers are gaining a solid reputation. Factory production quality is improving, warranties are getting better, customer service departments have more UK staff, the battery management systems are getting smarter, and so on.

The cost of battery storage generally varies in relation to storage capacity. An 8 kWh battery will cost more than a 4 kWh battery. This is certainly true when you compare different models from the same manufacturer.

However, some manufacturers are able to deliver more storage capacity per Pound spent than others. Consider the Tesla Powerwall. It’s a 13.5 kWh battery. Fully installed, it costs in the region of £7,000 inc. VAT @5% (when put in at the same time as your solar panels – see VAT paragraph below). That gives it a price per kWh of storage of £519. That’s a very low number. At the moment in the UK, there are no other manufacturers that can match it.

Of course, you might not want to spend £7,000 on a battery. You identify with the 10 reasons above, but want to spend less money. Fortunately, there are products available at lower price points, such as from the UK company, Powervault.

There are also products costing more than £7,000. You might want to spend more to secure a prestige, very experienced European manufacturer like Sonnen.

The other thing to keep an eye on is VAT. Solar panels have 5% VAT added. Batteries attract VAT at 20%. However, if your battery is installed at the same time as the solar panels, the battery will have VAT applied at only 5%.

This can make a big difference. On a Tesla Powerwall, the difference can be £1,000 between 5% VAT and 20% VAT.

In the links below, you will find home battery storage products from established manufacturers. The first three links for Tesla Powerwall, Sonnen and Powervault take you to Electric Car Home pages where you can explore product details, watch videos, etc. The final two links for LG Chem and BYD take you straight to the manufacturer product pages.

Have a good look round, and if you then want to go to the next stage, read the section below to find local installers.

• Tesla Powerwall
• Sonnen
• Powervault
• LG Chem
• BYD

4. Find a Battery Storage Installer

If your appetite has been whetted, and you’re keen to get some proposals for battery storage, it’s time to speak to some local installation companies.

Remember: always get three quotes. Assuming prices are similar, dig a little deeper on Checkatrade, ask for references, etc. Sometimes it’s better to pay a little more if you feel comfortable with a particular company.

Ask the installers to justify the storage capacity of the battery they recommend. There is no point in having more capacity than you need. Equally, if you might need to store more electricity in the future, e.g. if you plan to buy an electric car, you might want more battery storage capacity than you need at the moment.

If you want to get both solar panels and battery storage at the same time, you might well choose one installer for both products. This makes sense. You can save on VAT. However, it’s very important to check the installer is competent for both solar panels and battery storage. Get up to speed on solar panels on our main Solar Panels page.