Batteries FAQs

How does a solar battery work?

A solar battery is storage for the excess electricity generated by your PV system, and a way for you to then use your own solar electricity at times when the sun isn’t shining.

During the day there are moments when your panels are generating too much energy to be used within the property, and conventionally this excess electricity would be exported back to the grid. That changes with a battery system. Now, when you wouldn’t be using all your generated electricity this will go into the battery, stored ready to be used as soon you will need it. When the sun goes down, you will start to use the electricity in the battery instead of taking it from the grid.

Find out more about solar battery systems.

What are the benefits of batteries Storage

The main benefit is that you can store the excess electricity produced from your solar PV array during the day and use the electricity at night. Instead of getting 5p/kWh from your supplier for exporting electricity back to the grid, you use it yourself, thereby saving you the 15p/kWh you would have had to pay to import grid electricity. So it reduces your energy bills. There are other benefits to batteries

  • many people find it rewarding to import as little grid electricity as possible;
  • to be more self-reliant;
  • distrust of electricity utilities;
  • a buffer of energy in an emergency.

Where does the energy of my panels go first – the house or the battery?

The appliances in the house will use the electricity first – any leftover will go to the battery. When the battery is full you export back to the grid in the usual way, or you can divert this power to heat the hot water or into electric vehicles.

How big does my battery need to be to supply all my home energy?

Most houses use around 10 kWh of electricity per day. If you have solar, some of that energy will come directly from the solar panels in the day. So around 7 kWh of storage would be enough to run your house all night. We recommend a bit of a buffer for a few rainy days as well as potential for the battery to degrade over time. A battery of 10 kWh of storage would be more than sufficient for most residential applications. However, this can vary depending on your demand, so good battery system design is all about sizing the battery according to your circumstances.

Click here to find out more about how solar battery systems.

If I don’t use all the battery energy during the night will I have it the next day?

Yes, if you don’t use it all that energy will remain in the battery for use the next day. However, because the battery is partially charged already, there will be less space in the battery to capture the next day’s solar energy. The best battery economics come when you are fully charging and discharging the battery every day, so that you are fully utilising the battery storage capacity you have paid for.

Will a battery take me completely off-grid?

Possibly, but it’s not something we recommend in the UK. If the battery works off-grid, it can potentially take you off grid too. However, you will need a really big solar PV system to supply enough energy in the winter, or some other form of back-up generation. For this reason we don’t recommend going off grid in the UK.

In other locations, such as southern USA where solar resource is much more even over the course of a year, going off-grid is increasingly possible.

Can a battery work off grid in the UK if there’s a power cut?

Yes, there are battery systems that can operate off grid in the event of a power cut, and then reconnect when the grid comes live again. There are a limited number of options for grid tied battery systems that can operate in the event of a power cut but Victron and SMA offer this capability, and Tesla are launching in Q2 2018. This is because of the electrical regulations that govern all generation equipment connected to the grid. Put simply, if there is a power cut, operatives need to work on the wires safely, and they can’t have generators making it live. In a power cut, the battery system needs to discharge to your house, but not anything at all to the grid, and this control as to be absolutely 100% reliable.

We talk about this issue in more detail here.

Does the Tesla Powerwall work off grid?

The Tesla Powerwall does not currently work off grid in the UK, for the regulatory reasons described above. Tesla have developed this capability and it operates in other countries, but they are working on a solution with the regulators to bring an off-grid compatible Powerwall to the UK.

Click here for an installer’s view of the Tesla Powerwall 2.

Can I have a battery without solar/ charge it directly from the grid?

It certainly is possible for AC coupled batteries to simply charge from the grid using off-peak cheap electricity. Currently this includes “Sonnen”, “LG chem” and “Maslow” and we expect Tesla to follow suit shortly.

How much is a Tesla Powerwall?

£6,935 when install with a solar PV array, and £8,600 when installed itself as a retrofit to an existing solar installation. The differences here are the amount of VAT payable (5% when part of a solar system, 20% when retrofitted), and the project management costs required.

What is the return on investment for batteries?

This is a complicated calculation as it depends how the household uses the battery, and if they manage to fully utilise the battery capacity on a daily basis. The payback of a well-sized battery system is of the order of the lifetime of the battery unit itself (10 -15 years).

We are seeing customers wishing to utilise more of their own solar generation, and becoming more independent from the grid. A battery is a broadly cost neutral way of doing this, but until prices reduce (and they are reducing rapidly) they aren’t a means of generating a huge financial return on the investment.

How Long will batteries last?

Most modern, lithium ion batteries have a 10 year warranty, (for example Tesla Powerwall and LG Chem batteries). As this type of residential solar battery technology is relatively new to the market there is little evidence of what will happen after 10 years. Although the batteries could last 20 years or longer we recommend budgeting for a replacement after the 10 year warranty is up. Some of the older manufacturers like Sonnen have a 10 year warranty but an expected 20 year lifespan. The cost per kWh of storage are much higher for batteries like Sonnen as these batteries are predicted to achieve more cycles.

What size are batteries?

This will vary depending on the make and capacity of the battery, but can be quite large, and we need to think carefully about where to locate them. Some commonly used models include:

  • Tesla Powerwall 13.5kWh, 1150mm x 755mm x 155mm
  • LG Chem 9.8kWh, 744mm x 206mm x 907mm
  • Sonnen 4-8kWh, 660mm x 1400mm x 360mm

Is my battery fireproof?

Yes, all batteries meet stringent fire safety regulations, and can be used indoors.

How do I dispose of my batteries?

After the life span of the batteries the battery will be picked up by the manufacturer of the batteries and dispose them for you. They have to meet WEEE regulations which govern the disposal of electrical goods, in the same way you would for a washing machine etc.

For more info on how home solar battery systems work, click here.

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