Solar Back-up Batteries & Power Cuts

solar home illustration

Did you know that not all solar batteries can provide you with back-up electricity in a power cut? In fact, for safety reasons, it's more common that they don't have this capability. Here's what you need to know about solar batteries and power cuts.

Solar panels with a solar battery

When you don't use all the energy generated by your solar panels during the day, a solar battery can store the excess so you can use it at another time. For example, at night or on particularly cloudy days when your panels aren't generating as much energy. This means you can use even more of your renewable and free electricity and buy less from your supplier.

While solar batteries can maximise your energy savings and lower your carbon footprint even further, many people assume that a solar battery will also be able to provide you with back-up energy during a power cut. While there are some solar battery systems which do have this feature, it is far more common that a solar battery cannot help you in a power cut.

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Solar batteries and power cuts…what usually happens?

The reason why solar batteries often won't provide your home with back-up power is due to the safety risks involved in doing so. Your solar panels and battery are connected to the main grid. During a power cut engineers will be working on the grid and if solar panels or batteries are in operation there is a risk the engineers could be electrocuted by the electricity being generated. This is why solar inverters are designed to automatically switch off when a power cut is detected.

However, if your solar battery has back-up functionality, you will be able to use your solar energy during a power cut…

Solar batteries with back-up power…how do they work?

Solar batteries with back-up power have a relay (a switch) which will automatically disconnect your electricity supply from the grid when it detects a power cut. This is called islanding. This relay is installed between your main fuse board and the incoming electricity supply. You'll have power for your home so you can continue to use your electronic appliances, appliances and any electric heating during the power cut.

Some systems provide an almost seamless transition from grid power to solar back-up power so you may not even notice that there has been a power cut. This feature is called UPS (Uninterruptible Power Supply).

Will your solar panels continue to charge the battery during a power cut?

This depends on the type of back-up system you have. Some lower cost batteries will automatically discharge their stored energy when a power cut is detected. This is in part due to the fact that the battery inverter is usually smaller than the solar inverter; allowing the energy in the solar inverter to flow into the battery could overload it. However, if the battery inverter is larger than the solar inverter, energy can continue to flow into the battery until it is fully charged.

How much electricity can a back-up solar battery provide?

This depends on the size of the inverter in the battery. In a solar battery back-up system, the battery needs to hold enough power for your everyday use while keeping some energy in reserve in case a power cut happens. The larger the capacity of the battery in kW, the more energy you can reserve for power cut back-up and the more appliances you'll be able to run during a power cut.

It's important to work out how much electricity you want to store for daily use and how much you want to keep in reserve in case a power cut should happen. If you don't keep enough energy in reserve you won't have enough for a power cut, but if you keep too much in reserve you may find you don't have enough available energy to power your home on a standard day.

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Usually, when the battery inverter cannot provide enough energy for an appliance to run, the supply is topped up using electricity from the grid. During a power cut this obviously isn't possible. If you're using a back-up supply and try to use more energy than it can provide, this may cause the inverter to trip, leaving you with no power during the outage.

If you want/need to be able to power a lot of appliances at once time during a power cut, you will need a battery with a large inverter. However, the larger the inverter the more expensive the battery will be, so an alternative and cheaper solution is to avoid using high power appliances while using your back-up supply. It is possible to separate your appliances onto different electricity circuits so that only the low-energy appliances are powered by the battery. This is usually a straightforward process which is not costly or complex.

Examples of Solar Batteries with Back-up Power

If you are interested in buying a solar battery which can provide electricity for your home during a power cut, you need to ensure you choose a battery which has this functionality as it is not a standard feature. Here are some examples of solar batteries you may want to consider.

Puredrive Energy PureStorage

Pure Drive is a British designer and manufacturer of innovative solar storage systems.

PureStorage residential battery is a Hi-Rate 4.8 kWh LiFePo4 battery which can both store excess solar energy and provide back-up power in the event of a power cut. When the system detects a power cut the battery will automatically power your appliances through a UPS which begins in less than under 20 milliseconds. The PureStorage battery is a smart battery and can be monitored and controlled using a free app or the website via a mobile device with an internet connection.

If you would prefer to keep the battery outside your home you can choose to include the weatherproof outside enclosure. The PureStorage has a 3 / 5 kW inverter and includes a 10 year warranty with 6,000 cycles for your peace of mind.

Tesla Powerwall with Back-up Gateway 2

The 13.4 kWh Tesla Powerwall 2.0 has been leading the international solar battery market for many years but until recently it did not have the back-up power supply feature. The company has now released the Tesla Powerwall 2.0 with Back-up Gateway in the UK and Germany.

The new gateway component looks like a small version of the Powerwall battery and will island the battery from the grid in the event of a power cut. In addition to this new feature, Tesla claims that the battery now has faster connectivity and a stronger connection to the monitoring system. The Powerwall has a 5 kW inverter which could cover a significant (if not all) of the electrical load in the average home during a power cut.

Should you choose a solar battery with back-up power?

There are some big factors to consider before you buy a solar battery with a back-up power supply. A solar battery will store surplus solar electricity for use at night, but is it important to you that you have some power during a power cut?

Firstly, solar batteries with this functionality are more expensive than standard batteries, partly because they need to have a high storage capacity to be able to power multiple appliances at once. If the battery is not large enough, you risk tripping the inverter which will essentially make the system pointless

Secondly, while power cuts are inconvenient, significant outages are rare in the UK, although homeowners in rural areas may find they have to deal with them more often than most. If you do decide on a battery with back-up, you need to consider whether or not the supply needs to be a seamless UPS.

When trying to weigh up the extra costs against the potential benefits, it's always best to speak to an MCS certified (or equivalent) solar installer as they will be able to advise you on the right battery for your home and provide tailored quotes for the installation. Send us an enquiry today and we'll put you in touch with up to 3 MCS certified (or equivalent) solar installers who will provide free no-obligation quotes today.

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