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Ensuring residential battery storage systems are sold responsibly

UK PV installers are taking on the challenge of adding storage to their offerings. Image: Rexel Energy Solutions.
I’ve had a few flyers through the door over the last couple of months offering me a PV-battery storage system. Sat at home nursing a broken leg I took the opportunity to ring one of the companies up to hear what they had to say.

The experience of that call, the flyers and a few other sales documents has led me to the conclusion that we need something in place to make sure that battery systems are sold appropriately – as I think there is the potential for customers getting charged loads extra for a battery “add-on” that doesn’t really give them a whole lot of benefit. Yes, some systems may be very good and wholly appropriate but there are factors I think merit particular consideration:

Potential for miss-selling: some systems may significantly reduce the financial returns of a PV system – they may cost significantly more and unbeknown to the customer, may reduce the feed-in tariff (FiT) income and may require expensive battery replacement(s).

A need for clear information: It can be very hard to compare different offerings. Customers need clear and comparable information to help them choose between nominally ‘similar’ systems. They also need to understand exactly what they are getting – for example they may think a system will work during a power-cut (a functionality that many customers want and expect) – but many systems don’t.

I have recently suggested to MCS that, where a battery is being offered as part of a PV installation, then a standard set of information needs to be provided to the customer. I think this information should include:

Battery information

Battery nominal capacity – expressed in a standardised way

Battery lifetime – how long is the battery expected to last?

Battery charge – discharge efficiency

System characteristics

Operating modes – clear information on how the system will operate

Effect on FIT income – could the system affect the potential FIT income?

Power cut operation – will the system provide power during a power cut?

"Giving clear information on a battery is not as easy as it sounds," says Martin Cotterell. Image: Fergus March.
Battery information

Giving clear information on a battery is not always as easy as it at first sounds, as there are many ways to express key characteristics. One way this could be addressed is requiring suppliers to use a standard text, perhaps something like: “This battery has a total capacity of xxxkWh (at the C20 rate). However, to ensure a good battery life, the system is designed to cycle the battery down to no more than xx% discharged – this means an effective usable capacity of xxxkWh – this equates to a 100W light bulb operating for xxhours.”

System characteristics

Basic system characteristics need to be provided to the customer. The two most obvious being whether a system will run loads during a power cut (see below); and also whether there will be any impact on FiT income. This latter point is important as some systems put the battery on the solar side of the FiT meter – meaning that the battery charge-discharge losses reduce the energy flowing out of the system and hence actually reduce the FiT income.

The relative importance of other system characteristics can depend on the purpose of the system. However, many customers (and some salespeople!) appear to get quite confused about what adding a battery to a system is intended to do and how the system will actually operate in practice.

Adding energy storage to a grid-connected PV system may be considered for many reasons, however, for domestic or small commercial system owners in the UK, two main reasons predominate: storing surplus solar PV energy for use later in the day and providing a back-up power supply in the event of a power cut.

Back-up power supply

In most customers’ minds, the addition of battery storage generates an expectation that the system will then be able to provide power during a power cut. However, for many systems on the market this is just not the case (they have simply not been designed to fulfil this function). Additionally, the size/capability of battery storage systems may significantly limit what loads can be run during a power cut and for how long (I have spoken to customers who expected the storage system to be able to run their entire house for a few hours – something the system simply wasn’t able to do).

Storing solar surplus

For many systems there will be times during the year when the power being generated by the solar PV system is greater than the power requirements of the building that it is connected to. During those periods – for a system without storage – all the surplus electricity gets exported into the grid. For systems with storage, some of the energy can get retained for use later in the day – an operation sometimes called “time-shifting”.

Without a good understanding of the scale of the available solar surplus, it is very difficult to properly design and specify a storage system. A diagram similar to that above is typically used to demonstrate the potential for adding battery to a grid connected PV system.

However, while this diagram looks great it, is an idealised situation some sites may have a solar surplus that is so small or infrequent that adding storage is simply not justified.

The diagram and the information it represents is important. Indeed I think it is key in determining if storage is right for a particular site. The information is also important for sizing a system. I don’t intend in going into the details here (that’s another blog!) – but if a supplier doesn’t have good information on the size and nature of the solar surplus at a particular site then it is going very hard to properly design and sell a system.

Storage systems are hard to write about in a brief blog – as it’s a complex area with many factors to consider. However, I am currently working on a PV-storage document for the National Solar Centre (NSC) which will look at the issues mentioned

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