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Dr Chris Jardine explains battery storage for domestic solar systems.

Battery storage for domestic solar systems is becoming ever more popular, with many in the industry tipping it as the ‘next big thing’.  So why are batteries such a big deal, and are they going to be part of the future?

Storage required

Before we look at the domestic case, it’s worth revisiting the need for storage within electricity systems in general.  For an electricity system to function consistently and keep the lights on, it is necessary to ensure that the supply of electricity exactly matches demand on the grid.  At present this is mainly achieved by ramping up the supply of fossil fuel generators (coal and gas) to ensure this balance is met.

It is important to note that intermittency arises from a number of sources.  Much of the focus falls on uncontrollable renewables such as onshore and offshore wind and solar PV.  But intermittency at the system level is equally, if not more so, driven by intermittency in demand –  people use vastly different amounts of electricity depending on the time of day and night.  The ‘intermittency issue’ is exacerbated by the fact that nuclear power is completely inflexible and cannot be ramped up and down at all to meet demand.  Combine all this together and you have a system-level problem.

At present, we do not have enough renewables in the UK for this to pose a massive issue.  However, in countries such as Germany and Denmark, where generation of wind and solar power is much higher, they are beginning to see certain times of year when supply from renewables outstrips demand, and the electricity price collapses to zero.  Conversely there will also be occasions when the opposite is true and the wholesale price of electricity rockets (though consumers are protected from seeing this).  That means storage of some sort is inevitable to help to ensure an adequate supply of electricity and to provide a more stable energy market.

The storage solutions

This is clearly a complex problem, with multiple causes and multiple stakeholders, but how best to solve it remains unclear.  There are a variety of different technologies that can be deployed, dependent on size (amount of power required), timescales (from summer to winter, daily, a matter of minutes), and actors (who should be responsible?).  Here is a by no means exhaustive list of possible solutions, in order of scale:


These increase the capacity of connections between our electricity network and the rest of Europe.  If we build more large interconnectors across into Europe, we can sell them electricity when we are in surplus, and vice versa.

Large hydro pumped storage

This already exists on the system and is extremely valuable at providing electricity at times of peak demand.  The Dinorwig Power in Snowdonia is a 1,728 MW hydroelectric scheme, that pumps water from a lower to an upper reservoir when demand (and price) is low, and provides a fast response to short-term, rapid changes in power demand by operating as a conventional hydro scheme.  However, the possibility of similar new schemes is low. (Although there is at least one new company we know of looking into it: Quarry Battery Company).

Grid-scale storage

There is an argument that grid-scale intermittency issues require grid-scale storage solutions.  Trials are currently underway of lithium ion batteries – the same technology as in a laptop – but at the MW scale.  Other solutions exist already. For example, the JET nuclear fusion site at Culham uses flywheels with rotors weighing in at 775 tonnes and measuring nine meters in diameter, to generate up to 400MW each. This too could be a cheap and effective means of providing large scale storage.  Vanadium flow batteries could also store huge amounts of energy over very long periods of time.

Domestic level storage

With advances in battery technology over the last few years it is now possible to integrate a battery either into your PV system, or into your home.  The battery can be used to store a small amount of energy, maybe 2 kWh, but this can be sufficient to store some energy from the solar system for evening and night-time use.

Inherent storage

Rather than use a dedicated purpose-bought battery system for helping smooth through intermittency, it makes better economic sense to use existing storage to fulfil the same functions.  The obvious example of this is to use an electric vehicle, which has a battery built in for transport purposes, to soak up excess renewable power, rather than purchase a dedicated battery system just for energy storage.  We might even use ‘vehicle-to-grid’ technology to discharge car batteries into the grid at times of need.

It’s clear that there’s a huge range of options out there and at present it’s unclear what solution, or solutions, will take off in the long-term.  That’s both because there’s some debate as to what the best technical solutions are, and also which ones will be seen as most desirable by utility, commercial and domestic markets.  Whilst economics ought to favour large schemes, much will depend on the policy measures put in place and the ability to use batteries to make a profit by buying low and selling high at the different quantities and timescales in operation. At present, however, only a few of the above described solutions have the possibility to run energy storage profitably.  One is the existing pumped hydro storage which has been operational for many years.  The other is by combining with domestic solar PV systems.

Home battery storage

For a solar PV system there is a difference in benefits to the homeowner, depending on whether the electricity generated is used in the home or not.  If you use the electricity yourself, you save about 15p/kWh off your electricity bill.  If you export the electricity you receive a payment of 4.7p/kWh.  There is therefore a financial incentive for solar households to use their own electricity rather than export it, and this creates an economic driver for the purchase of domestic battery storage.  Couple this with fears around security of supply, a desire to be autonomous from energy suppliers (essentially being off-grid whilst on-grid), and you have a set of financial and emotional reasons for seeing domestic battery systems take off.

Domestic solar/battery systems come in two types.  The first are those with batteries integrated with inverters, and most major manufacturers now have some kind of offering along these lines.  The other is a separate battery system that sits alongside the PV inverter, such as Moixa Energy’s Maslow unit.  Joju Solar are currently installing 30 of these Moixa batteries into social housing units owned by Oxford City Council. This is part of the pioneering Project ERIC – Energy Resources for Integrated Communities – which is investigating whether a group of linked batteries in a community can save money and make the National Grid more resilient.  We’re eagerly awaiting the first data from these sites so we can see how the systems perform.  These small modular storage units may have the economy of scale benefits that we have seen for solar PV – once you start installing at volume, then prices reduce dramatically.  So it may be that by the time other solutions come along, the idea of domestic battery storage is already firmly embedded.

What is certain is that energy storage is going to play a major part in future electricity systems; the decarbonisation agenda means that this is going to have to happen.  How this ultimately plays out is unclear, but it looks like solar homeowners could lead the way.


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