We get a lot of enquiries about using batteries to store electricity produced by domestic solar photovoltaic (PV) roof arrays during sunny periods, to save and use when demand for electricity rises above what the roof is producing (e.g. at night, when using lights, TV, etc).
However, at the moment we generally advise against battery storage unless it is unavoidable. There are a few reasons for this - not just the cost of batteries (currently high), but also to do with whether this is the best way to reduce carbon emissions or buffer electricity from renewable sources. If you have a grid connection, then sharing surplus electricity through the grid is most probably the best option as it reduces demand on power stations at peak times (see below).
There are an increasing number of companies selling batteries & the kit needed to divert surplus electricity from solar panels, but prices vary and it is worth looking carefully at what benefits (financial or other) you would actually get from battery storage.
Battery storage is a key part of an off-grid renewable energy system, in a remote location where connection to the electricity grid would be prohibitively expensive. For more on this see: Can I live off-grid using wind and solar power?
It is worth bearing in mind that the electricity from solar PV panels or a wind turbine will reduce carbon emissions if it goes out into the grid and is used elsewhere, and it may save more carbon than if it goes into batteries.
If excess renewable energy is fed into the grid, then it reduces the need to generate electricity from fossil fuel power stations. This is especially relevant in the case of PV panels as these produce electricity during peak demand time (the middle of the day) when many fossil fuel power stations are used to produce electricity. Replacing electricity use at night will not have the same benefits, as night production is generally from lower carbon sources.
The production of batteries (such as lead acid or lithium) requires energy and involves hazardous substances. A lot of the material can be recycled when the battery fails, but the additional environmental impact and energy use in manufacturing means adding batteries will increase overall carbon and other emissions.
For similar reasons as above, there is usually not any carbon saving from using electricity from solar PV to heat water. See the page Can I use solar PV panels to heat water?
If batteries are made in sufficient quantity, and if we can extract all of the raw materials (e.g. lithium) at low-enough cost, and if there is enough material actually available, then costs may possibly come down enough for a battery to recoup the financial cost.
However, at the moment this is not the case - when you calculate the energy a battery can store through its lifetime, the cost per kWh is much more expensive that buying electricity through the grid from large-scale generation.
It is important to look at the full installed cost (including wiring, fitting, etc), not just the battery itself. Also, an existing solar PV inverter might not be compatible with a battery - and a new compatible inverter would be a big additional cost.
Say a large lithium battery system costs about £5,500 (inc VAT) for 6kWh capacity, and has a quoted life of 4000 charging cycles at up to 85% depth of discharge. This then this means it can store: 4000 x 85% x 6kWh = 20,400 kWh (over 4000 cycles).
The cost per kWh would then be: £5500 / 20,400 kWh = about 27 pence per kWh if the full 4000 cycle lifetime is achieved.
At the moment grid electricity is about 15 pence per kWh, so a little over half as much as the battery cost.
The warranty for such batteries is not usually any more than 10 years, which may be less than 4000 cycles (there would be 3650 if one cycle per day). This will depend on the pattern of use - you may get more than one full charging cycle on a sunny day if using electricity during the day, but you may not do much battery charging on many dull winter days. If the batteries fail after the warranty but sooner than the cycle limit they would be much less cost-effective.
Lead-acid batteries are cheaper than lithium, but they don't last as long and need to maintain a higher level of charge. Overall, the cost per kWh stored is likely to work out about the same as with lithium-ion.
In the UK we don’t get enough sunshine year-round for a house to be self-sufficient in electricity use (not including any heating needs) from a solar PV roof & battery alone.
Even if you can produce & store enough in summer (which may not be possible), homes in the UK will still need to have a grid connection in order to have enough electricity though late autumn, winter and early spring.
Currently, the best sources of renewable energy in the UK are either wind or water power - and we need a grid network to distribute electricity from wind farms and hydro-electric power stations to houses. Electricity from wave & tidal power, geothermal, and other renewable sources will also all need to be distributed to houses and buildings via the grid.
Also, it's unlikely you'll have a battery big enough to store all of your surplus electricity on sunny days. On an average summer day in the UK, a 3.5kW PV system should generate about 12 or 13 kWh (units) of electricity (so more than this in good weather). Many lithium batteries being promoted at present have perhaps 5 to 7kWh storage capacity, so about half that generation amount. Over several summer days, much more electricity is likely to be generated than will be used in the house, and far more than the battery can store. A house would need a large string of batteries (both very expensive and taking up lots of space) to store all of their own solar electricity - otherwise a lot of the electricity will still be exported to the grid.
So homes will still need to keep a grid connection in almost all cases, and maintain a contract with an electricity supplier. A battery will be an additional cost on top of this.
Looking to the future
We think it makes more sense to share electricity through the grid at times of surplus, so that it can be used directly. A system of sharing renewable electricity through the grid, with some shared large-scale storage to buffer it, makes much more sense than all houses needing their own batteries.
When we looked at possible ways of heating houses as part of our Zero Carbon Britain research, we found that using heat pumps (ground source or air source) may be the best solution for many homes. We simply cannot produce enough wood fuel from the land area available in the UK to meet heating needs, but we could generate a lot of electricity from offshore wind, wave/tidal and other energy sources, and this could be used to power heat pumps in towns and cities.
If we do use heat pumps much more, then people will need to run these by buying in the renewable electricity from large scale wind/wave/tidal/etc during the winter months when heating is needed. So a good-quality low-carbon grid is going to be essential. Batteries storing relatively small amounts of electricity on sunny days (mainly in summer), won’t match up with either the energy demand of heating or the time of year of peak demand.
We think it is far better to invest in moving towards a robust grid based on a mix of power from wind, wave, tidal, hydro, solar, biomass, etc. And to buffer at times of low generation (and avoid blackouts) our Zero Carbon Britain model has some large-scale energy storage from biomass fuel. Integrating energy storage at a large scale (e.g. for whole towns) is cheaper than everyone having batteries, and gives the flexibility needed in both winter and summer.
To read more about how we could make a low-carbon grid with biomass storage work, see the summary 'report in short' or the whole report available to download from the ZCB website