PV battery storage seems to be surging in popularity as it has finally become properly viable for domestic customers, thanks to the reduction in capital cost and simultaneous increase in energy prices. The availability of PV battery options also looks to have nudged more homeowners into installing PV systems, as crucially this addition means more of the electricity generated is able to be used by the household.
MCS (Microgeneration Certification Scheme) released figures on 2nd May that demonstrate a significant uptick in solar PV installations – in January to March 2023 there were 50,719, a 114% increase on the number of installations for the same period in 2022. Additionally, PV battery storage had its strongest quarter to date, with 351 systems installed, more than half the scheme total to date (the first MCS-certified PV battery storage installation was registered in April 2022).
PV battery storage: more value, more convenience
As well as the attractive benefit of using more of the electricity generated, PV battery systems help to match the solar PV output to the required electricity usage within the home, so there is no need to consider the weather conditions when operating electrical goods. Those with photovoltaic systems will know the annoyance of needing to constantly monitor PV output to maximise usage, and of turning on the washing machine… only to find cloud then obscures the sun. The reality of weather conditions in the UK means that very few days are wall-to-wall sunshine, and therefore there is significant variation in output during the day – trying to match this to electricity usage within the home is extremely difficult.
The end result of this challenge is that a large proportion of PV-generated electricity ends up being exported to the grid for very little financial benefit. The amount of photovoltaic generation used in a home may be as low as 25% across the year if no PV battery system has been installed, particularly as the peak generation periods during the summer months often occur when people are out at work during the day. PV battery storage not only increases the proportion of solar PV generation that can be used in the home (which could increase to as much as 65% across the year with careful design), it takes away the issue of varying output during the day, and the inconvenience of having to make conscious decisions on when to run appliances.
Designing optimal PV battery storage systems
PV battery systems provide optimal performance when designed specifically to suit the property’s requirements. The first decision to make is whether you are planning a smaller system which is designed to maximise usage within the home, or a larger PV battery system which is designed to use for export tariffs – the specification and size is going to be very different depending on which objective you wish to fulfil.
Either way, the size of the PV battery needs to be considered in relation to the size of the solar PV array itself – there isn’t any point having a battery which is smaller than the peak photovoltaic output, which would result in the battery being filled in a single hour in summer.
For a system to use within the home only, look at how much excess PV generation is exported on a sunny spring day (March/April), or how much electricity is used in the home in a typical day, and use this as a minimum size. However, if electric heating is installed, bear in mind that it’s unlikely a large enough PV battery can be installed to meet all of the household electricity needs.
If you are planning a system that will be used with export tariffs, then the PV and battery sizing will depend more on the installation costs involved and the available export tariffs. If variable export tariffs are selected, then a larger battery could make the most of the peak export period by enabling the battery to discharge to the grid as well, but this would require the homeowner to manually monitor export tariffs during the day and manually discharge to the grid. Most people however will want a system that requires very little manual intervention, so a fixed rate export tariff is likely to be a better option – the PV is normally oversized to fill the battery (sized against the home loads) quickly and then maximise income through large exports during the summer months from the excess PV generation. There is, however, always a risk that a fall in energy prices, or the removal of higher export tariffs, will significantly affect the payback of the system and result in large amounts of unused generation.
Extra savings from off-peak energy rates
If you have access to low off-peak electricity rates, you could consider a system that will enable you to charge the battery from the grid overnight. This can provide a significant running cost savings on top of the stored PV generation, and significantly reduce the payback period. This sort of scenario is rarely included within the calculated figures from PV suppliers, but can provide cost savings throughout the year, so it is worth working out if the numbers stack up in this way.
On the subject of off-peak energy rates, if you have an electric vehicle and use cheap off-peak electricity to charge it, then do be careful to ensure the PV battery is set to not discharge during the off-peak hours, to keep that stored electricity for use in peak-rate periods, and make the largest possible cost savings.
PV batteries: size matters
It’s important to be aware that most battery manufacturers will set a minimum 20% discharge level (so you can’t use the last 20% of stored electricity in the battery) and a maximum charge level of 90-95% in order to protect the battery. A PV battery that is quoted as 8 kWh may therefore only have 5.6 kWh of usable storage, and it is this usable storage figure that must be used when sizing.
PV battery systems will also have maximum discharge rates, in terms of the maximum power that can be supplied without needing additional electricity from the grid. This means it’s important to know what the maximum loads will be within the home, to size the system correctly, and ensure that you get the full benefit from your investment. The maximum charge rate and discharge rate of the battery inverters also need to factored in if the battery is to be charged during off-peak periods, to ensure that enough cheap off-peak electricity can be stored in the time available.
Some PV battery systems can also be used to provide back-up power during power cuts, with either an automatic or manual changeover. This option may require some modification to the property’s existing wiring, so it’s important to establish whether back-up power is a requirement when reviewing your options initially, so you can properly specify and cost the system. The size of the PV battery is even more important here, as you may require a much larger battery system depending on how long the power cuts are likely to be, and the minimum battery storage level the property will require for any power cut event. This could require a much larger battery storage system, and a minimum discharge level of, say, 50%, to always guarantee electricity during a power cut.
Maximising PV use – realistically
It’s important to remember that even with PV battery storage you won’t be able to use 100% of the generated PV electricity within the home, and it’s likely that the battery will be full for lengthy periods of the summer with significant amounts still exported to the grid. For those who are willing to remember to take action on the sunniest days, some of that excess generation can still be used to charge electric cars, heat up hot water cylinders, heat swimming pools, run air con units and the like, to squeeze maximum value out of the solar photovoltaic array and PV battery system.
To find out more about how it’s possible to maximise solar PV value and convenience through the addition of an optimally designed PV battery system, call us on 01206 266755 or email firstname.lastname@example.org.