Specialist Installer of Solar Photovoltaic and Battery Storage Systems in Oxford and Surrounding Counties

Call us on: 07717 004240

or email enquiries@simssolar.co.uk

Battery Storage

Battery storage is the latest innovation to make news with manufacturers and distributors alike investing considerable effort to bring battery storage solutions to the market and into your home. Over the following pages we summarise the current state of the industry and try to give guidance as to whether battery storage may be right for you. This advice document will be updated as new information comes to light and new products enter the market place, so please do give us a call if you have any questions and allow us to help demystify the technology.

The Building Research Establishment has also launched a domestic guide which includes 20 questions to ask installers and will be of invaluable assistance to anyone thinking about investing in battery storage. We‘ve given some initial answers to these in the following sections to get you started.

The Basics – Is it Suitable for Me?

All PV-linked battery storage systems work on the principle that they store electricity that would normally be exported to the grid during daylight hours (i.e., when you are generating more electricity than you are using) and make it available when your demand exceeds the rate of PV generation. Some systems are more sophisticated than others and make this electricity available at any time of the day or night while others make it available only at night. The amount of electricity that can be stored will depend on duration and intensity of sunlight (irradiance) and the base load demand of the property (see below). This means that, generally speaking, the larger your PV system, the more ‘surplus’ electricity you are likely to have available to divert to storage. Equally, if you have a lower base load demand, you’ll have more ‘surplus’ electricity to store in the first instance. And as you might expect, there’ll be more potential to store energy during the summer months. To put this into context, on an overcast day in winter, a 4kWp system may generate less than 5kWh per day - unlikely to even cover a typical base load demand - whereas the same system in the summer may generate in excess of 20kWh per day and provide opportunities to store excess electricity for later use.

What is base load?

Base load is the amount of electricity you typically use when you’re not doing anything ‘extra’. For starters, your property will use energy just by being connected to the grid and having appliances on standby (let’s call this passive base load). Then there is the active base load made up from appliances such as fridges and freezers that are always on but consume occasional significant electricity (e.g., when the compressor is on). The base load of your property is relevant. A base load of 200w is not untypical - over the course of a year this would equates to 1,752kWh, over half of the amount that most energy comparison sites assume that a typical property uses each year. Or in hard cash, over £235 each year before you even flick a switch (assuming 13.5p per kWh).

If you are unsure what your combined passive and active base load is, take one meter reading from your incoming supply when you go to bed and then another when you get up. This will give an approximate overnight consumption - divide by the number of hours which have passed to give you an estimate of your base load consumption in watts. Doing this over a longer period when you’re not actively using power is likely to give a more reliable measure. If you already have a PV system you should remember that you’ll already be off-setting your demand so your base load may be higher than your readings suggest (depending on the time of day and year).

Battery Technology

The battery technologies you are most likely to come across will be variations of lead acid and lithium ion. Batteries based on a lead acid chemistry have been around for centuries; the technology is well understood and well proven with limited potential for further development. Lithium based batteries are newer, but widely used in electronic devices. The technology is under continued development with regards to storage capacity and affordability. Lead acid batteries used in storage systems are similar to those used in vehicles. Unlike those used in vehicles, the batteries for storage systems are optimised to provide moderate power over a long period of time. Lead acid batteries most likely to be encountered may be either vented (VLA) or valve regulated (VRLA). Lithium ion batteries used in storage systems are similar to those used in computers i.e., 18650 form factor. Some of the key features are set out here:

Lead AcidLithium Ion
  • Relatively cheap for a given capacity
  • For a given capacity they are heavier and larger than an equivalent lithium battery
  • They cannot be discharged (depth of discharge) to the same degree as lithium batteries (~50%)
  • They require more maintenance* and a strict protocol for charging must be followed
  • Hydrogen and oxygen gas is vented during charging*
  • More sensitive to hot temperatures Better for larger storage systems, that is, can add more batteries more easily
    *VRLA variant batteries less so
  • More expensive
  • Greater storage density
  • Greater depth of discharge achievable (~90%)
  • Battery charging and discharging must be carefully controlled
  • Longer service life
  • More sensitive to cold temperatures

Battery Life Expectancy

Generally speaking, a lithium battery will outlast a lead acid battery. Manufacturers will typically reference, as a product differentiator, the number of charging cycles that should be possible during the battery’s lifetime. This number refers to the number of complete round trip cycles (i.e., full discharge followed by full recharge) under laboratory conditions and doesn’t really reflect real life usage. In reality, a battery is more likely to undergo incomplete charge and discharge cycles. Manufacturers will also quote a warranty period; the manufacturer’s warranty will not cover abuse of the battery (for example, allowing the electrolyte level in a lead acid battery to fall to far). As with other battery powered devices, the working life of your battery storage system approaches its end as the batteries fail to hold their charge. At this point, batteries may need replacement and this really needs to be done by your installer. Lead acid batteries can be readily recycled. Under the WEEE directive, manufacturers of lithium batteries will need to provide a means of recycling.

System expansion

Although it’s easier to scale up a lead acid battery system at the design stage, expanding a pre-existing lead acid battery storage bank is not advised. Conversely, a lithium battery system can be expanded later on.

Battery capacity

Battery capacity is generally presented in two ways; for lithium batteries this tends to be in the form of kilowatt hours (kWh) and for lead acid batteries as amp hours (Ah) for a given time period, normally 10 hours (C10). 1kWh is the amount of energy used to run 10 x 100W light bulbs for 1 hr.

Battery Storage Systems

There are three basic solutions on offer:

  • DC coupled
  • AC coupled
  • Hybrid

DC coupled systems

DC coupled systems are those where the battery storage and management occurs between the solar panels and the inverter (the DC side). DC coupled systems offer better overall efficiency as all the voltage transformations remain on the DC side. A round trip efficiency of around 90% is possible. Examples of DC coupled systems include the StorEdge solution from SolarEdge whose solution incorporates the Tesla PowerWall. A key benefit of the DC coupled solutions which may also be drawback in some situations is that the stored energy must pass back through the inverter to be available for self-consumption, so unless you obtain the approval of your local Distribution Network Operator (DNO) to install an inverter which can output more than 16A, the output from the system will be limited to 16A or 3.68kW. In reality this is unlikely to be a significant issue; for example the maximum output of the Tesla Powerwall is 3.3kW but worth noting.

AC coupled systems

AC coupled systems are those where the battery storage and management occurs on the AC side of the inverter i.e., between the inverter and the incoming supply. AC coupled systems are less efficient as all the voltage transformations includes both AC and DC conversions. The round trip efficiency for these systems is around 80%. Examples include SMA’s Sunny Island solution and their forthcoming Sunny Boy Storage system. A key drawback is that DNO approval will be needed prior to installation. This approval is required because the combined output of the PV system and the battery storage system has the potential to exceed 16A. A key benefit of AC coupled system is that they are more readily scale able and configured to provide a back-up power solution in the event of grid failure. Additional control systems are needed to achieve this and these will require DNO approval and witnessing. For those concerned about short term power cuts (’brown outs’), AC coupled systems are good candidate solutions

Hybrid systems

Hybrid systems are those where the inverter and the battery management are housed in a single unit. An example of this includes the Fronius’ Symo Hybrid. This system is DC coupled and has the potential for the battery to be charged from the AC supply. Deciding whether an AC or DC coupled system suits you best may depend on where the existing inverter is located and where the incoming electricity supply enters the building.

Impact on Feed in Tariff Payments

Theoretically, there is potential for the FiT payments to be reduced for DC coupled systems due to the roundtrip inefficiencies of battery storage and energy losses that will occur before the Generation Meter. However, the inferior round trip efficiency of AC coupled systems means that any energy stored in the battery is not efficiently transformed back for later use, requiring more electricity to be bought from the grid to supplement demand. Sims Solar suggests that the reduction in FiT payment it is not really material in selecting a DC or AC coupled system.  

What else is needed?

All battery storage systems need to know when to start utilising the battery stored energy to meet demand. This requires an energy meter; a bi-directional meter that can detect when demand is exceeding the rate of generation and then call upon the battery to meet the shortfall in home generation.


A reliable battery back-up system needs a good understanding of property demand to avoid future disappointment. While software exists to predict the benefits of battery storage for a reliable assessment, Sims Solar recommends real life monitoring of household PV generation and existing self-consumption.

Now or Later

Prices and technologies will undoubtedly move in favour of the consumer. For this reason we’d normally suggest that battery storage is considered at the time of inverter replacement.

Exceptions to this are:

  • New Solar Photovoltaic Panel (PV) installations
  • PV systems where a SolarEdge inverter is already employed
  • Where an AC coupled system is desired


Useful Downloads

SMA Sunny Island

SMA Smart Energy

Fronius Symo Hybrid

SolarEdge StorEdge