Our guide to battery storage



Ever since we started installing solar PV systems we have been asked by customers about the potential for storing the generated electricity. This has traditionally been held back by prohibitive costs but there are now cost-effective options commercially available. This has resulted in high customer interest in a similar way to the early days of solar PV systems. The other parallel with early PV is that certain greedy, unscrupulous and (in many cases) incompetent companies are cold-calling, telling their lies and cashing in on the ‘gold rush’.

The intention of this guide is to dispel any myths and highlight the importance of knowing exactly what you’re buying. One battery storage system can be very different to another.

Why install battery storage?

A battery storage system may be used to enhance the self-use of generated electricity, provide backup in the case of a grid failure, or both. It can also be used without solar panels to store off-peak electricity and use it during peak charging times for those on Economy 7 or Economy 10 tariffs.

Types of battery storage

There are essentially two types of battery storage system – a.c. coupled and d.c. coupled.

AC coupled systems can be used with or without solar panels. They are connected to the electrical mains of the building and use a combined inverter/charger to charge the batteries when required and then supply mains power to support the electricity demand. This means that they will work with any brand of solar inverter, but the ’round-trip’ of the d.c. from solar panels going through the solar inverter, the inverter/charger, the batteries, then back through the inverter/charger results in marginally lower efficiencies than d.c. coupled systems. We install a.c. coupled systems such as Tesla Powerwall and Alpha Smile.

DC coupled systems charge the batteries directly from the solar panels. The energy is then fed through the solar inverter on demand, which is slightly more efficient. This requires either a charge controller that has to be compatible with the solar inverter, or alternatively a hybrid solar and battery inverter. We recommend a.c. coupled systems (see above paragraph).

Types of battery

The main types of battery on the market are lead-based and lithium-based batteries. Lead batteries include lead acid, AGM, lead crystal and other variants. Lithium batteries can be lithium ion, lithium iron, lithium ion phosphate (LiFePO) and others.

The pros and cons of each are summarised below:

Attribute Lead Acid Lithium Ion
Total Storage Capacity An individual lead-acid battery will typically have a gross storage capacity of 100Ah – 200Ah @ 12V or 1.2kWh – 2.4kWh. They may be connected in series for a higher voltage and/or in parallel for greater capacity at the same voltage. A typical lead-acid pack suitable for a residential grid-backup solution will be in the range of 8kWh – 25kWh depending on the length of time required to operate off-grid and the total power of the loads to be supported. Lithium Ion battery packs typically are supplied as self-contained units with a built-in battery management system (BMS). Gross capacities vary from about 2kWh up to 8 – 10kWh depending on the model and manufacturer. Some models may be connected in parallel, others may be extended with expansion packs and all need to be fully supported by the software in the battery charger/inverter chosen.
Daily Usable Capacity There is a close relationship between the amount of the total battery capacity that is used each day and the life of the battery as expressed by the number of cycles and typically it is recommended to only discharge a lead-acid battery down to about 50% of the total capacity, this if referred to as a 50% Depth of Discharge (DOD). This makes the storage capacity available for daily use only 50% of the gross storage capacity. Most lithium-ion batteries can be used daily down to about 90% of their gross storage capacity with little or no impact
on their lifetime in terms of number of cycles. This makes the storage capacity available, for daily use, 90% of the gross storage capacity.
Full Cycle Efficiency Lead-acid batteries tend to get less efficient the nearer to full capacity they reach. This either results in a low full cycle efficiency of less than 80% (if they are re-charged near to their full capacity) or the need to design the system to only use about 80% – 90% of their full capacity (in order to maximise their efficiency). Most lithium-ion batteries have a full cycle efficiency of around 90% – 95% even for a cycle from their full depth of discharge up to full capacity making them ideally suited for daily use applications like solar PV systems which need to use most or all of their retained energy in the evening/night and charge up again fully during the day.
Lifetime (Cycles) The number of cycles that a lead-acid battery can be used for is directly related to the amount of energy charged and discharged in each cycle. With a system configured to utilise 50% of the gross storage capacity on a daily basis a typical lead-acid battery will have a lifetime of 2,000 – 2,500 cycles. Allowing for some degradation over the life of the battery a useful lifespan of about 5 years in a well-designed system may be expected. A good quality lithium-ion battery may have a lifetime of 5,000 – 7,000 cycles which is considerably more than 10 years of normal usage. The built-in BMS will ensure that the battery condition is always maintained in optimum condition and a full 10 year life may be expected.
Cost The initial investment cost of a lead-acid battery will be relatively cheap when expressed as £ per kWh of gross capacity but all comparisons should always be done on a £ per kWh of usable capacity which makes a lead-acid battery twice as expensive as it may initially appear. The initial investment cost of a lithium-ion battery may be 2.5 – 3 times more expensive per kWh of gross capacity compared to a similar sized lead-acid battery but when comparing the £ per kWh of usable capacity the difference will be typically about 1.5 times as expensive. The lithium-ion battery will however last twice as long as the lead-acid so over a 10 year period the lithium-ion will almost always be a cheaper option with no need to renew the battery after 5 years.
Weight A lead-acid battery may weigh between 70kg and 80kg per kWh of usable capacity so a typical 5kWh – 6kWh domestic battery pack may weigh in excess of 350kg which may cause difficulty in locating a large battery pack in a residential property as a strong floor will be required. A good quality lithium-ion battery pack will typically weigh between 10kg and 15kg per kWh of usable capacity so considerably less than an equivalent lead-acid pack but a typical residential battery pack will still weigh 75kg – 100kg requiring some consideration as to where to place it.
Charge / Discharge Power Most lead-acid batteries can be charged and discharged relatively rapidly and when connected in parallel the total charge/discharge rate is in effect increased. In a typical solar PV system a lead-acid battery pack may be charged and discharged in 2 – 3 hours with a peak discharge rate much higher for short period of times. Most lithium-ion batteries have a relatively restricted charge/discharge rate often needing 3 – 4 hours to charge and a maximum discharge rate of between 1kW and 2kW for a typical residential system. A system utilising lithium-ion batteries therefore needs to be designed to take care to only connect essential loads to the circuit that will be powered from the battery pack.
Operating temperature Lead-acid batteries are significantly impacted by the ambient temperature and an increase from 20c to 30c can result in a 25% reduction in the lifetime as defined by the number of cycles and a 50% reduction in the lifetime as defined in years. Lithium Ion is less impacted by moderate temperature changes and ambient temperatures in the range of 15 – 30 degrees centigrade will not significantly impact the lifetime nor performance of the battery.

The choice of battery type is not a simple decision with many different factors to take into account but we would always recommend that a comparison is made using the above considerations and looking at the total cost over the life of the system and not simply choosing the lowest initial cost option which in many cases may be more expensive over the life of the system.