The choice of a battery is one of the most critical decisions that needs to be made when designing a
grid-backup or enhanced self-consumption solar PV system. The two main types of battery commonly
chosen for solar PV systems are Lead Acid and Lithium Ion with various different specific types and
products from many different manufacturers available on the market. The table below gives a summary
comparison of the key attributes of these two different battery technologies.


AttributeLead AcidLithium Ion
Total Storage CapacityAn 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 CapacityThere 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 of a lead-acid battery, 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 90% of the gross storage capacity.
Full Cycle EfficiencyLead-acid batteries tend to get less efficient the nearer to full capacity they reach which either results in a low full cycle efficiency of less than 80% if they are re-charged near to their full capacity or designing the system to only use about 80% of their full capacity in order to maximise their efficiency.Most lithium-ion batteries have a full cycle efficiency around 95% even for a cycle from their full depth of discharge up to full capacity making them ideally suitable 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 of a daily basis a typical lead-acid battery will have a lifetime of 2,000 - 2,500 cycles. Allowing for some degredation 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 battery management system will ensure that the battery condition is always maintained in optimum condition and a full 10 year life may be expected.
CostThe initial investment cost of a lead-acid battery will be relatively cheap when expressed as Rand per kWh of gross capacity but all comparisons should always be done a Rand 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 Rand 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.
WeightA lead-acid battery may weigh between 70kg and 80kg per kWh of usable capacity so a typically 5kWh - 6kWh domestic battery pack may weight 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 a equivilant lead-acid pack but a typically residential battery pack will still weigh 75kg - 100kg requiring some consideration as to where to place it.
Charge / Discharge PowerMost 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/dischage 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 utilsing 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 TemperatureLead-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 significant impact the lifetime nor performance of the battery.