Within the field of long-term storage of solar energy, lithium-ion batteries (batteries) by far excel others in energy density and cycle life. Data from the United States National Renewable Energy Laboratory (NREL) in 2023 show that lithium iron phosphate (LiFePO4) battery cycle life can be 3,000 to 5,000 times (with an 80% capacity retention rate), far exceeding the 300 to 500 times (with a 50% capacity drop) of lead-acid batteries. Take Tesla Powerwall as an example. Its total cost of ownership (TCO) of the lithium battery system it adopts over a 10-year cycle is $0.18 per watt-hour, a reduction of 48% from lead-acid batteries at $0.35 per watt-hour, and installation space requirements reduce by 62% (it only requires 0.8 cubic meters to deliver the same capacity, whereas lead-acid batteries require 2.1 cubic meters).
High-temperature suitability is another clinching factor. Measurements conducted in 2024 by the Australian Solar Energy Institute also reveal that capacity loss per year for LiFePO4 battery upon heating to 50°C is 0.8% whereas for lead-acid batteries it goes up to 12% at a heat of 50°C. Although the cycle life of REDOX flow batteries (such as all-vanadium REDOX flow batteries) is over 10,000 times, their energy density (15-25Wh/kg) is just 1/5 that of lithium batteries, thus the weight of the same 20kWh energy storage system is 800kg (whereas that of lithium batteries is merely 160kg), and the initial investment cost is 73% higher (about 600 US dollars /kWh for REDOX flow systems). The cost of lithium batteries is 350 US dollars per kWh.
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The charging and discharging efficiency directly affects the energy return. Lithium-ion battery round-trip efficiency has been estimated to be between 95% to 98% by the Fraunhofer Institute in Germany, while that of lead-acid batteries is only between 70% to 85%. Taking as an example an off-grid system with an average daily charging and discharging capacity of 50kWh, the lithium battery solution can store an extra 2,750 KWH of electricity per year, which translates into a 13% increase in the utilization of solar energy. Although sodium-ion batteries are 22% (approximately $270/kWh) less expensive than lithium batteries, their cycle life (2,000 cycles) and cold-weather performance (capacity falls to 65% at -10°C) limit their use in the long term and thus are suitable only for low-load temperate-climate applications.
Environmental compliance increases the cost of technological substitution. The EU’s battery Regulation (which came into effect in 2027) requires the recycling rate of lead-acid batteries to be above 95% with processing cost up to a maximum of 120 US dollars per ton (recycling cost of lithium batteries is only 80 US dollars per ton). The California Energy Commission’s 2025 report implies that the carbon footprint of Li battery solar systems (15kg CO2/kWh) is 46% less than those of lead-acid systems (28kg CO2/kWh). The project in Qinghai, China, uses the array of LiFePO4 batteries. The capacity retention rate is 91.2% after 10 years of operation, and three lead-acid battery packs have been replaced in the same decade, and the heavy metal pollution amount is 4.7 tons.
Market data also confirm the trend of technology iteration. According to Bloomberg New Energy Finance, in 2023 the 89% of new worldwide solar energy storage was accounted for by lithium batteries, while the contribution of lead-acid batteries fell to 6%. A classic example is an off-grid venture of a mining company in South Africa. After a transition to LiFePO4 battery, the rate of using diesel generator decreased from 8 hours per day on average to 1.2 hours, and the annual fuel cost amounting to 420,000 US dollars was saved. For ultra-long cycle (25 years +) situations, while the capacity attenuation rate of the flow battery (0.001% per time) is superior, its power density (0.1-0.2kW/kg) is only 1/10 of lithium batteries, and the maintenance cost (average 3% of equipment value per year) is five times greater than that of lithium batteries (0.5%). Overall advantages are still second-rate.
Empirical tests demonstrate that LiFePO4 battery is capable of outputting more than 90% of its rated power within the temperature range of -20°C to 60°C, while the power of lead-acid battery reduces significantly by 40% at -10°C. During the extreme cold weather incident in Texas in 2022, the rate of availability of power supply from solar systems with lithium batteries was 98.7%, while from lead-acid systems it was only 61.2%. With technological advancements like silicon anodes, the energy density of lithium batteries can reach above 300Wh/kg by 2025 (presently 130Wh/kg). At that time, the total cost of ownership (TCO) in 2025 will decrease even further to 0.12 US dollars /Wh, solidifying its leadership in the long-term energy storage market.