In 2025, your practical and proven choices for solar backup are between lead-acid and lithium-iron-phosphate (LFP) batteries. LFP batteries have dropped in price dramatically in the past 5 years. They are now the same price or less expensive than lead-acid batteries. LFP batteries last much longer than lead-acid batteries.
The main concern when choosing the best solar battery is safety. LFP batteries can suffer catastrophic failures, leading to thermal runaway, fire, explosions, and the release of highly toxic gases. LFP fires cannot be extinguished; they must burn themselves out.
By comparison, accidents and failures with lead-acid batteries are much less likely to cause property damage and bodily harm. Gel or AGM sealed lead-acid batteries are safer to use in living spaces because of the fire hazard associated with lithium batteries. Because of the chance of thermal runaway associated with lithium batteries, sealed-lead-acid, Gel, or AGM batteries are safer to use in living spaces.
Lead Acid vs Lithium Ion
Lead-acid Flooded, AGM, and Gel
Lead-acid batteries are available in standard flooded, absorbent glass mat (AGM), and Gel. All three types have the same basic lead-acid chemistry. The terms sealed-lead-acid and valve-regulated lead-acid can refer to flooded, AGM, or Gel.
Flooded Cell
Flooded-cell lead-acid batteries have a liquid electrolyte covering the lead plates. They are the original wet-cell battery design, which has remained almost unchanged since it was invented by French physician Gaston Planté in 1859. Most combustion cars use lead-acid batteries. Starting a car engine requires a short burst of 400 to 600 amps, and flooded cell batteries are excellent for this purpose.
This battery type is the least expensive and has a service life of 200 to 300 cycles, the shortest of all battery types. The service life is defined as the number of charge/discharge cycles until the battery capacity decreases to 80% of its original value. The battery is still usable after its normal charge cycle life, especially if it was oversized in the first place. However, it will hold a smaller total charge and may be marginal for engine starting. A flooded cell battery lasts about 3 to 5 years.
Flooded cell batteries tolerate a depth of discharge of about 50%. The depth of discharge (DoD) is the percentage of a battery’s rated capacity that it can be discharged and recharged without damage. In other words, a flooded cell battery rated at 100Ah has a usable capacity of 50Ah. Taking a battery deeper than 50% DoD can shorten its life.
Gel
Gel batteries are a newer design with the same basic chemistry as a flooded-cell battery. In this case, the liquid electrolyte is modified into a semi-solid gel by the addition of silica. Gel batteries tolerate deep cycling well but are not suitable for engine starting. They are spill-proof and vibration-resistant, but do not perform well at low temperatures. Gel batteries have the lowest self-discharge rate of all lead-acid batteries. They also have the longest life of all lead-acid batteries.
Gel batteries must be charged slowly and are easily damaged by overcharging. They require properly calibrated chargers. Make sure that the charger you are using can handle a gel battery. To learn more about solar chargers, click here.
Gel batteries are the most expensive type of lead-acid battery. Because they are deep-cycle, long-lived, safe, and maintenance-free, they are the best solar backup batteries in the lead-acid class.
Absorbent Glass Mat (AGM)
Instead of being filled with liquid electrolyte, AGM batteries have a fiberglass mat between the metal plates that are saturated with electrolyte. Think of a sponge saturated with water compared to a glass of water.
AGM batteries deliver more starting amps for their size than flooded-cell batteries. They also have a longer service life, although not as long as lithium batteries. They are vibration-resistant, perform well in extreme temperatures, and are maintenance-free. AGM batteries are commonly used in boats, racing cars, and aircraft.
AGM batteries are more expensive than flooded cells but still much less expensive than lithium batteries. They have a lower Ah rating than flooded-cell types of the same size and weight. This means that they are good at starting engines, but will have less total capacity as a house battery for running your appliances. While they can be charged faster, they are more easily damaged by overcharging. AGM batteries can be damaged by chargers intended for standard flooded-cell batteries. They can be used for both starting engines and as a deep-cycle house or solar backup.
Lithium-Ion
In 2025, Lithium-Iron-Phosphate(LFP) is the only lithium battery chemistry to consider for solar energy storage.
Understand Thermal Runaway
In thermal runaway, the battery releases energy uncontrollably, raising internal temperatures and increasing the risk of fire and explosion. It can occur when the battery is defective, damaged, overheated, or overcharged. This is why lithium batteries are restricted on commercial airlines.
LPF batteries are more thermally stable than other lithium chemistries. LPF batteries also have a reduced risk of fire during thermal runaway.
Fortunately, lithium batteries are much safer in 2025 than they used to be. Consider that they are in common use in all types of consumer devices and in electric cars.
Battery Management System (BMS)
To maximize safety, a solar system using lithium batteries must be properly designed for battery storage, overload protection, charger and inverter selection, and maximum safe electrical load. All lithium batteries require specialized protection circuits, called a battery management system (BMS), to monitor their power output, charging rate, and temperature to prevent internal damage and thermal runaway. The BMS can include components both internal and external to the battery. When you pick a battery, the BMS must be compatible with other components, such as the solar panels, inverters, chargers, and loads.
Lithium-Iron Phosphate(LFP)
Right now, LFP batteries are the best choice for solar energy backup. In case you are wondering why it is abbreviated LFP, it is because the chemical symbol for iron is Fe, as in the word ‘ferrous.
The Safest Lithium Battery
LFP batteries are one of the safest types of lithium batteries. Compared to other lithium chemistries, they have a higher thermal runaway temperature. Above the thermal runaway temperature, the “chemical short circuit” cannot be stopped, and the battery overheats and self-destructs.
When other types of lithium batteries experience thermal runaway, the internal chemical reactions release oxygen. This allows the battery to burn without air. LFP batteries do not release oxygen during thermal runaway. They may catch fire during thermal runaway if there is sufficient oxygen from the outside.
Specific Energy
LFP batteries have a specific energy of about 90 to 160 watt-hours per kilogram(Wh/Kg). While this is only a moderate energy density compared to other lithium chemistries, it is much better than lead-acid, which has a specific energy of about 30-50 Wh/Kg. This means LFP batteries are lighter and smaller than lead-acid batteries, but are bigger and heavier than other, more powerful, less safe lithium batteries.
Cobalt-Free
LFP batteries are cobalt-free. Because cobalt is unstable at higher temperatures, eliminating it from the battery chemistry improves safety. Also, cobalt is very expensive and has a large environmental impact. So being cobalt-free makes LFP batteries safer, less expensive than other lithium batteries, and better for the environment.
Charge/Discharge Life
LPF batteries have a charge/discharge life of up to 4000 cycles, which is longer than that of other lithium chemistries and much longer than that of lead-acid batteries. They can be run to 80% DoD without reducing their lifespan. Expect an LPF battery to last about 15 years.
Conclusion
To summarize, in 2025, your choices for a solar backup battery are either lead-acid or lithium-iron phosphate. The least expensive battery to buy is a flooded lead-acid battery. While the lithium-iron-phosphate (LFP) battery may cost more upfront, it is cheaper over its total lifespan because it lasts much longer.
Battery Parameters
Before purchasing a battery, determine how much electricity your system consumes daily. For more information on sizing solar energy systems, please click here.
Before you buy a battery, make sure you understand the following properties:
Rated Capacity and Depth of Discharge
The rated capacity is simply the battery’s named size in Ampere-hours. Keep in mind that energy calculations are in watts, which is amperes times volts. To find the usable energy in the battery, multiply the rated capacity by the Depth of Discharge (DoD) percentage. For example, if the recommended battery DoD is 50%, then the usable energy in a 100Ah battery is 100 x .5 = 50Ah. Many flooded-cell lead-acid batteries have DODs of 50%.
Maximum Current, Deep Discharge, or Multi-Purpose
Batteries can be designed to optimize the maximum current for starting engines or operating other heavy-draw equipment. Alternatively, they can be designed for deep cycles. Boats often have separate starting and house batteries. The starting battery provides the maximum cranking amps, and the house batteries keep the cabin appliances and lights going all night. Some AGM batteries are advertised as dual-purpose. Be sure to check the specs.
Charge-Discharge Lifecycle
A basic lead-acid battery lasts about 300 cycles before reaching 80% capacity. AGM and gel types may last into the high hundreds of cycles. LFP batteries last for thousands of cycles and usually have an 80% DoD.
Battery Management
Make sure you know the battery storage and operation requirements. Flooded cell, AGM, and gel batteries have different requirements for storage and charging. Overall, lead-acid batteries are simpler to manage and safer than lithium-ion batteries.
Most lithium batteries have a built-in management system that monitors battery temperature, voltage, output, and charge state. Charge controllers for lithium batteries should be matched to the specific battery and include a remote battery temperature monitor.
Safe Installation
Installation requirements vary depending on the battery. Basic flooded cell lead-acid batteries require ventilation. Many batteries must be protected from extreme temperatures. Make yourself aware of the installation guidelines and regulations for your specific application.