Battery types for off-grid: lead-acid, AGM, and lithium compared
The battery bank is usually the most expensive component of a small off-grid solar system, and the one with the shortest lifespan. Choosing the right chemistry determines whether the bank lasts 3 years or 15.
Flooded lead-acid (FLA)
The oldest and cheapest rechargeable battery chemistry. A bank of 6V golf cart batteries (Trojan T-105, Crown CR-220, or equivalent) is the traditional off-grid starter.
Cost per kWh of rated capacity: the lowest of any battery chemistry.
Usable depth of discharge (DoD): 50%. Discharging below 50% shortens life dramatically. A 200 Ah bank at 12V (2,400 Wh rated) gives 1,200 Wh usable.
Cycle life at 50% DoD: 800-1,200 cycles. At one cycle per day, that's roughly 2-3 years of daily cycling. Weekend-only systems or systems with oversized arrays (the battery rarely hits 50% DoD) last 4-6 years.
Maintenance: FLA batteries require periodic watering (topping up with distilled water as electrolyte evaporates during charging). Every 2-4 weeks, check the cells and add water to the fill line. They also need equalization charges (controlled overcharge to desulfate the plates) every 1-3 months.
Cold performance: Capacity drops roughly 20% at 0°C and 40% at -20°C. Discharged FLA batteries can freeze (a fully charged battery freezes at -70°C; a 50% discharged battery freezes at -10°C). Insulate or bring indoors in cold climates.
Best for: Budget builds where maintenance time is available. The cheapest upfront path to a working battery bank.
AGM (absorbent glass mat)
A sealed lead-acid variant. The electrolyte is absorbed in fiberglass mats between the plates instead of sloshing as a liquid.
Cost per kWh of rated capacity: roughly double FLA.
Usable DoD: 50% (same as FLA for longevity; technically AGM survives deeper discharge better than FLA, but regular deep cycling still shortens life).
Cycle life at 50% DoD: 600-1,000 cycles. Slightly less than premium FLA batteries at the same DoD.
Maintenance: None. Sealed, no watering, no equalization needed (though periodic equalization from a smart charger helps longevity). Can be mounted in any orientation. No acid spills.
Cold performance: Similar to FLA, 20% capacity loss at 0°C. Same freezing risk when discharged.
Best for: Systems where maintenance isn't going to happen reliably (remote installations, greenhouses visited weekly) or where the battery must be indoors and acid fumes are unacceptable.
LiFePO4 (lithium iron phosphate)
The modern choice. Heavier upfront cost, dramatically better lifecycle economics.
Cost per kWh of rated capacity: highest upfront, but dropping year over year.
Usable DoD: 80% routinely, 90-100% occasionally. A 200 Ah bank at 12V (2,400 Wh rated) gives 1,920 Wh usable, versus 1,200 Wh from lead-acid at the same capacity.
Cycle life at 80% DoD: 3,000-5,000 cycles for quality cells (EVE, CATL, BYD). At one cycle per day, that's 8-14 years. Some manufacturers spec 6,000+ cycles to 80% retained capacity.
Maintenance: None. No watering, no equalization.
Cold performance: LiFePO4 loses capacity in the cold (roughly 20% at 0°C), similar to lead-acid. The critical difference: LiFePO4 must not be charged below 0°C. Charging at freezing temperatures causes permanent lithium plating on the anode, which irreversibly reduces capacity. Most quality LiFePO4 batteries have a built-in BMS (battery management system) that prevents charging below 0°C. If your battery doesn't, add a low-temperature cutoff relay on the charge line.
Cost per cycle: This is where LiFePO4 wins despite the higher upfront cost.
When you factor in the deeper usable discharge (80% vs 50%) and the 3-4x cycle life, lithium costs roughly half as much per actual use-cycle as lead-acid. Over the lifetime of a system, the lithium bank also needs zero replacements in the same period where lead-acid needs 2-3 replacement sets.
Best for: Any system where upfront budget allows. The total cost of ownership is lower, the usable capacity per dollar is higher, and the maintenance is zero. For daily-cycling solar systems powering growing equipment, LiFePO4 is the default recommendation in 2026.
Other lithium chemistries
NMC (nickel manganese cobalt) and NCA (nickel cobalt aluminum): Used in EVs and power walls (Tesla Powerwall is NMC). Higher energy density than LiFePO4 (more Wh per kg), but shorter cycle life (1,000-2,000 cycles), higher thermal runaway risk, and more expensive. Not recommended for DIY off-grid builds; the safety margin is thinner than LiFePO4 and the cycle life advantage of LiFePO4 matters more than energy density in a stationary application.
Sodium-ion: Emerging chemistry. Cheaper raw materials than lithium, better cold performance, no lithium supply-chain risk. Cycle life is currently 2,000-3,000 (between lead-acid and LiFePO4). Not widely available to consumers in 2026 but worth watching; CATL and BYD have announced production lines.
Sizing the bank
The battery bank calculator on this site computes the required capacity for a given daily load and number of autonomy days (days of cloudy weather the bank must cover without solar input). The general formula:
Required rated capacity = (daily load Wh x autonomy days) / (DoD x system voltage)
Example: 2,300 Wh/day load, 2 days autonomy, LiFePO4 at 80% DoD, 48V system: (2,300 x 2) / (0.80 x 48) = 119.8 Ah at 48V (or 5,750 Wh rated).
For lead-acid at 50% DoD: (2,300 x 2) / (0.50 x 48) = 191.7 Ah at 48V (or 9,200 Wh rated).
Lead-acid needs 60% more rated capacity than LiFePO4 for the same usable storage because of the shallower safe discharge depth.