A lead-acid battery bank costs less than lithium on the day you buy it. But a battery is a store of energy you draw on every day for years, so the fair question is what each type costs for every kWh it gives back. Measured that way, the answer usually flips. For the technical differences, see our LiFePO₄ vs AGM battery guide.

The label is not what you get

The share of a battery's rated capacity you can safely use is its depth of discharge (DoD):

• Lead-acid, AGM and gel: about 50%.
• Lithium (LiFePO₄): 80–100%, depending on the battery and your settings.

A common 48V lead-acid bank is four 12V 100Ah batteries. On paper it holds 4 × 12V × 100Ah = 4.8kWh, but at 50% you can only use about 2.4kWh. A 51.2V 100Ah lithium battery holds 5.12kWh and at 90% gives about 4.6kWh. To get that 4.6kWh from lead-acid, you would need a 4.6 ÷ 0.5 = 9.2kWh bank, nearly twice the size.

Cycle life and replacements

Each drain and recharge is one cycle. A home solar battery usually does one a day: 365 × 10 = 3,650 cycles in 10 years.

• Lithium (LiFePO₄): typically 3,000–6,000 cycles.
• Lead-acid, AGM and gel: typically a few hundred to about 1,000 cycles, or roughly 2–4 years in a solar system.

Even a lead-acid battery rated for 1,000 cycles is used up in 1,000 ÷ 365 = about 2.7 years of daily use. Over 10 years that is 3,650 ÷ 1,000 = 3.65, so four banks: the first and three replacements. With a 2–4 year life, expect three to five banks in 10 years.

A lithium battery rated for 6,000 cycles covers 3,650 with room to spare. One rated for 3,000 reaches its rating after 3,000 ÷ 365 = about 8.2 years, then carries on at reduced capacity, since the end of a rating usually means around 70–80% of original capacity, not a dead battery. Either way, it is one battery for most or all of the decade.

The sum that matters: cost per kWh delivered

Cost per kWh delivered = total battery cost over 10 years ÷ total kWh delivered over 10 years

Total kWh delivered = usable kWh per cycle × number of cycles. The example below uses made-up round numbers: example figures only, not our prices. Use real quotes for your own sums.

• Lead-acid example: a 5kWh bank costs R10,000. At 50% DoD it delivers 2.5kWh a cycle, or 2.5 × 3,650 = 9,125kWh over 10 years. Even at the top of the range, about 1,000 cycles each, you need four banks: 4 × R10,000 = R40,000. Cost per kWh: R40,000 ÷ 9,125 = about R4.38.
• Lithium example: a 5kWh battery costs R20,000, twice as much. At 90% DoD it delivers 4.5kWh a cycle, or 4.5 × 3,650 = 16,425kWh over 10 years. Rated for 6,000 cycles, one battery does the decade. Cost per kWh: R20,000 ÷ 16,425 = about R1.22.

Here the battery that costs twice as much up front works out at less than a third of the cost per kWh, and delivers nearly twice the energy every night. Your numbers will differ, but once you count usable capacity and replacements, lithium usually wins.

Efficiency and charging speed

Lithium typically gives back over 90% of the energy you put in. Lead-acid loses more, often around 15–20%, so it needs more panel power to fill.

Speed matters too, with only about 4.5–6 peak sun hours a day. Lithium accepts a strong charge almost all the way to full. Lead-acid slows right down near the top, and if it is not fully charged regularly, sulphate builds up on the plates and capacity is lost for good. On a short winter day, it often never gets there.

Weight and space

A 12V 100Ah lead-acid battery typically weighs about 25–30kg, so a four-battery bank weighs about 100–120kg (4 × 25 = 100, 4 × 30 = 120) for just 2.4kWh of usable energy. A single 51.2V lithium battery gives nearly twice the usable energy in one unit that is much lighter.

Maintenance and balancers

The four batteries in a 48V lead-acid bank drift apart over time and the weakest drags the bank down, so you need a battery balancer, one more part to buy. Flooded batteries also need topping up with water and good ventilation, as they give off gas while charging. Sealed AGM and gel types skip the water but still need balancing.

A lithium battery has a built-in battery management system (BMS) that balances the cells and protects against overcharging and deep discharge. There is nothing to top up, but you must set the correct battery type on your inverter.

When does lead-acid still make sense?

• Tiny, backup-only systems: if it only runs the Wi-Fi and lights during the odd outage, it does few cycles, though it still needs replacing every few years.
• A short-term need: such as moving house within a year or two.
• A working bank you already own: use it until it is tired, then replace the whole bank. Never mix lead-acid and lithium in one bank, and change your inverter's battery settings when you switch.

Choosing a lithium battery

Our 48V inverters pair with our own 51.2V lithium batteries in 5.12kWh and 16kWh sizes, and we also stock batteries from Sunsynk, Pylontech, Dyness and Ecco. Most can grow with you: start with one and add more of the same model in parallel later. For the full comparison, read our LiFePO₄ vs AGM battery guide, and for runtimes see how long a 5kWh lithium battery will last. Need help choosing? Contact us.

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