Lithium versus lead-acid batteries: the real cost over ten years

Walk into any solar shop in Nairobi and you will be offered two battery options at very different prices. The lead-acid battery costs a fraction of the lithium one. For a customer comparing two figures on a quotation, the choice looks obvious.

It is obvious — but not in the direction the price tag suggests. This article does the arithmetic properly.

The two numbers that decide everything

Battery cost is not the purchase price. It is the purchase price divided by the total energy the battery will deliver across its life. Two properties determine that:

Cycle life — how many times the battery can be charged and discharged before its capacity falls below a useful threshold.

Depth of discharge — how much of the rated capacity you can actually use without damaging it.

These two numbers are where lead-acid and lithium iron phosphate (LiFePO4) diverge sharply.

Lead-acid: the honest specification

A good deep-cycle lead-acid battery, treated well, delivers somewhere around 500 to 800 cycles. Treated badly — repeatedly discharged deeply, left partially charged, or run hot — it can fail in under 200.

Critically, "treated well" means not using more than half of it. Discharging a lead-acid battery beyond 50% shortens its life dramatically. So a 200Ah 12V lead-acid battery, nominally 2.4kWh, gives you about 1.2kWh of usable energy per cycle.

Kenya presents a further problem: heat. Lead-acid life falls sharply with temperature. A battery in a hot store room or an unventilated cupboard in Mombasa will not achieve what the datasheet promises in a European laboratory.

LiFePO4: the comparison

A LiFePO4 battery delivers 3,000 to 8,000 cycles depending on cell quality — the batteries in our catalogue are rated between 3,000 and 8,000. It can be discharged to 90% or beyond without harm.

So a 5.12kWh LiFePO4 battery gives about 4.6kWh usable per cycle, and will do that thousands of times.

It also tolerates heat far better, requires no ventilation for hydrogen gas, needs no topping up with distilled water, and — importantly for anyone who has bought a battery that turned out to be tired — reports its own state of health through a battery management system.

The ten-year arithmetic

Let us size both options for the same job: a household needing 5kWh of usable storage per day.

Lead-acid. At 50% usable depth, you need 10kWh of nominal capacity to get 5kWh usable. That is roughly four 200Ah 12V batteries. At 600 cycles — a fair figure for daily cycling in Kenyan conditions — that bank lasts about 1.6 years before it needs replacing. Over ten years you buy the bank six times.

LiFePO4. At 90% usable depth, you need about 5.6kWh nominal — a single 5.12kWh wall unit gets close, or one slightly larger unit covers it. At 6,000 cycles, daily cycling gives you over sixteen years of life. Over ten years you buy it once.

Even if the lithium battery costs four times as much on the day, buying one of them instead of six banks of the alternative is not a close contest. And that ignores the labour of replacing a battery bank six times, the disposal of the old ones, and the periods of degraded performance before each replacement.

Cost per stored unit — the figure that matters

Divide purchase price by (usable kWh × cycles) and you get cost per kilowatt-hour delivered over the battery's life. Run that calculation on any two batteries you are comparing and the answer becomes unambiguous. Lithium typically comes out at a fraction of lead-acid per unit delivered, despite the higher sticker price.

This is the number to ask any supplier for. If they cannot tell you the cycle life and the recommended depth of discharge for what they are selling you, they do not know what they are selling.

Where lead-acid still makes sense

Honesty requires acknowledging the cases where lead-acid is defensible:

• Very infrequent cycling. A battery that discharges a handful of times a year — a genuine emergency-only backup — may never reach its cycle limit, so cycle life stops being the deciding factor.
• A hard upfront budget ceiling. If the choice is a small lead-acid system now or no system at all, the lead-acid system is producing value while you save.
• Starting batteries. Lead-acid remains the right chemistry for engine starting, which is a completely different duty.

For daily-cycling solar storage, which is what almost every household and business installation actually is, the case for lead-acid has largely gone.

What to check before you buy lithium

Not all LiFePO4 is equal. Four things to establish:

1. 1Cell manufacturer. Cells from established manufacturers such as EVE or CATL behave predictably. Unbranded cells may not match their claimed capacity, and mismatched cells within a pack shorten its life.
2. 2Cycle rating and the retention threshold it assumes. "6,000 cycles" should be stated alongside the capacity retained at that point — commonly 80%.
3. 3Battery management system. A proper BMS protects against over-charge, over-discharge, over-current and temperature extremes, and communicates with the inverter. This is not optional.
4. 4Voltage class and inverter compatibility. A 51.2V low-voltage battery cannot be connected to a high-voltage inverter input, or the reverse. This is the single most expensive mistake in Kenyan solar, and it is entirely avoidable.

A warning about mixing

Do not mix old and new batteries in one bank, do not mix chemistries, and do not mix brands unless the manufacturer explicitly supports it. A bank behaves like its weakest member — a tired battery will drag down healthy ones, and you will replace the whole bank sooner than either would have failed alone.

If you plan to expand later, buy an inverter and battery family that supports expansion, and add capacity within that family.

The short version

Lead-acid is cheaper to buy and considerably more expensive to own. For anything cycling daily — which is nearly every solar installation — LiFePO4 wins on cost per unit delivered, on tolerance of Kenyan heat, on maintenance, and on the simple fact that you install it once.

Every battery in our storage range is LiFePO4, and each product page states its usable capacity, voltage class and cycle rating so you can run this arithmetic yourself.