Solar batteries: lithium vs lead-acid

Before comparing technologies, confirm that you genuinely need storage. On a grid-connected system with net metering, batteries are not needed in order to save: the grid acts as backup and offsets on your bill the energy you exported.

Adding batteries purely to save usually worsens the return. Every kWh that passes through a battery suffers charge and discharge losses and consumes a fraction of the equipment's service life: that kWh comes out more expensive than the same kWh offset through the grid.

Batteries are justified when what you want is continuity — frequent outages, critical equipment that cannot stop — or when there is no reliable grid, as on many farms. When you do need them, size them for the priority loads, not for the whole installation.

Almost every misunderstanding when quoting storage comes from confusing four different parameters.

Energy, in kilowatt-hours (kWh), determines how long you can sustain a load. Power, in kilowatts (kW), determines which loads you can switch on at once. A battery can have energy for the whole night and still fail to start a pump if its power output is low.

Usable depth of discharge is what fraction of the nominal capacity you can use without penalising the equipment's life. It is the parameter that most distorts comparisons, because it is where the two technologies differ most.

And round-trip efficiency is how much of the energy that goes in comes back out. What is lost there is paid every day for the whole life of the system.

In stationary storage the most common chemistry is lithium iron phosphate, known by its acronym LFP, chosen for thermal stability and long life rather than energy density — which matters in a vehicle, not in a plant room.

Its practical traits: it allows deep discharges, usually 80 to 90% of its capacity; it withstands far more cycles than lead; its round-trip efficiency is around 90-95%; it takes up considerably less space and weight; and it needs no periodic maintenance.

It also incorporates an electronic management system that watches each cell, cuts out on overcharge or excessive discharge and balances the pack. That system is part of the product and explains much of its price: it is not a battery, it is a battery with its controller.

It is the classic technology of off-grid storage, with more than a century of history, well known to any installer and with a clearly lower entry price.

Its main limit is depth of discharge: routinely going below 50% dramatically shortens its life. In practice, a lead battery of a given nominal capacity delivers considerably less usable energy than its label suggests. It also withstands fewer cycles and its round-trip efficiency is lower.

Within lead there are variants: flooded cells require topping up with water and careful ventilation, while sealed AGM or gel types remove that maintenance in exchange for a somewhat higher price. All of them are sensitive to being left discharged for long periods, which degrades them permanently.

Lead-acid is cheaper to buy, but its shorter service life and lower usable depth of discharge mean that, over the years, lithium usually works out cheaper per unit of energy genuinely used.

The sum that levels the comparison is simple: multiply the nominal capacity by the usable depth of discharge and by the number of guaranteed cycles. That gives you the total kWh the battery will deliver in its life. Divide the price by that figure and you get the cost per usable kWh, which is comparable across technologies.

That sum often reverses the apparent result, and the more intensive the use, the more strongly. For daily backup, lithium wins comfortably; for backup a few hours a year, lead can still make sense because cycle life stops being the limiting factor.

Bear in mind too that cycle life is declared down to a residual capacity — usually 70 or 80% of the original — not until the battery stops working. And budget for replacement within the life of the solar system: the panels last 25 years, the battery bank does not.

Sustained heat accelerates the ageing of almost any chemistry. Across much of Colombia this is not a minor detail: where the bank is located has a real effect on how many years it lasts.

The sensible choice is a ventilated place, shaded and protected from rain, never under uninsulated metal roofing or in a closed room that traps heat. Each technology declares its operating temperature range without penalty; it is worth checking that the planned location falls within it.

On safety, flooded lead batteries release gases during charging and require adequate ventilation of the enclosure. Lithium iron phosphate is among the most stable chemistries and has no such requirement, although like any electrical installation it must comply with the applicable regulations, with its protections and an accessible disconnect.

The ones that most often lead to buying less battery than you think or replacing it ahead of time.