Lithium vs lead-acid batteries

The short answer

For a solar system that cycles every day, a lithium iron phosphate battery (LiFePO₄) almost always costs less per unit of energy delivered over its life than lead-acid. You can use nearly all of its capacity, it lasts many times more cycles, and it needs no maintenance. That is why every Elumina system uses LiFePO₄ storage.

Lead-acid still has a place: occasional backup that is rarely used, or very tight budgets where the upfront figure is the only thing that matters. For daily solar use it is usually the more expensive choice in the end.

Depth of discharge: how much you can actually use

A battery's label shows its nominal capacity. You cannot safely use all of it. Depth of discharge (DoD) is the share you can draw down without shortening the battery's life.

• Lead-acid: about half its rating is usable for daily cycling. Discharging deeper wears it out much faster.
• LiFePO₄: nearly all of its rating is usable.

So a lead-acid bank needs to be roughly twice the nominal size to deliver the same usable energy each night. A quote that compares a "10 kWh" lead-acid bank with a "10 kWh" lithium battery is not comparing like with like.

Cycle life: how long it lasts

One cycle is one full discharge and recharge. A solar battery does roughly one cycle a day, so cycle life is effectively lifespan.

• The LiFePO₄ batteries we fit are rated for 6,000 cycles or more. At one cycle a day that is over sixteen years. They carry a five-year warranty.
• Lead-acid batteries are typically rated for somewhere between a few hundred and around 1,500 cycles, depending on type and how deeply they are discharged. In daily solar use, that usually means replacing the bank several times over the life of the panels.

Lead-acid ranges are typical industry figures. Check the datasheet of any specific battery you are quoted, and look for the cycle rating at the stated depth of discharge.

The fair way to compare prices

Do not compare battery prices. Compare the cost of each unit of energy the battery will deliver over its life:

Run any two quotes through that sum. Because lithium gives you about twice the usable share and several times the cycles, it can cost two or three times as much upfront and still deliver each unit for a fraction of the lead-acid cost. Add the replacements a lead-acid bank will need over the years and the gap widens further.

Maintenance, heat and safety

• Maintenance. Flooded lead-acid batteries need their water topped up and their terminals cleaned. LiFePO₄ needs nothing beyond a periodic settings check, which our maintenance visits cover.
• Heat. High temperatures shorten the life of every battery, but lead-acid suffers more. Wherever you are in Kenya, install batteries in a shaded, ventilated spot, never in a hot roof space.
• Safety. LiFePO₄ is among the most thermally stable lithium chemistries, and each battery has a built-in battery management system that protects it from over-charging and over-discharging.
• Space and weight. Lithium is much lighter and smaller for the same usable energy, which makes wall mounting practical.

Reading the battery line on a quote

A good quote states, for the battery:

• the chemistry (LiFePO₄, or the type of lead-acid);
• the voltage and capacity, for example 48V 100Ah. Multiply them to get nominal kWh: 48V × 100Ah = 4.8 kWh;
• the usable capacity, or the depth of discharge it is designed for;
• the cycle rating and the warranty period, and who handles a claim.

If a quote just says "battery" or "200Ah" with no voltage or chemistry, ask. Our checklist for solar quotes covers the rest of the lines to check.

Common questions