Lithium or lead-acid: which one actually pays

In a backup quote the battery is almost always the most expensive line, so it is where people cut. The question is usually framed as “lithium or lead”, but it is incomplete: what matters is how many times a year you will use it. A battery that discharges deeply every night lives in a different world from one that works six times a year.

The two numbers that decide everything

Almost any battery argument resolves with two figures that rarely appear together on a datasheet.

Depth of discharge (DoD)

The percentage of capacity you can use without punishing lifespan. In lead-acid it is around 50 %: of a 2.4 nominal kWh battery, only 1.2 kWh is practically yours. In LiFePO4 it reaches 90 %, so from the same 2.4 nominal kWh you use 2.16 kWh.

Direct consequence: for the same useful backup you need nearly twice the nominal capacity in lead. That doubles the weight, the space, and much of the price advantage that seemed to be there.

Cycle life

A cycle is one charge and one discharge. A good-brand lead battery delivers 400 to 600 cycles at 50 % discharge. A LiFePO4 delivers 3,000 to 6,000 cycles at 80–90 %. If you lose power every day, one cycle a day means lead lasts between eighteen months and two years, while lithium passes eight.

The cost that matters: per delivered kWh

The honest comparison is not what the battery costs, but what each kilowatt-hour it will deliver before dying costs. The arithmetic is simple:

With reference prices in US dollars and two batteries of the same nominal capacity:

The gap is more than five to one in lithium’s favour, and that is without counting that over those eleven years you would have bought eight lead banks, with eight freight charges and eight installations. Counting replacements, lithium comes out cheaper in cash around the third year of daily use.

The arithmetic flips if use is occasional. If you only lose power a dozen times a year, those 500 lead cycles are forty years of service, and the battery will die of old age rather than of use: in that scenario lead costs a third as much and does the same job.

The deciding factor in hot climates: heat

Almost every cycle table is measured at 25 °C. A utility room in a hot climate passes 35 °C much of the year, and a battery bank under a zinc roof can pass 45 °C.

For lead-acid this is serious: there is a well-documented rule of thumb that every 10 °C above 25 °C halves service life. Those 500 catalogue cycles become 250 in a room at 35 °C. The battery that looked cheap gets replaced every nine months.

LiFePO4 tolerates heat far better on discharge, though it is not immune either: above 45 °C the BMS starts limiting current and degradation accelerates. In both cases the advice is the same, and it is free: cross ventilation, never directly under the roof sheet, never in direct sun, and clear space around the cabinet.

Maintenance, space and safety

Weight is not a minor detail. A lead bank of 5 usable kWh weighs close to 250 kg and usually needs a reinforced base; the LiFePO4 equivalent weighs about 60 kg and hangs on the wall. In a house with limited space, that decides the installation.

On safety it pays to be precise, because a lot of noise circulates: the battery fires that make the news almost always involve NMC chemistry, the one in electric cars and power tools. LiFePO4 is a different and markedly more stable chemistry, so much so that it is what portable stations and serious residential banks use. Even so it demands a real BMS — the underlying reason not to buy loose cells of dubious origin.

When lead is still the right answer

It is not a dead technology. It wins clearly in three scenarios:

• Occasional use. Fewer than two cycles a week. The battery will die of old age rather than of cycling, and lithium never gets to collect on its advantage.
• A hard upfront budget. If $400 is the difference between starting and not starting, a lead bank today beats a lithium bank next year.
• Remote sites with local service. In outlying towns someone repairs and recycles lead, and possibly nobody can diagnose a BMS. Local repairability is worth more than efficiency on paper.

If you go lead, choose deep cycle, not starting batteries. A car battery is built to deliver a lot of current for a few seconds, not to discharge slowly every night: in backup service it lasts months.

What to avoid

• “Lithium” with no surname. If the datasheet does not say LiFePO4 or LFP, ask the chemistry. NMC in a hot room with no thermal management is not what you want in your house.
• Starting batteries used for backup. Cheap today, replaced in six months.
• Loose cells with no BMS. The home-built pack costs more once the cells drift out of balance, and there is no warranty.
• Mixing old and new batteries in the same bank. The set behaves like the worst of them.
• Datasheets with no declared cycles. No cycles and no depth of discharge means there is nothing to compare.

The practical conclusion

If you lose power several times a week, LiFePO4 is the sensible decision, and not because it is fashionable: because of the cost of delivered energy, the tolerance to heat, and the absence of maintenance. Lead still makes sense for occasional backup or when the upfront budget is the only constraint that matters.

To find out how much capacity you need before comparing prices, run the calculator. And if you are still deciding between a portable station and a fixed system, continue with the guide on portable versus hybrid inverter.