How long does a solar battery last?

A home battery is warranted on two clocks at once: years and use, whichever runs out first. Which one you reach depends entirely on how hard you work it, and working that out takes five minutes.

The answer changes whether storage pays, so it is worth doing before you buy rather than discovering it in year eight.

The two clocks

Calendar ageing happens with time. Cells lose capacity sitting still, influenced by temperature and by the state of charge they are held at. A battery in a hot loft ages faster than one in a cool garage.

Cycle ageing happens with use. Each full charge and discharge takes a small amount off the total. More throughput, faster decline.

A warranty typically guarantees a minimum remaining capacity after a stated number of years or a stated amount of use, whichever comes first. Real examples show the pattern: Tesla’s Powerwall warranty guarantees 70% of original capacity at ten years, with unlimited cycles in solar self-consumption use but a throughput cap in other operating modes, while GivEnergy offers twelve years on approved residential installations, conditional on periodic health checks. That phrasing is the manufacturer telling you both mechanisms are real, and that how you operate the battery decides which clause governs you.

Working out which clock you will hit

Divide the warranted cycle or throughput figure by your expected use per year.

If you charge from surplus solar and discharge each evening, that is roughly one cycle a day, about 365 a year, 3,650 a decade. If you also charge overnight on a cheap tariff and discharge during an expensive period, the arbitrage pattern, you may be doing one and a half or two cycles a day, which doubles the decade’s throughput to over 7,000.

Worked: a unit warranted for 6,000 cycles or ten years. At one cycle a day the cycles last sixteen years, so the ten year clock governs. At two cycles a day the 6,000 cycles are gone in just over eight years, before the year clock, and if those second cycles were in an operating mode the warranty does not cover, the position is worse still. Read the mode clauses, not just the numbers.

Compare the result with the warranted year figure:

• Cycles run out first: you are using the battery hard, which is good. It is earning. Plan for replacement earlier than the year figure suggests, and put the replacement in the projection.
• Years run out first, by a wide margin: the battery is under-used. Either it is oversized for your consumption, or there is an opportunity you are not taking, such as tariff arbitrage.

That second case is worth noticing. A battery that reaches its year limit with cycles to spare was larger than it needed to be, and the spare capacity was paid for without ever earning.

Reaching the warranty limit is not failure

At the end of its warranted life a battery still works. It holds a guaranteed percentage of its original capacity, 70% in the Tesla example, and the useful question is whether that remainder still covers your evening: a 10 kilowatt hour unit at its floor still delivers 7, which comfortably covers a 6 kilowatt hour evening and falls short of a 9.

A unit that started comfortably above your overnight consumption may still be adequate after years of decline. One that was sized exactly to your needs on day one will not be.

This is a mild argument for sizing a little above your measured requirement, balanced against the stronger argument that unused capacity never earns. The balance point is usually a size that cycles hard today and still covers a typical evening at its warranted floor.

What shortens life, in order

Deep, frequent cycling. Unavoidable, since throughput is how it earns: a preserved battery is an unpaid one. Modern chemistries handle it far better than older ones.

Heat. The one thing genuinely within your control. Where the installer proposes to site the unit is worth discussing: a cool, ventilated space beats a loft that reaches high temperatures every summer, and the siting question costs nothing to ask on survey day.

Sitting at full charge. Holding a battery at one hundred per cent for long periods contributes to calendar ageing. Systems that charge to full early in the day and hold there until evening are doing slightly more damage than ones that fill closer to the time of use. Some allow this to be scheduled; ask whether yours does.

Chemistry matters

Most current home batteries use lithium iron phosphate, which offers longer cycle life and better thermal behaviour than the older nickel-based lithium chemistries, at the cost of slightly lower energy density.

For a home installation, where physical size is rarely the binding constraint, that trade is a good one. If a quote specifies something else, ask why, and read the cycle warranty beside the answer.

Putting it in a projection

The correct treatment is to reduce usable capacity year by year along the warranted curve and recompute what the battery can shift on that reduced capacity, which is how the degradation mechanics actually behave and how our calculator models it.

If your projection runs longer than the expected life, a replacement belongs in it at a realistic cost. A twenty-year savings table containing one battery and no replacement is not describing twenty years, and the omission flatters the exact years where the totals grow largest.

The question to ask a supplier

“What remaining capacity is guaranteed, after how many years and how much throughput, in the operating mode I will actually use, and where are you proposing to site it?”

Four specifics, all answerable from the warranty document and the survey, all of which go straight into the arithmetic, and none of which the brochure’s “lasts over a decade” was written to answer.