A solar battery ages on two clocks. Cycle life counts charge and discharge events, usually until the battery falls to a remaining capacity the test or the warranty defines. Calendar life counts years, including the years the battery sits at partial charge and does almost no work. Either clock can run out first. A pack that has used few cycles can still be at the end of its calendar life, and a pack that cycles hard every day can wear out the cycle counter while the calendar date still looks young.
| Clock | What moves it | What a warranty number is |
|---|---|---|
| Cycle life | Repeated charge and discharge, at a stated depth, rate, and temperature | A contract for that stated duty, down to a remaining capacity the contract names |
| Calendar life | Time, including rest, at a stated temperature and state of charge | A contract about age, which can end with cycles still unused |
A cycle is only defined when the depth is defined
One cycle, in casual speech, means empty to full and back. Warranties are stricter and narrower. They specify a depth of discharge, a charge rate, a temperature window, and the capacity left at the end, such as a percentage of the original nameplate. Change the depth and you are in a different test. A shallow daily swing and a deep backup swing are not the same cycle, even though a counter on a phone app may increment once for each. How depth and the charge stages interact is the subject of depth of discharge and charge stages.
Temperature sits inside the definition. A cycle counted in a cool test chamber is not the cycle the same cell performs in a hot closet. Manufacturers who publish more than one cycle figure are usually publishing more than one duty. Read the footnotes before you compare two numbers that happen to be large. If only one figure is printed and the depth is missing, you do not yet have a cycle life. You have a marketing integer.
Chemistry changes which duty is even plausible. The reasons lithium iron phosphate is chosen for stationary solar storage, and the limits that choice does not erase, are in why LFP batteries are used for solar storage. This page will not attach a universal cycle count to that chemistry or to any other. A round number in the thousands, repeated across brochures, is not a measurement of your pack.
The lab cell is not the pack you bolt to the wall
A cycle result on a single cell, held at a fixed temperature, with copper tabs the test lab welded, describes that cell in that fixture. A packaged battery adds a BMS, busbars, fuses, a case, and cell-to-cell differences. The pack's warranty is the document that survived that assembly. Quoting the cell paper as the pack's life copies a result across a boundary the paper did not cross. If the pack warranty is silent, the cell paper still does not fill the silence.
Calendar aging continues during the months a backup battery waits at a high state of charge, which is exactly how some outage designs are programmed. The cycle counter barely moves. The years do. A warranty written only in cycles says nothing obvious about that wait, unless a separate calendar clause is printed. Read both clauses. How to find them on the sheet is part of how to read a battery datasheet.
Use the contract you were given
Ask for the warranty PDF that matches the serial number, and mark the depth, the end-of-life capacity, the temperature, and the calendar years. If the installation will cycle daily for bill management, confirm that this duty is the duty the warranty describes, not a backup-only duty with a different depth. If the installation will sit full for outages, confirm the calendar clause and the recommended storage state of charge. Those two questions select different lines on the same document.
Throughput in kilowatt-hours is a third way some contracts count use. It can be derived from cycles and depth, and it can also be written as its own cap. Treat it as whatever the contract says it is. Do not convert it back into a cycle count by assuming a depth the contract never stated. When a seller offers to translate the warranty into a single years-of-life figure for every climate, ask them to show the arithmetic and the clause. If they cannot, keep the clause and discard the translation.
