The pack you buy is not the pack you own in year twelve

The warranty on each pack here carries both a calendar term and a throughput limit — 15 years ^[1] and 6,000 cycles ^[2] for the larger unit. Cover ends when you reach either one, and which arrives first is set by how hard you cycle the pack.

That matters because it interacts directly with the strategy you chose. A pack bought for outages and held near full most of the time accumulates few cycles, so its calendar term ends long before its cycle count does. A pack cycled every evening for bill savings is spending that account steadily, and can exhaust it while the calendar warranty still has years left on paper.

Multiplied out, the warranted cycle count times the day-one usable capacity gives 60,000 kWh for this pack ^[3]: our own arithmetic rather than a figure the warranty states, and an upper bound on the energy delivered within the warranted cycles, because capacity fades as they are used. Multiply that by the widest margin a kilowatt-hour can earn on your tariff’s published prices and you have the ceiling the payback tool prints beside your net cost: the most this model lets the pack earn on arbitrage within its warranted cycles. It is not a limit on what the pack can do once the warranty ends.

The retention curve is ours, and so is its anchor. We could not obtain a published capacity-versus-cycles curve for these packs, so rather than copy one from a competitor or invent a manufacturer figure, we assumed that a pack worked to its full warranted cycle count arrives at 70 percent of day-one usable capacity ^[4] and drew a straight line to it. That works out to 0.00005 of capacity per cycle ^[5]. Neither figure is a warranty term. The maker’s U.S. limited warranty documents for these packs, as published when we retrieved them on 2026-10-05, state their own capacity-retention terms, set against nameplate capacity at fixed points in the warranty period rather than along this curve, so read the document that applies to your activation date (Enphase lists them at enphase.com/warranty/us), not this assumption, for what is actually promised.

Anchored to a thirty percent loss over the full warranted cycle count and applied in a straight line. The real shape of the fade for these packs is not published on the pages we cite, so the capacity shown for any single year may be above or below what a particular pack keeps.

The practical effect on a payback figure is modest but real and it always runs one way: later years earn less than earlier ones, so a projection built on a flat annual saving reports a payback date that arrives slightly before the true one. Our schedule applies the fade year by year rather than averaging it, which is why the yearly rows in the payback worksheet decline instead of repeating.

Not “how long is the warranty” — that gets you a number of years and tells you almost nothing. Ask three things instead: what retained capacity percentage is guaranteed at the end of the term, what cycle count sits alongside the years, and whether the daily cycling your savings depend on is inside the terms or outside them.

The third question is the one that catches people. A warranty written around a backup-duty pack and a system sold on daily arbitrage savings can be quietly inconsistent, and the inconsistency only surfaces at the point of a claim. Get the answer in writing before the deposit rather than after the fade.