The pack you buy is not the pack you own in year twelve
Payback arithmetic quietly assumes the battery is the same machine every year. It is not. Capacity declines with use, and a warranty that reads fifteen years can expire in eight if you work the pack hard enough. Both effects push the payback date later, and neither appears in most projections.
Usable capacity in year 20
7.8 kWh
That is 78 percent of the day-one figure, and the calendar term runs out first in year 15. The fade curve is our own assumption; the term and the cycle count are the manufacturer’s.
| Year | Cumulative cycles | Usable kWh | Retention | Warranty |
|---|---|---|---|---|
| 1 | 230 | 9.94 | 99.4% | covered |
| 5 | 1,150 | 9.48 | 94.8% | covered |
| 10 | 2,300 | 8.91 | 89.1% | covered |
| 15 | 3,450 | 8.33 | 83.3% | covered |
| 20 | 4,600 | 7.76 | 77.6% | expired |
Anchored to a thirty percent loss over the full warranted cycle count and applied in a straight line. Real cells fade fastest early and then flatten, so this understates year-one capacity slightly and overstates late-life capacity slightly.
How hard you intend to work it
One a weekday is about 250. Two a day for an evening peak and an overnight top-up is about 730.
Two limits, running at different speeds
Quoted from the source document, retrieved 2026-08-06
“Supported by a 15-year limited warranty (6,000 cycles), the IQ Battery 10C offers dependable performance”
Both limits sit in one short sentence and the bracket is the half that gets dropped in conversation: the years and the cycles are conditions joined by whichever arrives first, so the useful reading is not fifteen years of cover but fifteen years of cover unless you use it, and how hard you use it is a decision you make rather than a property of the pack.
A storage warranty typically carries both a calendar term and a throughput limit — 15 years [1] and 6,000 cycles [2] for the larger unit here. Cover ends when you reach either one, and which arrives first is entirely determined by how you use the thing.
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 and will die of old age with cover intact. 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 usable capacity gives the total energy the manufacturer stands behind: 60,000 kWh for this pack [3]. Divide that by what a kilowatt-hour of throughput earns on your tariff and you have the absolute ceiling on lifetime arbitrage income — the figure the payback tool prints beside your net cost.
What we assume, said plainly
The retention curve is ours. 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 anchored a straight line to the one thing the warranty implies: a pack worked to its full warranted cycle count arrives at 70 percent of day-one capacity [4]. That works out to 0.00005 of capacity per cycle [5].
Anchored to a thirty percent loss over the full warranted cycle count and applied in a straight line. Real cells fade fastest early and then flatten, so this understates year-one capacity slightly and overstates late-life capacity slightly.
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.
The question to ask the installer
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.
- Feed it into the paybackThe yearly schedule with the fade already subtracted.
- What a cycle earnsBecause cycles are the currency being spent here.
- Reading the quoteWarranty terms, scope exclusions and the questions to ask.
Questions this page gets asked
Which limit runs out first, years or cycles?
Divide the warranted cycle count by how many cycles you complete a year. At one a weekday the 6,000 cycles here take about 24 years, so the 15-year term arrives long before them. Cycle twice a day and the same count is gone in about 8 years, and the term never comes into it.
Is the fade curve on this page a manufacturer figure?
No, and we say so on the tool itself. The pages we retrieved publish a warranty term and a cycle count but no capacity retention curve, so the shape of the decline is our own assumption, anchored so a fully cycled pack ends its warranty having lost thirty percent. Real cells fade faster early and then flatten, so a straight line understates the first years slightly and overstates the last ones.
Does aggressive cycling for arbitrage shorten the life?
It converts calendar life into throughput. Each cycle is a small withdrawal from a finite account, so a pack worked hard for bill savings reaches its cycle limit years before a pack held mostly in reserve for outages. That is a real cost of the arbitrage strategy and it belongs in the comparison rather than in the footnotes.
What does the warranty actually promise?
Read the document rather than the datasheet. Most storage warranties guarantee a retained capacity percentage at the end of the term, subject to both the year count and the cycle count, and they are usually voided by operating outside the stated temperature range or by unapproved modifications. The number on the marketing page is the headline, not the obligation.
Provenance for every figure printed above
s4.enphase_10c.warranty_years— Enphase Energy product page — https://enphase.com/store/storage/iq-battery-10c (retrieved 2026-08-06)s4.enphase_10c.warranty_cycles— Enphase Energy product page — https://enphase.com/store/storage/iq-battery-10c (retrieved 2026-08-06)s4.enphase_10c.usable_kwh— Enphase Energy product page — https://enphase.com/store/storage/iq-battery-10c (retrieved 2026-08-06)a1.end_of_warranty_retention— solarbatterypayback.com — our own labelled assumption, disclosed at the point of use and on the methodology page — https://solarbatterypayback.com/methodology/ (retrieved 2026-08-06) [our assumption — no direct source]a1.loss_per_cycle— solarbatterypayback.com — our own labelled assumption, disclosed at the point of use and on the methodology page — https://solarbatterypayback.com/methodology/ (retrieved 2026-08-06) [our assumption — no direct source]