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The degradation trap: what a battery warranty can and cannot tell you.

A retained-capacity warranty tells you something important about a manufacturer's promise. It does not, on its own, tell you how a battery will age in each intervening year, how much energy it will deliver over its life, or how much money it will save. Those are different questions requiring different evidence.

What the percentage means

Start with the capacity basis, then the date.

For an illustrative 9.0 kWh starting usable capacity, a 70% endpoint corresponds to 6.3 kWh. That multiplication is valid only when the instrument defines its percentage against that same capacity basis. Nominal capacity, usable capacity and delivered AC energy are not interchangeable.

Arithmetic example, not a product forecast

70% of 9.0 kWh is 6.3 kWh. The path is not specified.

Starting usable capacity9.0 kWh

Illustrative year-10 endpoint6.3 kWh

9.0 kWh × 0.70 = 6.3 kWh

The diagram deliberately leaves the years between the two states unplotted. A straight line would invent an annual path. A curved line would invent a different one. Neither is measured evidence, and neither becomes a contractual annual guarantee simply by ending at the published percentage. An endpoint also cannot settle what remedies apply earlier in the term; the complete instrument and applicable rights need to be read.

The exact Australian record

Powerwall 3 has an endpoint, not a published five-year checkpoint.

Tesla's AU/NZ warranty, Revision 1.16 effective 6 May 2026, names 13.5 kWh at installation and 70% retention at ten years. It states unlimited cycles for the listed self-consumption, time-based-control and backup applications, and a 37.8 MWh aggregate-throughput limit for other applications. It does not publish an 80% year-five checkpoint. Registration, connectivity and other conditions apply. Read the governing instrument, rechecked 19 September 2026.

A second limit

Calendar years and cycle limits can end at different times.

The Australian warranty register records both a calendar term and an explicit cycle limit for the examples below. The control changes one assumption: the number of warranty-counted cycles per day. It does not simulate ageing, convert unlike cycle definitions into equivalents, or account for other coverage conditions.

Before drawing a curve

Lifetime energy needs an operating model, not a warranty endpoint.

A lifetime-energy estimate needs at least a defined usable-capacity trajectory, cycle schedule, depth of discharge, measurement boundary, efficiency, downtime and operating conditions. A savings estimate additionally needs tariff, dispatch and cost assumptions. None can be recovered from a ten-year retention percentage alone. That is why the earlier MWh and dollar ranges are no longer shown.

For a buying decision, identify the exact model and market, the capacity against which retention is measured, each stated checkpoint, the earlier-of limits, and the remedy including labour and freight. Chemistry and independent testing provide separate evidence; neither silently rewrites the warranty. No linear annual capacity guarantee is inferred here for sonnenBatterie Evo.

Continue to the living battery-warranty comparison for the wider catalogue, or the Australian conditions register for exact instruments and source dates. The published rubric keeps those evidence fields separate.

Sources and correction record

• Tesla Powerwall AU/NZ warranty Rev. 1.16, pages 1 and 3; exact link beside the claim above.
• Australian warranty-conditions register. Each cycle illustration links its exact instrument and preserves the register's retrieval date; it is not represented as a fresh source check.
• 23 August 2026: Australian sonnen linear-degradation classification withdrawn.
• 19 September 2026: Tesla midterm claim, below-floor example, lifetime-energy ranges and savings claim withdrawn. The article now distinguishes an endpoint, a conditional duration calculation and a measured ageing curve.