When a battery warranty actually ends

Battery warranties are unusually easy to check, which makes the gap between what they say and what people believe they say more interesting than it would be otherwise. These are not confidential documents. Tesla and Enphase both post the current terms as public PDFs, and both run to seven pages. I read both. The useful parts are on the first two.

Two vendors is not a survey, and I picked these two because their terms are openly published and because between them they cover a large share of American residential installations. Everything below is quoted from those documents. If you own a different battery, the specific numbers will differ. The shape of the question will not.

The first surprise is the definition of success

The Tesla document is headed "Ten Year Limited Warranty." Under that heading, the promise is in two parts:

(1) Your Powerwall will be free from defects for ten years following its initial installation date; and (2) Your Powerwall will have an energy capacity of 13.5 kWh on its initial installation date, and will retain energy capacity as shown in the table below.

The table sets the retained capacity at 70 percent at ten years, expressed, per the document's own footnote, as a percentage of the 13.5 kWh rated capacity. Do that arithmetic and the guaranteed floor at year ten is 9.45 kWh.

Nothing about that is scandalous. Lithium cells age, all capacity warranties are written this way, and 70 percent over a decade is a defensible engineering target. The point is narrower and it matters at the moment of purchase: a Powerwall that has lost 30 percent of its storage in ten years has performed exactly as warranted. If a household sized its backup around 13.5 kWh and expected 13.5 kWh in year nine, that expectation was never in the contract.

Enphase writes the same promise with two points instead of one. Its IQ Battery 5P terms say the battery will maintain the ability to store and discharge at least 70 percent of nameplate at the end of the tenth year, and at least 60 percent at the end of the fifteenth. A declining curve, stated as a curve. That is more informative than a single endpoint, and it is the better disclosure of the two.

The second surprise is that the clock is not a clock

This is the part that does not survive contact with the brochure.

Enphase defines its warranty period for the battery like this:

The Limited Warranty Period begins on the Activation Date (defined below) and ends on the earlier of (a) 15 years from the Activation Date or (b) 6,000 discharged cycles.

Whichever comes first. Six thousand cycles at one discharge a day is about sixteen and a half years, so for a household using the battery the ordinary way, storing solar during the day and drawing it in the evening, the calendar runs out before the cycles do and the fifteen-year number is honest. Run it twice a day, which is what happens when you stack solar self-consumption against a time-of-use tariff or enrol in a utility program that calls on the battery, and six thousand cycles arrives in a little over eight years. The warranty on that battery is not a fifteen-year warranty. The arithmetic is mine, not Enphase's, and it is the whole reason the cycle clause exists.

Tesla handles the same risk differently and more subtly. Its capacity table has two rows. For solar self-consumption, time-based control, and backup, all three defined in the document's footnotes, the operating limitation is "Unlimited cycles." For the second row the limitation is 37.8 MWh of aggregate throughput, and the row is described as:

Any application not listed above, or any combination of applications that includes one not listed above

Read that second clause carefully, because it is doing real work. It is not only unlisted applications that fall into the throughput bucket. It is any combination that includes one. A household running ordinary solar self-consumption plus one thing that is not on the list moves the whole installation out of unlimited cycles and under a hard energy ceiling.

How hard a ceiling? 37.8 MWh divided by 13.5 kWh is 2,800 full-depth cycles, measured, the footnote specifies, at the battery AC output. At one full cycle a day that is about seven years and eight months, comfortably inside the ten-year period. Again, my arithmetic. The number is not hidden, it is simply expressed in megawatt-hours rather than in years, and megawatt-hours are not a unit most homeowners convert on the spot.

I wrote about that units problem in a different context in Report 095. It recurs because it works. A quantity stated in the wrong unit for the reader is technically disclosed and practically invisible.

The third surprise is geography

Tesla publishes a separate warranty for its European region. Same product, same 13.5 kWh, same 37.8 MWh throughput figure. The retention promise is different:

For Powerwalls installed in Austria, Belgium, France, Germany, Ireland, Luxembourg, Netherlands, Switzerland or the UK:

Under that heading, both rows guarantee 80 percent at ten years rather than 70. Ten percentage points of the pack, which on 13.5 kWh is 1.35 kWh, or a guaranteed floor of 10.8 kWh instead of 9.45.

I am not going to pretend to know why. Consumer-protection law in those markets is stricter than in the United States, which is the obvious hypothesis, but it is a hypothesis and I did not verify it, so treat it as an observation rather than an explanation. What is checkable is the fact itself: the same hardware carries a materially different promise depending on which side of the Atlantic it is bolted to, and the American side is the weaker one.

The clause nobody reads is about your Wi-Fi

Tesla's warranty is conditioned on the battery being online. The document is candid about why, and about the consequence:

If your Powerwall is not connected to the Internet for an extended period, or has not been registered with Tesla, we may not be able to provide important remote firmware upgrades. In these circumstances, we may not be able to honor your full ten-year Limited Warranty.

It then sets a floor, and the floor is the number worth remembering:

Even if we can't honor your full ten-year Limited Warranty for the above reasons, we will always honor your Limited Warranty for at least four years following the date your Powerwall was installed for the first time

Four years, not ten. The document also says Tesla will try to contact you if the connection drops for an extended period, and that language is hedged throughout with "may," so this is a stated risk rather than an automatic forfeiture. But a homeowner who puts the battery on a guest network that gets rotated, or installs at a property that sits empty for a season, or simply never registers the unit because the installer did not walk them through it, is carrying a term they do not know about.

Enphase treats connectivity as a recommendation rather than a condition. Its clause says the products "should be continuously connected to the internet," with an explicit carve-out for interruptions outside the owner's reasonable control, and the stated purpose is remote diagnosis and firmware updates. Same engineering rationale, materially weaker obligation on the owner.

Who pays the electrician

Here the two documents diverge more sharply than anywhere else, and the difference is measured in real money.

Tesla's remedies clause commits to labour reimbursement: if Tesla repairs or replaces and the work is performed by a Tesla Certified Installer, the warranty includes reimbursement of reasonable labour costs to that installer, per Tesla's standard policies.

Enphase excludes it, in terms that leave no room:

This Limited Warranty does not include any cost of labor or other costs related to (i) un-installing Covered Product; (ii) re-installing a repaired or replacement product, or (iii) the removal, installation or troubleshooting of the Covered Owner's electrical systems.

A free replacement battery that you pay a licensed electrician to swap is not the same product as a free replacement battery. Which of those you have bought is decided by one paragraph on page five that nobody reads at the kitchen table. Note that many installers sell their own workmanship warranty on top of the manufacturer's, which is precisely the gap it exists to fill, so this is a question to put to the installer rather than only to the manufacturer.

Four more terms worth knowing

Repair does not restart the clock. Tesla states that the remainder of the original period applies to a repaired or replacement product, and that under no circumstances will the original warranty period be extended because of a repair or replacement. Enphase is slightly more generous, continuing coverage for the remainder of the original period or ninety days from receipt of the repaired unit, whichever is later.

A refund is valued at claim time, not purchase time. Tesla's remedy options include refunding the market price of an equivalent product at the time of the claim, at Tesla's discretion. Enphase's refund option is the actual purchase price less reasonable depreciation based on use. Battery prices have fallen steadily, so both formulations point the same way: a cash-out in year nine is priced at year-nine value, not at what you paid.

Selling the house is not automatic on both. Tesla's terms allow a subsequent owner who provides proof of ownership to make claims. Enphase requires the transferee to submit a Change of Ownership Form and pay a transfer fee within thirty days of the transfer, and states that submitting the form is required for continued coverage. If a home sale is anywhere in your ten-year horizon, that is a real difference.

Neither is for life-safety loads. Tesla's terms say in capitals that the Powerwall is not intended for use as a primary or backup power source for life-support systems or other medical equipment, and disclaims liability for such use. If the reason you are buying backup is a home oxygen concentrator or a medical refrigerator, that sentence is the most important one in the document and it is on page six.

Why this beat cares

I help design the AI battery-cycling systems for a veteran-owned (HUBZone) energy-storage integrator. I do not own that company and earn nothing from this link. Full policy here.

The reason I read warranties for fun is that a cycling policy is, in engineering terms, a warranty-consumption policy. Every decision about how deep to discharge, how hard to charge, and how often to answer a price signal is a decision about how fast you spend a finite budget that somebody has already written down in a contract. On the grid side that budget is explicit and the operator models it. In a house it is the same physics with nobody watching the meter.

That is why the throughput clause is the one I would put in front of a homeowner first. It is the clause that turns a behaviour into an expiry date. A battery cycled once a day and a battery cycled twice a day are the same hardware with different lifespans, and one of these two documents says so in cycles and the other says so in megawatt-hours, but both of them say so. I have written before about what actually wears these cells out, in Report 036, and about the rest of the cost that a headline price leaves out, in Report 132. Warranty terms are the same lesson arriving as a legal document instead of a datasheet.

What I could not confirm

Warranty documents are versioned, and yours is not necessarily the one I read. The Tesla United States terms quoted here are Revision 2.6, effective 6 May 2026. The European terms are Revision 1.16, effective 3 December 2025. The Enphase terms are the United States, Puerto Rico and Canada version effective 27 December 2024, and that document says in its own first paragraph that it applies to products activated on or after that date unless a newer warranty is subsequently posted. Your coverage is governed by the version in force for your unit, which may be older or newer. Check the date on your own copy before relying on any figure here.

The cycle and throughput conversions are mine and are deliberately simple. 2,800 full-depth cycles from 37.8 MWh, roughly seven years eight months at one a day, and roughly eight years and two months to reach 6,000 cycles at two a day, all assume full-depth cycles and take no account of partial cycles, how a manufacturer counts a partial discharge, efficiency losses, or real usage patterns. They are order-of-magnitude illustrations of what the clauses bind, not predictions about any installation.

I did not open the Enphase IQ Battery 5P datasheet, so this report states its capacity retention as a percentage of nameplate without asserting the nameplate figure. I did not read the warranty terms of any other manufacturer, and two vendors is not a market survey. I have not tested either product, have no relationship with either company, and nothing here recommends or disparages either one.

I am not a lawyer and this is not legal advice. Both documents carry state-law carve-outs, Tesla's includes an arbitration agreement with a thirty-day opt-out, and consumer warranty law varies by jurisdiction, so the enforceability of any particular clause is outside what I can assess. My hypothesis about why European terms are stronger is unverified and is flagged as such above. My published research is in microwave spectroscopy and had nothing to do with either product.

The signal

Ask three questions before you sign, and ask them of the warranty document rather than the brochure. What percentage of the original capacity is guaranteed, and in what year. What ends the coverage first, the calendar or a usage limit, and what is that limit in a unit you can picture. And who pays the electrician.

All three answers are printed, free, and roughly two pages into a PDF the manufacturer publishes on purpose. The years on the front of the brochure are the one number in the whole document that does not, on its own, tell you when anything ends.