Everyone says '10 years of warranty'. We read the fine print.

If you ask five solar battery companies for a quote, all five will tell you "10 years of warranty". It's the industry standard. And that's exactly why the number no longer tells you anything.

What separates a good warranty from a bad one isn't the number of years. It's three things that almost never appear on the quote, but do appear in the warranty document — that PDF nobody reads until something goes wrong.

We downloaded and read the full official documents from Tesla, BYD, Huawei, LG, SolarEdge, Pylontech and sonnen. Everything below comes from those documents, and every claim links to the original PDF so you can check it yourself.

The three questions you should ask

When they tell you "10 years", ask:

1. How much capacity do you guarantee will be left at the end? Market answers range from 60% to 80%. On a 15 kWh battery, that difference is 3 kWh of usable capacity.
2. Is there a total energy limit? Almost all of them have one, and in many cases it runs out before the 10 years are up.
3. What happens if my internet goes down? The answer will scare you more than you'd think.

Let's go through them one by one.

1. "10 years" ≠ "10 years at 100%"

No lithium battery keeps all of its capacity. They all degrade. The relevant question is what floor the manufacturer guarantees you, and here the market is quite split:

A 60% warranty means this: if you buy 10 kWh and after ten years you have 6, the manufacturer has fulfilled its obligation. It owes you nothing. And 6 kWh might no longer get you through the night.

The case that's hard to explain: Tesla gives less warranty in Spain

Tesla's European warranty document (official PDF, revision 1.15) has a table broken down by country. Austria, Belgium, France, Germany, Ireland, Luxembourg, the Netherlands, Switzerland and the United Kingdom get 80% guaranteed retention at 10 years.

Spain, Italy, Malta, Portugal and Slovenia get 70%.

Same product, same price, ten fewer warranty points. The technical reason is climate: more heat, more degradation. The reason is understandable; the fact that it appears in no sales brochure, less so.

2. The trap almost nobody explains: total energy

Here's the most important fine print of all. Most warranties don't say "10 years". They say "10 years or X MWh, whichever comes first".

This second limit — the throughput, the total energy the battery can deliver over its guaranteed lifetime — is what causes many 10-year warranties to never actually reach 10 years.

Let's run the numbers with normal household use in Spain, around 300–350 cycles a year (practically one cycle a day):

Look at the last column. With daily use, SolarEdge's warranty ends between years 8 and 9, not at 10. BYD's, shortly after. There's no legal deception here: the document says so. But the brochure says "10 years" and the document says something else.

Two honest exceptions:

• Tesla states "Unlimited cycles" for solar self-consumption and backup. No cap. The 37,8 MWh limit only applies if you use the battery for arbitrage or grid services.
• sonnen guarantees 10.000 cycles, which at 300 cycles a year comes to 33 years. In practice, the real limit is the 10 years. And its warranty covers all components, including power electronics and the inverter, not just the cells.

And a technical point you should demand

When a manufacturer tells you "X cycles", ask: cycles at what depth?

Are two 50% discharges one cycle or two? Without defining it, the figure means nothing. We checked this across the nine documents we read: none of them defines depth of discharge when talking about cycles. That's why serious players in the industry have been shifting to expressing it in MWh, which leaves no room for interpretation.

If someone sells you cycles without telling you the depth, they're not giving you a figure: they're giving you a feeling.

3. The warranty as a leash

This is the part people least expect. In many cases, your warranty depends on keeping the manufacturer connected to your installation.

Read the Ampere Energy row again. It's a Spanish manufacturer, and it's written on its own FAQ page: three months without an internet connection and you lose the entire warranty. It isn't reduced: it's voided.

The approach is the same everywhere: telemetry is offered as a service and applied as a condition. If your router breaks, if you switch providers, if you're away for six months, the manufacturer reserves the right not to respond.

4. Other things we found

BYD counts the warranty from the factory, not from your home. The Warranty Start Date is six months after the cell's production date. If the battery has spent half a year in the distributor's warehouse, it arrives at your home with part of the warranty already used up. What's more, it must be installed within one month of that date or it's excluded.

Pylontech offers you more years in exchange for less warranty. On the US series, the base warranty is 7 years at 70% capacity. If you register the product, they extend it to 10 years… at 60%. You gain three years and lose ten points of retention. Presented as an improvement.

BYD and Pylontech offer pro-rated compensation. If the battery fails in year 8 of 10, you don't get a new battery: you get compensation proportional to the life it had left. In the case of Pylontech Force-H, with linear depreciation over 10 years: a failure in year 8 is worth 20% of the value.

Victron, an excellent brand in electronics, gives 3 years on lithium batteries (2025 policy), extendable to 10 by paying around 10% of the investment. It's consistent: batteries aren't their core product. But it's worth knowing if your installer proposes a full Victron system.

The comparison, in one table

Total guaranteed energy per kWh of capacity. The higher the number, the more room before it expires from use:

sonnen objectively has the best warranty on the residential market. They cover all components, guarantee 80% and their cycle limit never bites. It's worth saying even though they're a competitor: if budget isn't a constraint, their warranty is the benchmark.

How to read yours before you sign

If you're considering a purchase, ask for the warranty document before signing anything and look for five things:

1. The retention percentage. If it isn't there, that's bad. If it is and it says 60%, now you know what you're buying.
2. The energy limit (MWh) or cycle limit. Divide it by your estimated annual consumption and see how many years you get. If it's fewer than 10, the real warranty is that figure, not the one on the brochure.
3. When it starts counting from. Installation, purchase, manufacture or shipment? The difference can be months.
4. What happens without internet. Look for the words "monitoring", "registration" or "connection". If it depends on them, you have a conditional warranty.
5. How they compensate. Repair, replacement or pro-rated compensation? The three aren't worth the same.

And a common-sense question: who will be around in ten years to answer for it? LG has withdrawn from entire residential markets. A warranty is worth exactly what the company that signs it is worth when you need it.

What we do

We're covering this briefly at the end, because that's not what this article is about.

Our commercial warranty is 10 years at 80% capacity, with 108 MWh of total energy on the 15 kWh model (216 on the 30 and 432 on the 60). That's 9.000 cycles at 80% depth of discharge — and we say so with the depth included, because without it the figure would mean nothing.

On connectivity we've done the opposite of what the market does: telemetry can only add years, never take them away. The 10 years are unconditional; if you keep the system connected, they extend to 12. And the condition is nine months a year, not permanent connection, because a router that's broken for three weeks shouldn't cost you anything.

We can offer this for a practical reason: we see the batteries in real time and detect abnormal degradation weeks before it becomes a failure. It pays off for us that you stay connected. But it's a deal, not a toll.

And we repair module by module. If one part fails, that part is replaced and the rest keeps working. No pro-rated compensation for age.

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Methodology. All data comes from the official warranty documents published by the manufacturers, linked one by one throughout the article, consulted in July 2026. The equivalent-cycle figures were calculated by dividing the guaranteed energy by the usable capacity stated in each document. The depletion-year estimates assume 300–350 full cycles a year, the typical range for a residential self-consumption installation in Spain. Conditions can change: always check the manufacturer's current document before buying.