Battery warranties: what the small print actually constrains
A battery warranty usually has three limits running at once — years, cycles and total energy through the battery — and it ends when the first of them is reached. The way of using a battery that earns most is also the way that reaches those limits soonest.
Three limits run at once, and the warranty ends at the first one reached.
Years, cycles, and total energy through the battery. Which limit binds depends entirely on how you use it — and the usage pattern that makes a battery pay best is the one that reaches the cycle and throughput limits soonest.
Three different warranties, often confused
A battery quote may involve three distinct promises from three different parties. They fail in different ways and it is worth keeping them apart.
The product warranty, from the manufacturer. Covers the hardware against defects for a term. Attaches to the product, so it survives your installer going out of business — but it generally requires the installation to have followed the manufacturer’s instructions.
The capacity or performance warranty, also from the manufacturer. Covers degradation: a promise that the battery will still hold at least some stated share of its original usable capacity at the end of the term. This is the one with the years-cycles-throughput structure.
The workmanship warranty, from your installer. Covers the installation itself — the mounting, the cabling, the commissioning. Only as durable as the company that gave it, which is why the question of who stands behind it matters more than its length.
The three limits, and which one binds
Capacity warranties are normally written as a set of limits with whichever comes first attached. In shape:
- a term in years — the calendar limit
- a number of cycles — how many times the battery may be filled and emptied
- an energy throughput — the total kilowatt-hours that may pass through it, often written in MWh
The mistake is to read the years and stop. Take a battery with 5 kWh of usable capacity, warranted for ten years or 10 MWh of throughput, whichever comes first, and work it two ways:
| How it is used | Units through it each year | Throughput limit reached after |
|---|---|---|
| Stored solar only, typical household | about 990 | over 10 years — the calendar limit binds |
| Cycled most days on a time-of-use tariff | about 1,500 | under 7 years — the throughput limit binds |
The first figure is MCS’s own storage uplift for a typical 4 kWp household, from MGD 003. The second is what near-daily cycling of a 5 kWh battery looks like. Same product, same paperwork, and the warranty ends more than three years apart.
Charging from cheap overnight electricity is the single biggest improvement to home battery economics we know of — see a battery without solar.
It is also the usage pattern that consumes the cycle and throughput allowance fastest.
That does not make it a bad idea. It makes it a calculation you should do before you buy: work out the units per year for the way you actually intend to use it, and check which limit that hits first.
Retained capacity: the number that is easy to miss
A capacity warranty does not promise the battery keeps working. It promises it still holds at least a stated percentage of its original usable capacity.
That percentage varies by product, and it is worth stating what it means in practice: a battery you sized at 5 kWh of usable capacity is not a 5 kWh battery in year ten. If you sized right at the ceiling of what your household could use, you have less headroom than you think. The sizing arithmetic already argues for staying below the ceiling; degradation is the second reason.
Ask for the figure, in writing, as a percentage of usable capacity — not nameplate.
What MCS requires your installer to tell you
MCS’s battery installation standard, MIS 3012, contains something genuinely useful here. Clause 4.2.1 sets out minimum technical information that must be communicated in writing to the client before the point that the contract is awarded. The list includes:
- the warranties applying to the system and its storage capacity, expressly covering degradation, number of cycles and energy throughput
- how the system indicates its current usable capacity or state of health — so you can tell whether it has dropped below the warranted level
- for the more complex system classes, the warranties provided for each discrete component
- usable storage capacity in kWh accounting for the maximum allowable depth of discharge
- round-trip efficiency, and the energy lost if charged from grid electricity
- the current cost of additional or replacement batteries, cells or monoblocs
- whether it can run in island mode during a power cut, and the maximum load in kW if so
- end-of-life and recycling arrangements
- where the system is to be remotely controlled by a third party, the terms of that arrangement, the terms for terminating it, and any early-termination penalties
That last one matters if a grid services or virtual power plant scheme is part of the pitch.
This is a pre-contract requirement, not a handover one. If a quote does not contain these things, the answer is to ask for them before signing — not to sign and hope they arrive with the paperwork.
The standard is issue 1.0 and its own internal date is 22 November 2021, though the copy MCS currently publishes sits at a 2025 address. We cite the date printed inside the document, for reasons we learned the hard way.
And at handover
MIS 3012 also requires a document handover pack, and specifies a minimum: copies of all commissioning forms and checklists, the maintenance requirements and available maintenance services, and the manufacturer user manuals and warranty details.
Alongside it, a signed document from the contractor confirming the installation meets the standard, listing the key components installed, giving the performance estimate, and stating the recommended interval for the first periodic inspection. The installation must be registered on the MCS Installation Database within 10 working days of commissioning, and the certificate sent to you.
Keep all of it. A capacity claim years from now will turn on being able to show what was installed, when, and to what standard — see selling a house with solar for the other reason the pack matters.
Your rights are not only the warranty
A warranty is a contractual promise on top of your statutory rights, not instead of them. Under the Consumer Rights Act 2015 goods must be of satisfactory quality, fit for purpose and as described, and services must be performed with reasonable care and skill. A battery that fails prematurely may be a breach of those terms regardless of what a warranty document says.
We are not lawyers and this is not legal advice. But it is worth knowing that “the warranty has expired” is the end of one argument, not of all of them.
The questions to ask, in order
- What are the three limits — years, cycles, throughput — and which binds for how I intend to use it?
- What retained capacity is guaranteed at the end of the term, as a percentage of usable capacity?
- How does the system report its state of health, and can I read it myself?
- What voids it? Specifically: grid charging, third-party control, depth of discharge, ambient temperature at the proposed location.
- Who honours the workmanship warranty if you cease trading, and is it insurance-backed?
- What does a replacement cost today, and are the cells or modules individually replaceable?
- Is the warranty transferable if I sell the house?
Sources
- MIS 3012: The Battery Standard (Installation), issue 1.0 Read in full. The document's own internal date is 22 November 2021, which is the date we cite — the folder in the URL says 2025. Clause 4.2.1 lists the minimum technical information an MCS contractor must give in writing before the contract is awarded; clauses 6.2 and 6.3 set out the handover pack. MCS restricts reproduction, so we cite clauses and paraphrase rather than reproduce.
- Consumer Rights Act 2015 Cited by link. Goods supplied must be of satisfactory quality, fit for purpose and as described; services must be performed with reasonable care and skill. These rights sit alongside any warranty and are not replaced by it.
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