What a 5-Year Warranty Actually Covers — and What ≥70% SoH Over 20 Years Doesn't

Battery storage warranties are three documents wearing one name. A buyer reading a single headline figure — "20-year warranty", "70% SoH guaranteed" — is usually reading across all three without the boundaries being drawn, and the boundaries are where the money is.

This post separates them, using our own published terms as the worked example, and is explicit about which parts are included and which are priced.

The three instruments

Base warranty covers defects in the hardware. If a pack, a rack, a cooling unit or a control board fails within the covered period, it is repaired or replaced. This is a product warranty in the ordinary sense, and it is bounded by both time and use.

Ours is 5 years or 6,000 cycles from the date of site acceptance, whichever comes first.

Performance guarantee is a different promise entirely. It does not say the equipment will not break; it says the equipment will still hold a defined fraction of its original capacity after a defined period under defined usage. This is the instrument that carries the state-of-health number.

Ours is an extended guarantee to 70% state of health over 20 years, at one cycle per day — 330 cycles per year — at 25 °C. It is available as a priced option, not as part of the standard cover.

O&M is a service contract: preventive maintenance, remote monitoring, capacity testing, spares and response times. It is the instrument that determines whether a fault covered by the other two is actually found and fixed quickly. We covered its structure in what a 10-year BESS O&M contract should contain.

Conflating these three is the single most common source of surprise in storage procurement. A base warranty does not guarantee capacity. A performance guarantee does not cover a failed contactor. Neither of them sends an engineer.

"Whichever comes first" is doing real work

The base warranty is bounded twice, and which bound arrives first depends entirely on how the asset is run.

At one cycle per day, 6,000 cycles takes over sixteen years — so the five-year limit binds, and the cycle count never comes into it. At two cycles per day, 6,000 cycles arrives in roughly eight years, and the five-year limit still binds first. But the cycle bound is not decoration: it exists because a plant cycled aggressively is consuming design life faster than the calendar suggests, and a warranty priced against a calendar assumption cannot absorb an arbitrary duty.

The practical implication for a buyer: state your expected duty cycle in the enquiry. A supplier pricing against one cycle a day and a buyer planning two are agreeing on a number that means different things to each of them.

Cycles per year is the hidden variable in the SoH guarantee

The performance guarantee is where the assumption does the most work, and where it is least often read.

Our extended guarantee is stated at 330 cycles per year. Across 20 years that is 6,600 cycles. Note what that means: the performance guarantee's total cycle exposure is close to the rated cycle life of the cells, which is how the two can coexist. Change the annual cycle count and the arithmetic stops working.

At 660 cycles per year — two a day, a plausible duty for a merchant arbitrage asset — the same 20-year period implies 13,200 cycles, which is more than twice the rated envelope. A guarantee written against 330 cycles per year does not cover that usage, and no honest supplier would extend it unchanged. What happens instead is augmentation: capacity added mid-life to restore the delivered figure, which is a cost that belongs in the model from the beginning rather than a surprise in year eight.

This is why a capacity guarantee without its cycle assumption is not a guarantee. The number that matters is not 70%; it is 70% at 330 cycles per year at 25 °C, and a competing offer at a different annual count is not comparable.

What a capacity guarantee does and does not commit us to

Read precisely, a performance guarantee commits the manufacturer to a remedy if measured capacity falls below the stated threshold under the stated conditions. That is narrower than most buyers assume, in four specific ways.

It is conditional on usage. Operating outside the stated depth, rate, temperature or annual cycle count moves the asset outside the instrument.

It is conditional on measurement. The capacity test method, its measurement point, the state-of-charge window, ambient correction and tolerance band all change the result. The method must be agreed at contract, not at site — a point we make in every proposal, because discovering it at site is how capacity disputes begin.

It is a remedy, not a prevention. The guarantee says what happens if the threshold is missed. Typically that is augmentation or replacement to restore the figure, and the form of the remedy should be written down.

It is bounded by the conditions being met on both sides. Maintenance performed, firmware current, environment within envelope. This is the practical reason the O&M contract and the performance guarantee are usually best held by the same party.

Why we publish the extension as priced

The most useful thing we can say about our own terms is the part that sounds least like marketing: the 20-year, 70% SoH guarantee is a priced option and we quote it as one.

The alternative — folding it into the headline and recovering the cost elsewhere — is common enough that buyers have learned to discount long warranty claims generally. A guarantee carried for twenty years is a real balance-sheet obligation for the supplier; it has a cost, and a supplier who presents it as free has either priced it silently or has not thought about it. Neither is reassuring in a counterparty you need to still exist in year fifteen.

Stating it as an extension also gives the buyer a decision they can actually make. Some projects need a twenty-year firm capacity figure because their offtake agreement demands one. Others are better served by standard cover plus provisioned augmentation space, which is cheaper at the outset and defers the cost to when it is needed. Those are different commercial structures and the buyer is better placed to choose than we are.

What to ask

Five questions resolve most of the ambiguity in a warranty package, and they are worth asking in writing:

• What is the base warranty period, and what is its cycle bound?
• What is the performance guarantee threshold, over what period, at how many cycles per year, at what temperature?
• Is the performance guarantee included or priced separately, and if priced, at what?
• What is the capacity test method, and is it defined in the contract?
• What is the remedy if the threshold is missed — augmentation, replacement, or liquidated damages?

A supplier who answers all five in a table is describing a real instrument. The broader version of this list is in five questions to ask any BESS supplier, and the surrounding procurement structure is in our BESS procurement guide.

Our full published terms — cycle life basis, operating window, design life and certification status — sit on the product pages. For warranty terms against a specific duty cycle and offtake structure, talk to our team.