A warranty is a set of promises, not a number of years
When a supplier tells you a battery energy storage system (BESS) comes “with a 10-year warranty,” that single phrase hides several very different promises. Some of them cover the hardware if it fails. Others guarantee how the system performs — how much energy it will still store years from now, how efficiently it cycles, and how often it is available to run. A weak warranty leans on the reassuring year count and stays vague on the rest. A strong one puts hard numbers on every promise and accepts a penalty if it misses them.
For a buyer or developer in India, the warranty is where price meets reality. It is the document that decides who pays when capacity fades faster than expected — you or the supplier. This guide walks through what each promise should actually say.
Product warranty vs performance warranty
Almost every serious BESS warranty is really two documents stapled together, and confusing them is the most common mistake buyers make.
- A product warranty covers defects and outright failure — a bad cell, a faulty battery management system (BMS), a dead power conversion system (PCS). It promises the equipment was built right and works as delivered. On its own it is often short.
- A performance warranty is the long promise. It guarantees that the system will keep meeting defined performance standards — usually capacity retention — over a much longer horizon, commonly 15 to 20 years, under stated operating conditions.
You need both. A product warranty tells you the box will not fall apart; a performance warranty tells you the box will still do useful work in year ten. When you compare offers, read the two separately and note where each one ends.
The capacity-retention guarantee
This is the heart of the performance warranty. It states the minimum usable capacity the system will still deliver at the end of the warranty period, written as a share of the capacity it started with. This remaining fraction is called State of Health (SoH), and end-of-life values in real warranties usually land between 60% and 80%, depending on the manufacturer. A common shorthand is “70% capacity at year 10.”
Indian tenders make this promise concrete. In the Solar Energy Corporation of India (SECI) 600 MW / 1,200 MWh standalone-storage tender, the guaranteed usable energy is scheduled to decline from 1,200 MWh to about 840 MWh over 15 years — that is 70% retention at year 15, and the developer is contractually on the hook for it. We cover why capacity fades, and how the underlying curve behaves, in our companion guide on round-trip efficiency and degradation — the warranty is simply the legal wrapper around that physics.
The trick, as DNV notes, is not just the headline percentage but how it is measured: at what temperature, at what depth of discharge, over how many cycles a year. A 70% guarantee measured under gentle lab conditions is worth less than a 65% guarantee measured under your real duty. Always ask what assumptions the curve rests on.
Energy throughput: the other clock
Capacity retention is a promise about time. Energy throughput is a promise about use, and the two run on a “whichever comes first” basis.
Throughput caps the total energy (in megawatt-hours) the system can deliver over the warranty life. A warranty might read “10 years or 30 MWh of throughput, whichever comes first.” If you cycle harder than the supplier assumed, you hit the energy limit before the calendar limit, and the guarantee lapses early. This is why a warranty tied to your expected cycles per day is worth reading carefully — a solar-shifting duty of one cycle a day burns throughput far slower than a two-cycle arbitrage duty.
Round-trip efficiency and availability guarantees
Two more numbers separate a real guarantee from marketing.
- Round-trip efficiency (RTE) is the share of energy you get back after storing it. A serious warranty guarantees a minimum RTE, not a best-case figure. SECI standalone tenders specify a minimum RTE of around 85% on a monthly basis (some contracts require 86%), with penalties below that.
- Availability is the share of time the system is ready to charge or discharge when called. Indian tenders commonly require 95% availability on an annual basis, and some demand as high as 98%.
Both matter because a battery that is efficient but often offline, or available but leaky, quietly costs you money the headline capacity number never shows.
Liquidated damages: the promise with teeth
A guarantee only means something if missing it costs the supplier. That is the job of liquidated damages (LDs) — pre-agreed penalties written into the contract.
Indian tenders are unusually explicit here. In SECI standalone-storage contracts, LDs for availability below 95% can run to twice the capacity charges for the capacity not made available, settled monthly. For efficiency shortfalls, one published structure levies a penalty per unit of excess conversion loss when RTE falls into the 70%-to-85% band. When you read a private warranty, look for the same mechanism: does a missed guarantee trigger a defined remedy, or just a promise to “use best efforts”? The difference is the whole value of the document.
Exclusions: how a warranty quietly dies
Every warranty carries a list of operating conditions and exclusions, and breaching even one — sometimes inadvertently — can let the supplier deny a claim. Common ones include:
- Temperature caps. Charging outside a stated band (for example below 0°C or above 50°C ambient) can void coverage. In India’s heat this is not a footnote — cooling design is what keeps you inside the band.
- Cycle and C-rate limits. Exceeding the assumed cycles per year, or charging or discharging faster than the rated C-rate, can breach the terms.
- Maintenance and data-logging duties. Many warranties require documented maintenance and continuous operating data; fail to log it and the supplier can argue the fault is yours.
Because these conditions do so much of the real work, read the exclusions before the headline. The chemistry you choose also sets how much thermal headroom you have to stay inside them — see our note on LFP vs NMC chemistry and the safety standards a well-built system meets.
Augmentation obligations
If capacity naturally fades toward about 70%, how can a 15-year contract promise steady usable energy? Through augmentation — adding fresh battery modules at planned points to top the system back up. The question a warranty must answer is who is obliged to do it. A strong performance warranty makes holding the guaranteed usable-energy line the supplier’s contractual duty, with augmentation as one accepted way to meet it. A weaker one leaves augmentation to you while still calling itself a “guarantee.” We explain the economics of augmentation versus over-sizing in the round-trip efficiency guide; the warranty is where you pin down whose problem it is.
What this means for you
Do not accept “10-year warranty” as an answer. Ask every supplier to separate the product warranty (defects, and for how long) from the performance warranty, then pin down each performance promise: the capacity-retention curve with its assumed cycles per year, temperature and depth of discharge; the energy-throughput cap; the minimum RTE and availability; the liquidated damages if any of these are missed; the exclusions that can void cover; and the augmentation obligation that keeps usable energy where you need it. These are exactly the terms serious Indian tenders enforce, and they should shape any private deal too — read this alongside our guides on what a BESS is and how to size a BESS for your site. Our own containerised and C&I storage products are built on LFP cells with these guarantees in mind. To translate a warranty into lifetime cost for your load and tariff, run the numbers in our savings calculator or talk to our team.
Warranty and tender terms change by notification and vary by supplier, contract and duty cycle; the specific SECI figures cited here are drawn from particular tenders and may differ in the current version. Verify current warranty terms and tender conditions before financial decisions.