C&I Battery Storage Capacity Degradation Warranty Checklist

A capacity-degradation warranty protects usable kWh only when four things are written into it: the measurement conditions, a year-by-year minimum capacity curve, a funded remedy if measured capacity falls below that curve, and exclusions that match how the system will actually be dispatched. A term sheet that promises "75% at year 10" without those four elements documents an intention, not an obligation — and the shortfall risk stays with you, the buyer.

This page is a procurement-engineering desk review of clause structure. It contains no field degradation data, no customer results and no supplier-specific figures: where a number is missing, the checklist tells you what to demand in writing instead of what to assume. A useful caution comes from a Norton Rose Fulbright edited transcript of an Infocast Storage Week panel (June 2019, United States), where a 10-year guarantee of 75% of year-one capacity was described as providing no value to a host C&I customer if the system is not dispatched correctly — the panel noted that even 150% of year-one capacity at year 10 delivers no value under poor dispatch. That is a 2019 industry discussion point, not a current market standard, and it frames the central lesson: capacity on paper and savings at the meter are two different products.

1. Lock the usable-capacity and SoH definition before you compare retention numbers

Retention percentages are only comparable when the test that produces them is identical. Ask for the measurement protocol in the warranty annex, not in a sales deck.

• Beginning-of-life (BOL) baseline: require the test method, ambient and cell temperature during test, C-rate for charge and discharge, depth of discharge, state-of-charge window, cutoff voltage, and whether auxiliary loads (HVAC, controls, fire detection) are inside or outside the measured boundary.
• Round-trip efficiency assumptions: confirm whether the guaranteed capacity is DC at the rack, AC at the PCS terminals, or AC at the point of interconnection. Each step adds losses; a warranty that does not state the boundary is unenforceable in practice.
• SoH method: ask whether state of health is validated by a field capacity test, a standardized full discharge test, or a software model extrapolated from BMS data. All three can be legitimate — but the method, the dataset, the calibration records and the test cadence must be auditable.
• "Usable capacity" definition: require the term sheet to say whether the number refers to nameplate, measured BOL capacity, or a warranted minimum at the meter. Later-year deliverables should be checked against the guarantee, not only against year-one performance.
• Data rights: secure access to raw test data and the option of third-party verification. Reject an SoH report with no stated conditions, no instrument calibration and no responsible party named.

2. Compare the annual guaranteed curve, not a single year-10 headline

A single endpoint hides the shape of the risk. Ask for the full curve, because the years that matter commercially are usually the middle ones, when savings are modelled but capacity has already faded.

• Request a year-by-year minimum capacity table with a tolerance band, measurement frequency, baseline-adjustment rules and temperature-correction method.
• Confirm whether the curve is indexed to measured BOL capacity or to nameplate — and what remedy applies if the BOL test itself underperforms the datasheet.
• Ask what happens when a measurement lands inside the tolerance band: retest window, number of permitted retests, who witnesses, and which result governs.
• Check for curve resets. If a module replacement or augmentation restores capacity, does the guaranteed curve step back up, stay flat, or continue from the original schedule?

3. Price the shortfall remedy — and read the oversupply clause

This is where most of the financial risk sits. A warranty that promises a remedy but not a mechanism is a promise to negotiate later, when you have no leverage.

• Remedy hierarchy: require the order of remedies — repair, module top-up, cash compensation, liquidated damages, or warranty extension — plus response times and who bears labour, logistics, crane access and downtime.
• Trigger and evidence: define the shortfall threshold, the measurement window, the evidence pack the buyer must submit, and the dispute process. Supplier-controlled software output should not be the only admissible proof.
• Oversupply: if delivered capacity exceeds the guaranteed curve, does the surplus offset future shortfall, create a credit, or carry no contractual value? Get an answer before signing — silence usually favours the supplier.
• Caps: check whether aggregate liability is capped, and whether the cap is expressed as a percentage of contract value, a fixed sum, or the price of replacement modules.

The 2019 panel's 150%-at-year-10 observation is worth keeping in mind here: surplus capacity is not a substitute for a dispatch strategy that converts stored energy into bill savings. A generous capacity guarantee cannot repair a mis-sized or mis-dispatched system.

4. Decide who funds augmentation before year 8

Augmentation is rarely an emergency; it is a planned cost that either appears in the original quote or appears as an unbudgeted capital call in the middle of the asset's life.

• Trigger definition: capacity below the guaranteed curve, a throughput or energy-throughput limit, or an end-of-useful-life threshold — and which one governs.
• Funding mechanism: require it in the original commercial terms — priced option, per-kWh top-up rate, escrow or reserved budget, or supplier-funded top-up with stated conditions. "To be agreed at the time" is not a funding mechanism.
• Secondary effects: how augmentation affects warranty extension, site access, permitting, interconnection limits, fire-safety documentation and planned downtime.
• Credit backing: verify who stands behind the future obligation — parent guarantee, insurance, service agreement — and whether that backing survives a change of ownership of the supplier.

5. Stress-test exclusions against your dispatch and O&M plan

Exclusions convert a warranty into a conditional warranty. Read them against your load profile, not against the datasheet.

• Expected exclusion triggers: ambient and cell temperature limits, C-rate, depth of discharge, SoC window, cycle or throughput caps, round-trip efficiency thresholds, auxiliary load assumptions, firmware versions, EMS dispatch logic, and O&M compliance (filter changes, thermal management checks, software updates).
• Use-case mismatch: demand-charge management, time-of-use shifting, backup-only standby and hybrid dispatch each produce a different degradation profile. Confirm the warranty's assumed dispatch envelope in writing and compare it with your intended control strategy.
• Application dependence: the same 2019 panel stated that a storage warranty always depends on the application — a defined cycling requirement and system life can be guaranteed, but only for that defined duty. Require the supplier to name the assumed cycling and dispatch envelope explicitly.
• Operational carve-outs: cure periods, remote dispatch rights, grid outages, curtailment, tariff changes and third-party software changes — clarify each before signature.

6. RFQ clause checklist to paste into your inquiry

7. Verify the warranty term sheet before signing

• Match the model and part number in the warranty to the datasheet, the type-test report and the proposed bill of materials. Divergence here voids coverage later.
• Request a sample SoH report and a sample augmentation calculation, then run both against your own load and dispatch profile.
• Confirm who performs testing — supplier, independent lab or owner-appointed third party — and who pays when a retest is disputed.
• Record every missing figure as an open verification item in the RFQ response matrix. Do not fill gaps with assumed cycle life, retention percentages or augmentation costs; an assumption made at tender becomes a dispute at year six.

Common buyer questions

Does a 10-year C&I battery storage warranty guarantee usable capacity?

Only if the warranty defines usable capacity at a stated measurement boundary, conditions and test method, and attaches a remedy with a named responsible party. A term length alone guarantees nothing about delivered kWh.

How is state of health measured for a capacity warranty claim?

Acceptable methods include a witnessed field capacity test, a standardized full-discharge test, or a validated software model from BMS data. Require the method, test conditions, calibration records and cadence to be specified in the annex, with a third-party verification option.

Who pays for augmentation if capacity falls below the guaranteed curve?

That depends entirely on the funding mechanism negotiated at contract stage — priced option, per-kWh top-up rate, escrow or supplier-funded top-up. If the mechanism is not in the original quote, treat the cost as buyer-borne until proven otherwise.

What happens if the battery delivers more capacity than the warranty curve requires?

Ask explicitly whether surplus offsets future shortfall, generates a credit, or has no contractual value. Most term sheets are silent, and silence generally favours the party drafting the document.

Get the warranty-term review sheet and a capacity-guarantee quote

Send us your load data, target dispatch strategy and the supplier's draft warranty table. We will return a clause-level review flagging where the guarantee is unmeasurable, where the shortfall remedy is unfunded, and where the exclusions conflict with your intended operation.

Request our C&I ESS warranty-term review sheet and a quote with an explicit capacity-guarantee table. Bring the BOL test method and your dispatch envelope; the guarantee should be written around them, not around a headline retention number.

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