Three in four BESS sites over-report availability, new research finds

•         Across 50+ operating BESS projects, most sites over-report availability, deliver less dispatchable energy than their capacity tests imply, and age more slowly than warranty curves assume, though aging is not what erodes the overbuild

•         The median site enters its first year with 0.5% operating headroom above nameplate energy, before any aging

•         57.6% of downtime comes from small, recurring outages; the power conversion system is the largest single source of unavailability, at 36.4%

•         Unavailability and cell imbalance leave the median site delivering 6.9% less dispatchable energy than its capacity test implies, equivalent to seven battery containers’ worth on a state-of-the-art 500 MWh site

•         On a representative project and financing model, a 2% reduction in revenue takes a 1.30x debt service coverage ratio to roughly 1.26x and a 15%levered return to near 14%

Aachen, Germany, 21 September 2026 – Battery energy storage system (BESS) sites report a median 96.7% availability. Measured on whether components respond when called, it is 94.1%. Published research today sets out what gaps of that kind do to coverage ratios and returns.

Banking BESS:financing lessons for lenders and investors, from 50+ operating battery projects, a new report from ACCURE Battery Intelligence with support from U.S. Bank, covers 4 GW of installed power capacity and 10 GWh of energy capacity across key markets including ERCOT and CAISO in the United States, the UK and Germany. It tests the availability guarantees, capacity tests and warranty degradation curves on which BESS financial models are built.

Almost no headroom at the start of year one

Owners build in headroom to absorb losses and degradation. The median project carries a DC overbuild of 20.2% above nameplate energy; conversion losses take 7.6 percentage points, usable capacity losses 5.2, and the operational performance gap 6.9. This means the median site enters commercial operation with 0.5% operating headroom above nameplate, and some start below it. None of that is caused by aging.

Reported availability is higher than measured

The median Reported Site Availability is 96.7%, in line with the conventional 97% target. But those values come from reporting formulae that vary from contract to contract, and often count a component as available whenever it reports an 'online' status rather than when it responds. Counting a component as available only when it participates as the energy management system (EMS) calls on it, the median falls to 94.1%. The gap averages 2.6 percentage points, three out of four sites over-report, and at the tenth percentile it reaches 6 percentage points. Contractual Availability, which excludes downtime caused by Excuse Events, is higher again.

These deviations in what the assets deliver, not just in how formulae count them, cost money. Applied to a representative BESS project and financing model, a 2% reduction in revenue moves a loan sized at 1.30x debt service coverage ratio to roughly 1.26x, and takes about a point off the sponsor’s levered internal rate of return.

The power conversion system is the largest single source of unavailability at 36.4%, ahead of balance of system, racks and containers. Most of that downtime comes from small, recurring outages that early detection and predictive maintenance can catch.

Dispatchable energy in live operation

The median site delivers 6.9% less dispatchable energy than its capacity test implies, driven by unavailability and stranded energy from cell imbalance. This is the aforementioned 'operational performance gap', not a further loss. Errors in state of charge estimation sit outside that figure and carry their own cost, which can reach $1 million or more per GWh of installed capacity a year.

Slower aging does not create spare headroom

State of health runs 1.8 percentage points above the average warranty curve in the early years, and degradation averages 1.6% a year against the 1.7% warranted. But a warranty curve is a commercial decision, not necessarily an accurate picture of how the battery ages, and because the supplier pays when an asset falls short, the curve carries a buffer. And with 0.5% operating headroom at the start, it is conversion, usable capacity and operational performance losses, not degradation, that consume the overbuild first. An asset aging more slowly than modeled while running below potential is underused, not ahead.

Darya Rüwald,Director of Battery Intelligence at ACCURE and co-author of the report, said: “Lenders and investors rely on contracts, warranties and capacity tests to build their models. Those documents flatter the asset on availability and capacity, and carry commercial buffers on aging. The median site enters its first year with half a percent of margin above nameplate. That is where every financing case should start. None of this is a reason to avoid BESS investments; the asset class is maturing fast and the grid needs more of it. But trust is easier to extend when the baseline is measured performance rather than contractual assumption. That is what we have put on the table.”

Kai-Philipp Kairies,CEO and co-founder of ACCURE, added: “Every maturing asset class reaches the point where measured field performance, not the contract, sets the baseline forfinancing. Solar and wind made that transition; storage is arriving there now and can get there faster.”

Scott Butin, Vice President, Infrastructure Finance at U.S. Bank, said: "As the energy storage market continues to mature, access to high-quality operational data is increasingly important, and independent research can help inform industry discussions, improve understanding of battery storage performance and support the continued growth of the market. While every market participant is influenced by anecdotal project experiences, we appreciate ACCURE's supplier-and operator-agnostic analysis across a wide range of assets and the breadth of findings presented in one place.”

The full report is available here

About ACCURE Battery Intelligence

ACCURE Battery Intelligence pairs an award-winning battery analytics platform with hands-on performance engineering for grid-scale battery storage. The platform combines artificial intelligence with physics-based battery models to give owners and operators an independent, cell-level view of their assets, and ACCURE's performance engineers work alongside clients to act on it: capturing the revenue their assets were built to deliver, lowering operating costs and mitigating risk. Founded by battery researchers from RWTH Aachen University, ACCURE is the assurance and outperformance layer for grid-scale BESS, supporting more than 24GWh of capacity worldwide. We work with battery owners and operators, and the insurers and financial institutions that back them, across Asia-Pacific, Europe, Latin America and North America.

About U.S. Bancorp

Headquartered in Minneapolis, U.S. Bancorp is the parent company of U.S. Bank National Association, the fifth-largest commercial bank in the United States. Our three major business lines serve 15 million clients globally, and our team of nearly 70,000 people invest our hearts and minds to power human potential every day. Ranked 110th on the Fortune 500, we are deeply respected for our culture and long-term stewardship and admired for our diversified business mix and product capabilities.

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