Availability: Why this BESS performance metric matters so much, and how to calculate it

Power availability centres around the BESS being able to deliver a certain percentage of rated power and ensures that the power conversion systems (PCS), transformers, and switchgear are operational.

On the other hand, energy availability ensures the state of charge (SOC) of the BESS is within the agreed operating window and that the BESS can sustain the contracted discharge duration, be it 2 hours, 4 hours, 8 hours or otherwise.

For control and communication availability, BESS owners need to ensure the energy management systems (EMS) and SCADA systems are online and functional so that the BESS can receive dispatch instructions.

BESS availability is calculated by taking the eligible operating hours minus the unavailable hours. This value is then divided by the total eligible operating hours and multiplied by 100 to get a percentage. The eligible operating hours are all the potential hours that a BESS could be online for in a given time period, for example, 1 year or 8,760 hours. The equation is as follows:

For example, if there were 175 down hours in the year, then the availability would be just over 98%, as shown below:

The 98% here is shown as an example as it fits within the typical window of availability of the average contract. Most contracts require a system availability of at least 97-99%, BESS availability of at least 98%, PCS availability of at least 99%, and an auxiliary system availability of at least 99% over the course of a year. Scheduled maintenance of 1-2% outage is typically exempt from penalties in a contract.

These percentages are stated within contracts because they are important for ensuring that any BESS installation can participate in grid services such as energy arbitrage or peak shaving. 98% is the target for many BESS installations because the drop from 98% to 97% increases the downtime by 50%, even though the availability only drops by 1%. This is because the actual downtime hours increase from 175 hours per year to 263 hours per year.

Timing also matters as well. A system that runs for 362 days without problems but goes down for three days at the peak of summer, when wholesale prices and capacity payments are at their highest, could actually perform worse than another BESS with lower availability that stays online during critical time periods when prices are high.

BESS can become unavailable for a number of reasons. It can be equipment failure such as the PCS, transformer, cooling system, fire suppression system, or the BMS, or it can be because of safety-related shutdowns, power/energy de-rating, unplanned outages, or control and communication failures.

It only takes a single component to cause BESS downtime, but there is also some ambiguity in calculating downtime and BESS availability—for example, is it from when the fault is detected or after a formal notification process? This distinction can lead to hours of uncompensated downtime.

Why BESS availability matters

At the core, BESS availability matters for owners because the more uptime a BESS has, the more energy it can discharge to the grid throughout the year. For BESS owners with grid contracts, greater uptime and more energy supplied essentially mean more money and a better ROI on their energy investment from energy services, grid support, or capacity markets.

So, for utility-scale and commercial projects, BESS availability is directly linked to the project value and failure to meet the contracted guarantees leads to penalties such as liquidated damages. Not meeting the targets can mean a lot less energy in the grid.

It was noted above about the extra downtime hours of just 1%, however, we can also look at it from the perspective of energy lost. If we take a more extreme example, where an 8GW installation only has a 93% availability instead of the target 98%, this means that 400MW of capacity is lost each year.

However, it does go beyond the ROI for asset owners. Those who have financed their BESS often have availability guarantees, capacity tests, and warranty curves underwritten into their finance contracts.

While availability and capacity are not often tested for financiers during operation, when it comes to refinancing, evidence of availability and capacity may be required, so BESS owners do need to keep track of availability over the years of their installation’s operation. A high availability represents more revenue and lower risk.

Ensuring that degradation and efficiency are verified and not assumed, and that warranty entitlements can be evidenced, means that the people funding BESS installations will have more confidence in funding future BESS projects.

Challenges with BESS availability

There are different challenges with BESS availability. The first is keeping the BESS online for as long as possible. There are so many critical components that can affect it, and it only takes one of them to fail. This includes cell failures, rack and module failures, BMS failures, PCS failures, dispatch errors, grid signal issues, thermal control systems, auxiliary power supplies and fire detection and suppression systems.

As noted above, there is often a period between failure and official notification, meaning that many BESS have a lower true availability than officially stated. This can mean that BESS owners lose revenue unexpectedly, and the grid might not get as much backup energy as expected, which could lead to energy being sourced elsewhere to plug the gap.

In a 2025 battery intelligence report, battery data analytics provider Accure reported that almost one in five (19%) BESS projects underperformed due to hardware component failures. So, while all the projects passed their initial power and energy performance tests (capacity, duration, ramp rate, response and settling time, and round-trip efficiency) for the offtake agreement, they didn’t meet their obligations during operation.

These initial performance tests are called pre-Commercial Operation Date (COD) tests, but they only characterise the capabilities of the BESS and not the ongoing ability to remain available to respond to grid dispatch operations.

The US Department of Energy (DOE) has identified this issue and said that the availability cannot be derived from meter data alone. All these challenges with pre-COD testing not being carried on throughout the BESS lifetime feeds into the above-mentioned issues surrounding financing and warranty and a lack of true availability data when it comes to refinancing.

How the challenges are being addressed

According to Accure’s report, 57.6% of BESS downtime comes from small, recurring outages. These issues are easy to detect with early detection systems, so one of the best solutions for ensuring that the BESS remain online and for calculating the true availability of the BESS during its lifetime is to use advanced energy management systems that can monitor the components, SOC, and thermal loads in real-time.

Understanding the health of components also helps BESS owners pre-emptively plan maintenance (predictive maintenance) to prevent unexpected downtime and maintain high availability. Proactive management is seen as the way to both circumvent downtime and ensure a high BESS availability while also ensuring that the true availability can also be measured throughout an asset’s lifetime to build confidence between asset owners and investors.

One company involved in this space is FlexGen, which has stated that its energy management systems have helped all its customers to maintain an annual availability of 98% across its energy storage sites. Flexgen also stated that during Winter Storm Uri, its energy storage portfolio managed to retain a 99.7% uptime. The software platform from FlexGen provides real-time information on performance and predictive analytics to detect issues, identify the causes, and resolve them.

Another example is DNV and Fluence. Fluence provided fleet-level availability data, the calculation methodologies incorporated into the company’s contracts, and operational data for select projects for DNV to independently verify.

The verification showed that Fluence has achieved a 98.7% MW-weighted availability in the BESS installations, with 99.3% across fleets of 50MW and larger. This validation while the BESS is active provides customers and investors with performance guarantees to build confidence and trust. The high availability has been partly attributed to another method for minimising downtime: a modular design, which reduces downtime risks and maintenance complexity.