​On September 17th, the Spanish government finally published the ministerial order establishing Spain's capacity market. Following European Commission approval, the final mechanism remains largely unchanged from the draft proposal discussed in this article. Regardless, four changes matter for developers, particularly for co-located assets with no charging rights.

1. Capacity market stress hours: a ceiling instead of a band

The original proposal required stress hours to account for between 5% and 10% of hours in a calendar year. The final order drops the floor and keeps only the 10% maximum.

This change gives REE full discretion over how many hours it selects, up to the 10% ceiling, rather than forcing a minimum. As we covered in a previous article, batteries’ de-rating factor is affected by the number of stress hours. The lower the amount, the higher the de-rating factor for batteries, particularly low-duration storage.

2. Do resources have to be available or do they have to generate?

For a standalone battery, the final document clarifies that the asset must be “available to deliver” during stress hours, not necessarily generating. In the draft, only renewables were required to generate during these periods. However, the final document extends this requirement to co-located batteries that cannot charge from the grid.

This difference in requirements makes capacity market participation more complicated for co-located BESS in the capacity market. While standalone batteries can simply remain available during stress hours and provide ancillary services, co-located batteries must instead dispatch at its required firm capacity.

No charging rights could lead to underdelivery

Without charging rights, this requirement creates a real risk of not meeting the firm capacity obligation and face penalties. In this case, the battery would need sufficient charging generation from its renewable asset to provide firm capacity during stress hours.

To test this risk, we simulated a co-located solar and battery project without grid charging rights during a period of unusually low solar generation in Spain (January 21st to February 2nd, 2026). The solar plant and battery have the same power capacity, while the battery has 8 hours of storage and a de-rating factor of 0.81. In the simulation, each day’s three highest-priced hours were treated as stress hours, when the battery has to deliver its firm capacity.

​​