A solar PV plant generates precisely when all the others do too. That is why it earns less and less for its energy. A battery does not change when the sun rises, but it does change when the output is sold.
The problem: sunshine is worth little
In 2024, solar PV investment in Spain virtually ground to a halt. The cause was not technical: it was price. With so much solar capacity installed, the middle hours of the day are flooded with supply and the day-ahead market price falls to zero or close to it.
The 2024 market data we compiled at BK sums it up well:
- 26% of the year's hours priced below €10/MWh.
- 68% of days with a spread of more than €50/MWh between hours of the same day.
- 25% of days with prices above €90/MWh.
For a solar plant, the result is an average captured price of around €35/MWh in 2024, with two full months below €10/MWh captured, during which it produced almost 20% of its annual energy. On top of that comes curtailment: energy the plant could produce but the grid will not accept.
The problem is not a lack of energy. It is a surplus at midday and a shortage at eight in the evening.
The day still has expensive hours. On 23 September 2026, for example, the day-ahead market cleared at €15/MWh at 13:00 and €263.9/MWh at 20:00.
- Charge
- 13:00–15:00
- Discharge
- 20:00–22:00
- Capturable spread
- €232/MWh
What a 2-hour battery does
A battery next to the plant, at the same point of connection, stores part of the midday output and exports it to the grid in the highest-priced hours. With a 2-hour duration (for example, 1 MW and 2 MWh) it covers the evening peak, which is where most of the value lies.
- Raises the captured price: energy is sold at evening prices, not midday ones.
- Reduces curtailment: what the grid cannot take at midday is stored instead of being lost.
- Adds a night-time cycle: the battery can also charge in the cheap night hours and discharge at the morning peak.
- Reduces volatility: the plant depends less on the price in solar hours.
All without applying for new grid access capacity: the plant and the battery share the export capacity already granted.
The numbers
We simulated the effect of adding a 2-hour system to a solar PV plant with each year's real prices, with arbitrage and a night-time cycle only: no balancing services or capacity. The additional revenue grows every year as price spreads widen and zero prices appear on OMIE.
On returns, the example we used in our 2024 studies is clear. At that year's prices, a solar plant without a battery achieved an IRR of 1–2%. The same plant hybridised, with a larger investment, rose to 3–5% on that capital. With capacity revenue, the estimated range moved to 5–12%.
Indicative figures. They come from 2024 BK Energies studies for small PV plants (around 1 MWp) and a 2-hour system. Every plant has its own generation profile, point of connection and costs; a site-specific study is needed before investing.
How much does it cost?
In our 2024–2025 ranges, installing the energy storage and the technical modifications cost between €0.22 and €0.30/Wh, with the cost per Wh falling as the system grows. On top of that come permitting to move from PV to PV + batteries (€15,000–30,000 per plant, depending on size) and a guarantee of €20,000/MWh, recoverable after construction. Equipment prices change fast: ask us for an up-to-date figure.
The capacity mechanism
The third revenue stream is the one that can move returns the most: getting paid for having firm capacity available when the system needs it. It already works in other European markets. In Germany, in September 2024, the combination of different ancillary services markets paid up to €19,000/MW in a single month.
In Spain, the capacity mechanism is still going through approval, and its timeline depends on final sign-off and the auctions that are called. What we do know is how these markets usually work: the first operational assets capture the best prices. Having the plant hybridised and running before the first auction is the best position to be in.
Regulation is now on side
The main obstacle to hybridising plants with regulated remuneration was losing part of that income when storing energy. RD 917/2025 solves this: since 1 January 2026 the plant is paid for all the energy it generates, even if part of it goes to the battery, and it keeps its grid access and connection permits.
How it is done
- Study. Actual plant output, price curve, curtailment and point-of-connection limits. These determine the battery's power and duration.
- Permitting. Hybridisation application, consumption point, simplified environmental impact statement (DIA), guarantees and licences. For operating plants, a land-use review.
- Execution. Engineering, supply, installation and commissioning, with the EMS and power plant controller (PPC) integrated.
- Operation. The EMS decides every day when to charge and when to sell, using price and generation forecasts, with the option of manual operation.
See a real example: the 5 MWh hybridisation in Ocaña (Toledo). Or see how we approach PV + BESS hybridisation.