Wholesale drives the upside. aFRR protects the downside.

When market conditions get worse, a co-located battery doesn't just earn less. It earns differently.

That's the answer to the second question of our case study with Fluence. In the strongest of our five market price scenarios, green and grey battery storage both earn around 85% of their revenue on day-ahead and intraday. In the weakest, aFRR makes up 35% of green storage revenue and almost half of grey storage revenue.

While wholesale drives the upside of a co-location project, aFRR protects the downside.

In the worst case, that's what makes the difference. With wholesale only, grey storage reaches 8,4% IRR, exactly as much as green storage. With aFRR, it gets to 10,3%.

But aFRR is filling up fast. Prequalified battery capacity has grown from about 0,3 GW to 1,2 GW, against demand of around 1,9 GW (ISEA, RWTH Aachen). Prices are already falling. So far this year, positive aFRR cleared 40% lower than in the same months of 2025, negative aFRR 18% lower, based on our analysis of the German auctions.

Our model already assumes aFRR prices halve by 2035. But in a weak market, every battery has the same reason to move into aFRR at the same time. That's why I think a downside case needs its own aFRR stress test, not just lower power prices.

The two storage types also earn in different ways when prices turn negative. Green storage charges from curtailed wind and halves the wind park's curtailment. Grey storage gets paid to charge from the grid instead, so the wind park is curtailed almost as much as without storage. Green storage captures curtailed wind, and grey storage captures negative prices.

So green storage depends on its wind park, grey storage on the market. How the choice between green and grey changes the right storage size is what I'll look at in the next post.

The full case study is available here.