Holiday Monday, 25 May. Wall-to-wall sun across Belgium, temperatures in the high 20s, industrial demand at the floor. By every textbook in the room, this should have produced hours of deeply negative imbalance prices. It didn't. The imbalance price spent most of midday hovering near the day-ahead, and even turned positive in a handful of quarter-hours when most prosumers were braced to be paid to stop injecting.

That mismatch matters. A lot of Belgian prosumer and aggregator positioning for May rested on a simple bet: solar peak plus low demand equals negative prices equals profitable curtailment or battery cycling. That bet underperformed last week. We think the structural reasons it underperformed will keep underperforming through summer.

Cooling has quietly become the dominant midday load

Heat changed the demand curve. With temperatures in the high 20s across the Benelux, residential and tertiary cooling lifted weekday afternoon load by a meaningful chunk versus a mild May day. Most analysts still model Belgian summer load curves on pre-2022 baselines, and they underestimate how fast HVAC penetration has grown in office stock and newer residential. The midday "shoulder" prosumers used to count on for solar-cannibalisation windows is shrinking, fast.

BESS arbitrage is now setting the floor

Belgian and Dutch battery fleets have crossed the threshold where they meaningfully shape midday prices. When day-ahead clears near zero, batteries charge aggressively. When the imbalance signal goes long, they bid into it. Around midday on 25 May, Elia's aFRR and imbalance signals showed a system that wasn't structurally long: every MWh of surplus solar found a willing buyer in a battery somewhere on the perimeter. Three years ago that buffer didn't exist. Today it sets the floor.

Cross-border flows did the rest

Sunny Belgium also meant sunny Netherlands and partly sunny Germany, so intuitively you would expect interconnectors to be useless. Not last week. France was sitting on its usual spring nuclear maintenance shoulder, UK midday demand pulled north-bound on Nemo Link, and Alegro provided an extra eastbound vent toward Germany. The Belgian zone had more places to send a surplus MWh than the "everyone is sunny, nobody wants it" view captures. Solar overflow that would have stayed home and crashed local prices got absorbed by a wider sink.

What this means for the negative-price thesis

We are not saying negative hours are gone. Belgium will still see them, especially on weekend low-load days when cooling is off and exports are constrained. But the easy short-imbalance trade, where any sunny holiday is assumed to produce hours of negative pricing, is structurally weaker than it was even twelve months ago. Three implications for Belgian energy managers and prosumers:

One concrete action this week: re-model your imbalance P&L on the last 60 days of Belgian quarter-hour data before locking in a summer hedge. The market that produced your current strategy isn't the market you are trading in this week.