The European Union’s transition to clean energy marked a milestone in May, when solar panels generated more electricity than all of the bloc’s coal plants for the first time — and that’s before summer sun boosts production even further.

While the furious expansion of solar generation bodes well for efforts to replace fossil fuels, the breakthrough also exposed flaws in the energy system. Power prices turned negative during some of May’s sunniest days as grid operators struggled to handle the surge.

“This summer will be something we’ll have to look at like it’s a postcard from the future,” said Kesavarthiniy Savarimuthu, analyst at BloombergNEF. “The biggest message will be: we’re not ready.”