Do renewables always make power cheaper? In the UK, it depends on how much is on the system. Using causal machine learning on 2018–2024 market data, researchers find wind’s causal effect is U-shaped: at low penetration, +1 GWh can reduce prices by up to £7/MWh, the effect weakens mid-range, then strengthens again at higher penetration. The impacts have grown over time as renewables’ share rose, providing an evidence base for market design and support policy. These findings support adaptive support schemes and capacity planning that reflect changing marginal value rather than one-size-fits-all assumptions.