The arXiv preprint (May 2026) models UK and EU systems at 2023 and 2030 snapshots using cycle-resolved attribution on hourly dispatch data, revealing that batteries can cut system costs by 4-11% while raising CO2 in 38% of cycles through coal displacement avoidance. Long-duration storage captures rarer cross-day co-benefits, reducing both metrics in 72% of operations. This preprint methodology relies on synthetic 8760-hour simulations without real-time market bidding frictions, a key limitation versus empirical datasets. Related work in Nature Energy (2024) on German storage confirms similar hourly emission spikes from price-driven cycling, while a 2023 Imperial College report on UK capacity markets shows policy incentives favor short-duration assets that amplify the cost-emissions split. Mainstream coverage misses how 2030 decarbonization expands co-benefit windows by 2.3x, implying targeted carbon pricing on storage dispatch could close the gap faster than aggregate capacity targets.