Dunkelflaute: when the wind dies and the sun's gone
A renewable grid's hardest hours are the cold, dark, windless spells when wind and solar generate almost nothing for days, and the system leans entirely on firm power and imports. This view auto-detects the worst such spell in the last year of German data and shows what happened. The grid-stress counterpart to the storage and build-out stories.
A Dunkelflaute is a cold, dark, windless spell when wind and solar together all but stop. Wattlas counts one when combined renewable output stays below 10% of demand for at least 24 hours.
There have been 14 such spells on record since 2021, and 1 in the 12 months to 8 August 2026.
The worst on record ran 97 hours in 2022, with renewables covering as little as 4% of demand at its floor.
Questions
What is a Dunkelflaute?
A cold, dark, windless spell when wind and solar together all but stop — Wattlas counts one when combined renewable output stays below 10% of demand for at least 24 hours.
How many Dunkelflaute events has Germany had?
14 on record since 2021; the worst ran 97 hours in 2022.
The worst low-renewable spell in the data, hour by hour. Wind and solar (the thin top slivers) all but vanish; coal, gas, biomass, hydro and imports carry the load; the day-ahead price climbs and spikes each evening.
Hours when wind+solar covered less than the threshold of demand, by month. Isolated low hours happen year-round — summer nights count — but the genuinely stressful events are the sustained multi-day spells, which cluster in the dark half of the year.
The generation mix on a normal stretch versus during the Dunkelflaute. Wind and solar collapse from a large share to almost nothing; coal, gas and net imports balloon to cover.
A Dunkelflaute ("dark doldrums") is a stretch of cold, overcast, windless weather when wind and solar generate almost nothing — often for days. The grid doesn't fail; it leans on firm power (gas, nuclear, hydro) and imports. These hours are why a high-renewable system still needs firm capacity, storage or strong interconnection — not an argument against renewables, but the engineering reality they have to plan for.
Every sustained low-renewable spell detected across five years of German data, longest first. The November and December 2024 events — when day-ahead prices spiked toward €1,000/MWh — sit near the top.