Stromatem: ten years of power, every quarter hour.
Each turn a year, across the band the day. You can watch the sun breathe, the wind in winter, and the share of wind, sun, biomass and hydro rise from 34 to 59 per cent.
Finding
One day
Questions for ten years of power
Today and tomorrow
Forecast of the transmission system operators for wind and sun against the load forecast; biomass and hydro come on top. Tomorrow’s forecast appears around 6 pm (CET).
What the spiral shows
One third became six tenths.
Renewable share of net electricity generation, calculated from every quarter hour since 1 January 2015.
- 33.7 → 58.9%
- Share of wind, sun, biomass and hydro in generation, 2015 and 2025.
- 58.5 GW
- Solar record: 13 August 2026 between 13:00 and 13:15.
- 131 h
- Longest dark doldrum since 2015: from 18 January 2015, wind and sun covered less than 10% of consumption for almost six days.
Cross-check: for 2025 the Bundesnetzagentur reports 437.6 TWh and 58.8% renewables; our calculation from the same data gives 437.9 TWh and 58.9%. The difference comes from later corrections and our rounding.
How to read the spiral
One turn is a year, the band is a day.
The spiral starts in the middle on 1 January 2015 and runs clockwise: New Year at the top, spring on the right, summer at the bottom. After each turn the next year lies outside. Across the band runs the day – midnight on the inside, noon in the middle, midnight again on the outside. Each of the roughly 410,000 quarter hours is one pixel.
In the “Who supplies” view every quarter hour carries the blended colour of its sources. The sun draws a yellow stripe through the middle of each turn, wide in summer, narrow in winter – the breathing. Blue is wind, coming in gusts and mostly in winter. It stays dark where coal and gas supply the power: on still winter nights. From the inside out the spiral grows brighter.
Dark doldrums
How long the longest lasted depends on the definition.
A dark doldrum (Dunkelflaute) is not a measurement but a convention. We count periods in which wind and sun together covered less than a fixed share of consumption without a break, and only those of 24 hours or more.
Below 10%, the longest lasted 131 hours, in January 2015. Below 15% it is 140 hours in October 2015; below 20%, 262 hours from 15 January 2017 – almost eleven days. That is why figures in articles contradict each other: each uses a different threshold. The “Dark doldrums” view lets you change it.
Method
How 410,000 quarter hours become one picture.
- 1
On SMARD, the Bundesnetzagentur (German Federal Network Agency) publishes net generation per energy source and grid load for every quarter hour since 2015. We load the twelve sources and the load once and store them as one small file per year.
- 2
For each quarter hour we combine seven groups – sun, wind (onshore and offshore), biomass with hydro and other renewables, nuclear, lignite and hard coal, natural gas, other with pumped storage – and store their shares of generation, rounded to 1/250.
- 3
Renewable means what it means for the Bundesnetzagentur: wind, sun, biomass, hydro and other renewables. A year’s share is total renewable energy divided by total generation, not the mean of the quarter hours.
- 4
Your browser loads the years one after another (about 2.6 MB together) and draws the spiral itself. The days since the last archive update and the forecast are fetched from SMARD by our server and passed on; SMARD never learns your address.
- 5
“Greener than x% of all days” compares the chosen day’s share with every measured day since 2015. The greenest window today and tomorrow comes from the day-ahead forecast for wind and sun divided by the load forecast.
Limits
- What is counted is the feed-in to the public grid. Electricity that households use directly from their own solar panels is missing, as are railway and industrial grids.
- Very small generators are partly estimated; SMARD corrects values later. The spiral shows the state on the day of retrieval.
- The share of generation is not the German government’s target, which is based on gross electricity consumption. Since 2023 Germany has imported more than it exports; which mix is imported is not in the data. Hence the “share of consumption” switch.
- The forecast only knows wind and sun. The time window is right; the percentage is usually around ten points below the later share of all renewables. It does not see local grid bottlenecks.
Sources and licences
Where the data comes from.
- Data: Bundesnetzagentur | SMARD.de, licence CC BY 4.0 – generation per source, grid load, forecasts; converted by us to shares and days
- Cross-check: Bundesnetzagentur, press release on electricity generation and consumption 2025, 5 January 2026
- Legal basis of publication: Section 111d of the German Energy Industry Act (EnWG)
Questions
Frequently asked questions about the Stromatem.
How do I find my birthday?
Under “One day”, pick a date from 1 January 2015 or tap the spiral. The panel then shows the course of the day, the renewable share and how green the day was compared with all others since 2015. “Copy link” lets you share it.
Why is the middle of each turn yellow in summer?
The middle of the band is noon. In summer, solar panels then supply a large part of the power – in June 2025 about 38 gigawatts on average between 12:00 and 13:00, ten years earlier just under 17. In the morning and evening the stripe narrows; in winter it almost vanishes.
What is a dark doldrum?
A period in which wind and sun supply hardly any power, usually in winter under high pressure and fog. How long it lasts depends on the threshold: below 10% of consumption the longest since 2015 lasted 131 hours, below 20% 262 hours.
Is the power from my socket as green as the spiral?
The spiral shows the mix on the German grid. Your tariff is a contract about certificates of origin; physically everyone gets the same grid power. Regionally it can differ when lines are congested.
How up to date is the data?
The archive runs to the last retrieval; our server fetches the days since from SMARD with about an hour’s delay. Grid operators publish tomorrow’s forecast around 6 pm.
A pattern hidden in your time series?
Measurements, consumption, generation, sensors: we make visible what long series contain – as a picture you can read and a tool you can use.