Hydro power

Harnessing the power of water flowing on rivers is easy. Put a waterwheel on the river, and the flowing water rotates the wheel. This is how watermills work. Some hydropower plants still work this way, so called "run of the river" hydropower plants.

Power output of a "run of the river" plant depends directly on the flow of the river. During dry seasons such plants may run out of power as flow in the river dwindles. But the plants also can't use all the flow from heavy rains - the water flows around the plant.

To REALLY harness the power of rivers, people dam the river, forming a reservoir upstream of the dam. With a dam and reservoir, a hydropower plant can adjust it's power output according to demand. Which is much more useful than power output depending on river flow. The reservoir stores water from rainy times, and is able to provide power even during dry times. Within the operating limits of the reservoir, of course. When a reservoir gets too full, floodgates are opened and water passes the dam - and when a reservoir gets too empty, the power output diminishes.

Explanation of the graphs

How much energy is stored in a hydropower reservoir is calculated by multiplying the volume of water currently in the reservoir with the drop height of the dam. This information is important for the grid operators. Hydropower plants in Europe report weekly their reservoir levels as MWh (MegaWatt hours) to the grid operators, and this data is made available in the ENTSO-E transparency platform.

The graphs on this website displays the ebb and flow of reservoir levels over the year, allowing you to compare the level to max, median, minimum and previous years level.

For end users of electricity, the reservoir level information can be used to estimate medium-term (about one year timeframe) electricity prices. Electricity production is transitioning to weather-dependent solar and wind power. The more full reservoirs are, the easier and thus cheaper it is for hydropower to provide backup when wind and solar isn't available. On the other hand, if reservoirs get empty during winter, electricity could become very expensive on cold windless winter days.