What do grid reserve, capacity reserve and security stand-by actually mean?

Our electricity supply has three guardian angels on the bench, ready to join in. In extreme situations, they safeguard the stability of the grid. Find out here which reserve is used when.

Illustration shows renewable energies, a factory and a residential building seen through a magnifying glass© BMWi

This is what it's all about: the grid reserve, the capacity reserve and the security stand-by have different roles but a common goal – to maintain the electricity supply.

Germany has one of the world's most secure supplies of electricity. One reason is that there are three clever back-up mechanisms to keep the supply going. Like a triple safety net, they make sure that our energy system remains stable and continues to function even as the energy transition advances. However, the various back-ups are all different, and the grid reserve, the capacity reserve and the security stand-by all have their own story to tell.

The grid reserve safeguards the electricity grid when demand is high

Our major transmission lines are at their busiest in winter. In the cold, windy months, the wind turbines in the north feed lots of electricity into the system, whilst the industrial heartlands in the south demand even more energy – especially when it's cold and gets dark early. To make sure that the powerlines don't have to shut down due to the sheer volume of electricity flowing through them, installations have to be curtailed in the north and ramped up in the south. These measures, known as 'redispatch', ensure that the grid can be operated safely. A grid reserve, also known as the 'winter reserve' or the 'cold reserve', is therefore formed each year, particularly for the colder six months of the year. It consists of power stations which are not currently operational or which have been notified for closure by the operators. They can step in quickly where needed. Their deployment is regulated by the Energy Industry Act and the Grid Reserve Ordinance. The major German transmission system operators (TSOs), who are responsible for grid stability, identify the future need for a grid reserve each year. Their findings are reviewed and confirmed by the Federal Network Agency. The grid reserve for the winter of 2020/21 amounts to around 6.6 gigawatts (GW).

The capacity reserve: for extreme and unexpected situations

When things get tight because something unexpected has happened, people are always happy to hear someone say: 'We've got some spare capacity we can use.' The same is true of the capacity reserve. It serves to guarantee security of supply even in extraordinary and unpredictable extreme situations. The capacity reserve is used when, despite free pricing, the supply of electricity on the wholesale market is not sufficient to cover the total demand for electricity. It is independent of the electricity market and thus delivers additional security for the consumers. The reserve consists of existing generating installations, storage facilities and demand outside the electricity market. The power plants in the capacity reserve are not allowed to operate actively on the electricity markets (ban on marketing), and are only permitted to increase their output in response to a demand from the TSOs. The TSOs can only make such a demand when there are no other alternatives available to offset imbalances between electricity take-off and feed-in. The capacity reserve is to provide a safety net consisting of another 2 gigawatts (GW) from the winter half-year of 2020/21. The capacity reserve is also regulated in the Energy Industry Act and the Capacity Reserve Ordinance. The TSOs concluded the first bidding round for the capacity reserve at the end of 2019. To this end, a total of 1,056 megawatts of generation capacity has been procured from the period from October 2020 until September 2022; a total of 2,000 megawatts was up for auction. This amount is to be reached in the next auction round. The current monitoring report on security of supply shows that electricity demand can be met at all times in the coming years. In order to ensure that the power supply remains stable as the energy transition progresses, the Federal Ministry for Economic Affairs and Energy currently monitors the security of the power supply; from next year, the Federal Network Agency will be responsible for this. The results are summarised in the monitoring report. It examines regularly (at least every two years) how the electricity market and the available power plants will develop in the coming years until 2030.

The security stand-by: if plans A and B are not enough

A double safety net feels really good. And, to make sure it can really cope with every situation, the 'security stand-by' is available as a third safety net. Established by the Electricity Market Act of July 2016, it consists of power stations which have been scheduled for closure in the course of the phase-out of lignite-fired power generation. For four years, it will be possible to reactivate these shut-down power stations in extreme situations before they are finally decommissioned. They can only be called on if all other measures and even the grid reserve and the capacity reserve are insufficient. That has never happened so far. The power stations will have a period of ten or eleven days in which to be reactivated in response to a demand by the TSOs. Eight power plant blocks with a total capacity of 2.7 gigawatts are on security stand-by. They account for 13% of installed lignite-fired capacity. These provisionally decommissioned power stations have ceased to generate harmful carbon emissions. It is estimated that 12.5 million tonnes of carbon emissions will be saved from the time when the security stand-by system was launched in October 2016 until the end of 2020.