Going from fossil to renewable electricity is going from production to harvesting, so of course renewables need storage. It is often said that renewable energy is sustainable, as in my two previous blog posts about PV and wind energy. But is the storage needed also sustainable? And how cheap is renewable energy plus storage?
This blog post is about how much storage is needed and for this we look at the three stages on the way to 100% renewable electricity. Currently, most countries have only a small share of renewables in their annual electricity mix and do not need storage. However, with the installation of more renewables, more storage will soon be needed. We need to be prepared for this and ready for sustainable storage.
Basic functioning of an electricity grid
In an electricity grid, electricity generation must be adjusted to the variable demand of the end consumers at all times. If the grid is mainly fed by nuclear and fossil power plants, this is how demand (the black curve) is met over time by different types of power stations (colours):
At the lower end is the relatively constant base load, while newer coal-fired power plants can supply the intermediate load with some flexibility, and fossil gas delivers highly flexible peak load.
A nuclear and fossil-fuelled power grid already has a high degree of intermittency forced by consumers. Dealing with intermittency is therefore not new to electricity operators.
France, for example, which has a high share of nuclear power, operates about 40% of its nuclear plants rather flexibly . In many other countries, however, intermittency is mainly managed with flexible gas supply and rather flexible coal combustion.
How renewable energies enter the electricity mix
In the meantime, two technologies have entered the market: the modernisation of an old technology by using composite materials (wind turbines) and a new electronic device (solar cells). In a capitalist world, it is impossible to keep these two technologies out of the market. Because they are modular and simple, they entered the market when they were still expensive, and prices fell as technological experience and production numbers increased. I compare this dynamic with the inventions of the movable printing press in Korea in 1234  and in Germany in 1450 . High society at the time tried to prevent printing so that other people could not spread their word so easily. But the printing presses were so simple that they broke through all barriers. Today, traditional energy companies cannot prevent renewables, even though the electricity grid is more regulated than most other markets. Electricity markets have become pluralistic, and the key to asserting interests tends to be not economic power and political ties, but trust in mitigating environmental problems .
For these and other reasons, this is what power generation looks like in more and more countries (this is again an example for Germany):
In winter, Germany still burns a lot of fossil fuels despite wind energy – but less in May and summer:
On this graph, the sun is like an undulating wave, the wind like mountains. This is not only smoothed by fluctuating fossil electricity, but also by the opening of national electricity markets to neighbouring countries, as the top panel of this graph shows: it displays exports and imports with Norway via the NordLink submarine cable. For this reason, consumption (black line) in Germany does not match total generation (upper edge of PV).
With the increasing share of solar and wind energy, electricity is no longer generated but harvested. This means that electricity generation, not just consumers, is also causing fluctuations in the grid.
This may sound difficult, but consider that in most countries electricity is allocated on the spot market one day in advance. Renewable fluctuations can be predicted very accurately one day in advance, and even longer.
Economics of power generation
To compensate for the volatility of renewables, fossil-fuel power plants have to shut down temporarily, which reduces their profits. Are you afraid they will go bankrupt and there will be blackouts when there is no sun or wind? The following graph shows the cost of generating electricity when less than full throttle is needed for all 8760 hours of the year:
This graph is based on real input data [10,11], not assumptions, from 2021 (before the war in Ukraine). It can be understood as follows. Gas-fired power plants are cheap to build. Their main cost is buying gas, and if the power plant is operated less frequently, less gas needs to be bought. Therefore, the cost of generating electricity remains relatively constant even with less hours of power generation per year. This means that gas-fired power plants earn less revenue, but remain competitive in the market.
In contrast, the curves of nuclear, wind and PV power plants are steep. This is because they are expensive to build but cheap to run (the radioactive waste repository is not included in this graph). If these power plants were temporarily switched off, they would age anyway and no money could be saved. Therefore, running them less frequently increases their cost of electricity generation, so the grid operators give them priority to keep electricity prices as low as possible. Since wind&solar and nuclear, and to some extent coal, are both characterised by steep curves, they tend to push each other out  (more on this below).
In the May graph above (Fig. 3), we can see that demand (black line) was lower than PV in the last third of May because coal was mainly generated for export, where it is more competitive in countries with less renewable energy. In fact, the generation costs of renewables are generally lower than many think [12,13] and the reasons for the quite high electricity costs in Germany are distribution, levies and taxes.
When an electricity market is not fully regulated by a government, renewables tend to push out nuclear and coal. Fossil gas becomes less lucrative, but remains competitive for providing short-term energy thrusts that cannot be provided by many other types of power plants.
Similar displacement processes can be observed also in other sectors of the economy . For example, programming an app costs money, but providing it to additional consumers costs little. This is the market power of so-called high fixed costs, but marginal costs close to zero.
If you take a closer look at the May graph above (Fig. 3), you will notice that on some days PV and wind can supply almost all the electricity during the midday period, even though – over the whole year – they produce about 50% of the total electricity.
Up to an annual share of renewable energies of about 50%, no electricity storage is necessary, because the fluctuations are balanced out by the fossil-fuelled power plants.
If more and more PV systems are installed in Germany, electricity generation over the midday period will soon exceed demand. Installing even more PV would require curtailing it during the midday period, and the annual share of renewables could no longer grow with more PV installations. The following graph shows that more PV electricity can be sold and consumed if it is stored in batteries for a few hours :
The “surplus” PV electricity (light yellow) is cheap and can be used in other sectors, e.g. for charging electric cars used for commuting, for heat generation and storage in industry and municipal heat grids, and for long-term storage (see below). This is called sector coupling and will play an important role in matching variable end-user demand to variable renewable electricity generation at any time – and thus sector coupling will improve supply security.
From all this you can intuitively deduce the following rule of thumb:
When the annual share of renewable energy rises above about 50%, short-term storage of renewable energy is needed [16,17] from day to night. Longer periods without sun and wind can still be bridged with enough fossil power plants and long-range electricity trading, up to an annual share of about 80% [18,19] renewables.
These exact percentages depend, of course, on the respective country and infrastructure.
As more and more countries increase their share of renewable energy, short-term storage is an emerging market with a whole range of players:
– a battery for households, which is often bought together with the PV system on the roof.
– the battery of an electric car is typically about 10 times larger than the household battery, and part of its capacity can be used for household use without affecting most car trips.
– Medium batteries for neighbourhoods that share solar panels.
– Large batteries that are often installed together with PV systems and wind farms or by regional authorities.
– Storage providers such as dams, off-river pumped storage or biogas, often operating across national borders.
– Other storage technologies  currently offer only small capacities and most of them are currently more expensive than batteries, hydropower or biogas, but they may enter the market as the energy transition progresses.
The following graph shows the cost of PV and wind power in Germany including the cheapest battery storage installed in 2021:
Since renewable electricity is cheap, storage can cost quite a bit without affecting competitiveness in the power grid.
However, this blog article is about estimating the storage capacity required so that we can examine in the next article, whether storage technology is sustainable. Therefore:
What amount of short-term storage is needed?
It may surprise you that the storage capacities needed cannot be predicted exactly. There is quite a lot of freedom in terms of which paths the energy transition takes in the different regions of the world . The question is, which of the many possible scenarios are cheaper, easier to implement, more acceptable, promoted by industry to increase sales, which ones require less materials like copper, and so on . This has the great advantage that the energy transition in each country only needs a science/technology-based framework, but does not need to be planned in too much detail. It can be kicked off with non-regret options that the framework provides, and will be tuned with ongoing adjustments and policy schemes.
Besides technological aspects, government regulations also play an important role. For example, a time-independent (flat) electricity tariff tempts PV-battery households to maximise self-consumption. This results in little use of the grid in summer, as in this modelled example from Western Australia:
The bottom panel shows that the introduction of flexible electricity tariffs, with prices based on supply and demand, may encourage households to sell electricity in the afternoon when prices are higher than at midday. This changes the battery capacity needed, which in turn spreads the grid load more evenly throughout the year – to the delight of the grid operators who need to earn money to keep the grid in good condition.
Another example of government regulation is how much electricity trading between neighbours and in districts is legally allowed so that citizens and citizen cooperatives can share in the profits. Different economic landscapes lead to different amounts of short-term storage.
By the way, household batteries for self-consumption do not significantly reduce the maximum grid load because they reduce grid load mainly at times when the grid is not heavily loaded . Therefore, some adjustment of the grid from centralised to decentralised power generation is still necessary. How much of this is necessary again depends heavily on the freedom mentioned above .
How much short-term power storage must be installed depends on which of the various possible paths is chosen for the energy transition. It also depends on whether the electricity tariff is fixed or flexible and how the storage in cars and industry is connected to the electricity grid (called sector-coupling).
In the most technically and economically likely scenarios, Germany needs between 25 GW and 350 GW of battery capacity, depending on the policy and infrastructure, with most of the detailed scenarios predicting between 40 and 100 GW:
Here, too, it is important to tune the entire system including sector-coupling. Thinking only about one part leads to irrelevant or nonsensical predictions .
The same is true for predicting the global energy transition . A detailed study  divides the planet into 145 sub-regions where most people live and calculates both renewable energy production from historical weather data and projected demand from consumers, industry and transport – with hourly resolution. This makes it likely that energy availability and the necessary energy storage are neither significantly overestimated nor underestimated, regardless of how much other sectors of the economy are electrified. The projected battery storage capacity is maximally 60 TW worldwide. We’ll save that figure for the next blog where we look at battery sustainability, but for now let’s look at
How to achieve 100% renewables?
On the way to 80% renewables, electricity is produced during midday and windy periods in surplus quantities, although batteries recharge during these times (we saw this in Fig. 5).
In contrast, during weather periods when little sun shines and little wind blows, it does not help much to install additional renewables:
In such times of little wind and little sunshine, about 20% of the total annual electricity generation is produced (depending on the country). This means that:
Once renewables account for about 80 % of annual electricity generation, long-term storage is needed to shift electricity from surplus periods to later periods with insufficient renewables (called: balancing power).
For example in Germany, mainly electrolysers will use surplus renewable electricity to split water into hydrogen and oxygen. The hydrogen will be stored where fossil gas is stored today: in washed-out salt caverns underground. In times when the supply of renewable energy is insufficient, gas-fired power plants will burn the hydrogen and convert it into electricity. All new gas-fired power plants should therefore be designed to also burn fuels other than fossil gas (so-called multi-fuel ICE or OCGT). In Switzerland, PV electricity can be stored as hydropower (no wind turbines are needed in Switzerland ). In Scotland, heat is suitable for long-term storage . Each country has its own specific mix of suitable long-term storage.
Long-term storage is economically viable because it provides only a fraction of the total electricity over the year . Only a limited amount of storage equipment must be installed (such as electrolysers or water pumps) because the periods of excess supply and hence charging are spread over a total of about 1/4 of the year. This keeps the investment relatively low. The balancing power plants run for about 20% of the time, so rather low efficiencies are affordable (from power-into-storage to power-back-to-the-grid). Since electricity is rather cheap [39,40] during the times of excess supply, balancing power is expected to increase the average electricity price by about 10% .
Such balancing power is the key to supply security [42,43,44]. Since there is hardly any such balancing power installed so far, many people are sceptical about supply security with 100% renewables.
Improving supply security in a 100% renewable power grid
With having a system of balancing power running, two issues must be cared for:
1. Renewable energy production may become imbalanced between seasons. For example, if there are not enough wind turbines installed for winter, the share of balancing power increases to more than 20%.
2. In winter, high-latitude countries sometimes experience prolonged periods of insufficient PV and wind energy, known as dark doldrums. This requires both sufficient long-term storage and a sufficient number of balancing power plants.
Let’s first look at the first point: seasonal imbalances.
For example in Germany, electricity demand is roughly the same throughout the year. Installing the same wind as PV capacity ensures relatively constant electricity generation across the seasons, of course with the year-to-year fluctuations indicated by the column groups in this graph:
Again, these year-to-year fluctuations will be absorbed by balancing power and sector-coupling (not shown). On a global level, PV and wind power complement each other over the seasons only in the regions coloured red in this graph (and the darker red, the better):
The blue colour indicates regions where solar and wind have occurred together over the last 20 years of weather data. This affects the ratio between wind and PV capacities that should be installed to minimise storage needs:
Regions coloured red require mainly PV and little long-term storage. Note that the majority of people live in these regions. However, in mainly rich countries at higher geographical latitudes, a significant amount of wind energy is beneficial and long-term storage is essential.
Finally, let’s look at the second issue: dark doldrums. Looking at the weather data of the last 21 years, bottlenecks occur as often as shown here (example for Germany):
Since solar energy in Germany fluctuates much less than wind energy, solar energy is quite reliable in summer, so that almost all outages of combined wind and solar energy occur in winter [49,50]. Such periods are typically caused by a high-pressure system over Central Europe, the British Isles or Iceland . As these are large-scale weather patterns, the grid connection to neighbouring countries does not help much, but such periods can be predicted well in advance. In the last 40 years, five such events in Europe lasted longer than 10 days . For example in Japan, dark doldrums occur mainly in late summer due to the monsoon .
In the case of Germany, dark doldrums will require either the expansion of existing salt caverns for more hydrogen storage or an adjustment in demand, possibly a combination of both: once every 10 years or so, the government must pay some big factories to cut production temporarily, people are encouraged to commute by public transport instead of private electric cars, and so on.
This is a legitimate concern, often raised by sceptics. But consider this: people in rich countries have always had to deal with periods of unfavorable weather. With 100% renewables, it won’t be the snowstorms in the US and ice rain in Europe that cause problems, it will be the long absence of wind, once every 10 years or so. Fortunately, these typical high-pressure weather conditions do not cause particularly cold temperatures, so heating with heat pumps needs only the usual amount of electricity.
The amount of long-term storage depends heavily on geography. In high-latitude countries, it depends on the balance between installed PV and wind power – and how these countries deal with dark doldrums that occur once in about 10 years.
Concerns about the supply risks of renewables often call for (nuclear) baseload. Let’s end this blog article with
The most convenient base load is so large that it runs the entire grid at night, so that mainly the daytime peaks need to be supplied with variable electricity. Having even this large base load in a grid with 100% renewables, what problems does it solve? Short-term storage for the evening is smaller but is still necessary. The same for long-term storage: because the base load requires less long-term storage, dark doldrums pose similar challenges as without baseload. What possible advantage of base load in a grid that is 100% renewable comes to mind? Perhaps that baseload reduces variability? Let’s take a look at that:
The base load obviously increases the variability much more. From night to noon, the variable power increases from 0% to 150%, while in a grid without base load it varies only between 50% and 80%.
Base load is a concept in a grid dominated by nuclear or fossil power plants. It should not be transferred to a grid dominated by renewables. What is beneficial in one system does not solve any significant problems in the other system. 100% renewable grids require balancing power, from times of oversupply to later times of undersupply, not base load.
In the next blog article, we look at the sustainability of storage. If storage is sustainable, 100% renewable energy systems are indeed sustainable.
Until then, I hope I have helped you to imagine the benefits and challenges of a climate-friendly electricity system. I have used many examples from Germany because this has been well studied. Where is the energy transition most likely heading to in your country?
Scenarios shown in Figs. 5 and 9 are calculated myself with the assumptions mentioned in the figure captions or discussions of the figures. Figs. 11 and 12 have been redrawn with Australia in the middle to avoid copyright infringement.
 Neon Neue Energieökonomik, Technical University of Berlin, and ETH Zürich, “Open Power System Data.” https://open-power-system-data.org
 C. Cany, C. Mansilla, G. Mathonnière, and P. da Costa, “Nuclear power supply: Going against the misconceptions. Evidence of nuclear flexibility from the French experience,” Energy 151, 289 (2018). https://doi.org/10.1016/j.energy.2018.03.064
 Wikipedia, “Movable type – Korea.” https://en.wikipedia.org/wiki/Movable_type#Korea
 Wikipedia, “Movable type – Europe.” https://en.wikipedia.org/wiki/Johannes_Gutenberg
 Wolfgang Gründinger, “What drives the Energiewende?,” Thesis, Humboldt-University, Berlin, 2015 https://www.researchgate.net/publication/315374888
 Tennet, “Load flow readings – Norway.” https://www.tennet.eu/electricity-market/transparency-pages/transparency-germany/network-figures/metered-load-flow-readings-norway/load-flow-readings-norway/
 Nord Pool, “Market data.” https://www.nordpoolgroup.com/en/Market-data1/#/nordic/table
 M. Yuan et al., “Would firm generators facilitate or deter variable renewable energy in a carbon-free electricity system?,” Applied Energy 279, 115789 (2020). https://doi.org/10.1016/j.apenergy.2020.115789
 C. Kost, “Stromgestehungskosten erneuerbare Energien 2021 [Electricity production costs of renewable energies in 2021],” Fraunhofer Institute for Solar Energy Systems ISE, 2021. https://www.ise.fraunhofer.de/de/veroeffentlichungen/studien/studie-stromgestehungskosten-erneuerbare-energien.html
 H. Wirth, “Aktuelle Fakten zur Photovoltaik in Deutschland [Current facts about photovoltaics in Germany]”, Fraunhofer Institute for Solar Energy Systems ISE, 2021. https://www.ise.fraunhofer.de/de/veroeffentlichungen/studien/aktuelle-fakten-zur-photovoltaik-in-deutschland.html
 P. J. Heptonstall and R. J. K. Gross, “A systematic review of the costs and impacts of integrating variable renewables into power grids,” Nature Energy 6, 72 (2021). https://doi.org/10.1038/s41560-020-00695-4
 G. Luderer et al., “Impact of declining renewable energy costs on electrification in low-emission scenarios,” Nature Energy 7, 32 (2022). https://doi.org/10.1038/s41560-021-00937-z
 D. Helm and C. Hepburn, “The age of electricity,” Oxford Review of Economic Policy 35, 183 (2019). https://doi.org/10.1093/oxrep/grz005
 I. M. Peters et al., “The role of batteries in meeting the PV terawatt challenge,” Joule 5,1353 (2021). https://doi.org/10.1016/j.joule.2021.03.023
 A. Zerrahn and W.-P. Schill, “Long-run power storage requirements for high shares of renewables: review and a new model,” Renewable and Sustainable Energy Reviews 79, 1518 (2017).http://dx.doi.org/10.1016/j.rser.2016.11.098
 M. Raugei, A. Peluso, E. Leccisi, and V. Fthenakis, “Life-Cycle Carbon Emissions and Energy Implications of High Penetration of Photovoltaics and Electric Vehicles in California,” Energies (Basel) 14, 5165 (2021). https://doi.org/10.3390/en14165165
 M. Child, C. Kemfert, D. Bogdanov, and C. Breyer, “Flexible electricity generation, grid exchange and storage for the transition to a 100% renewable energy system in Europe,” Renewable Energy 139, 80(2019). https://doi.org/10.1016/j.renene.2019.02.077
 M. Victoria, K. Zhu, T. Brown, G. B. Andresen, and M. Greiner, “The role of storage technologies throughout the decarbonisation of the sector-coupled European energy system,” Energy Conversion and Management 201, 111977 (2019). https://doi.org/10.1016/j.enconman.2019.111977
 M. S. Reza et al., “Energy storage integration towards achieving grid decarbonization: A bibliometric analysis and future directions,” Journal of Energy Storage 41, 102855 (2021). https://doi.org/10.1016/j.est.2021.102855
 L.J. Schwenk-Nebbe, J. E. Vind, A. J. Backhaus, M. Victoria, and M. Greiner, “Principal spatiotemporal mismatch and electricity price patterns in a highly decarbonized networked European power system,” iScience 25, 104380 (2022). https://doi.org/10.1016/j.isci. 2022.104380
 T. T. Pedersen, M. Victoria, M. G. Rasmussen, and G. B. Andresen, “Modeling all alternative solutions for highly renewable energy systems,” Energy 234, 121294 (2021). https://doi.org/10.1016/j.energy.2021.121294
 K. Say and M. John, “Molehills into mountains: Transitional pressures from household PV-battery adoption under flat retail and feed-in tariffs,” Energy Policy 152, 112213 (2021). https://doi.org/10.1016/j.enpol.2021.112213
 F.C. Matthes, F. Flachsbarth, C. Loreck, H. Hermann, H. Falkenberg, and V. Cook, “Zukunft Stromsystem II,” WWF, 2018. https://www.oeko.de/fileadmin/oekodoc/Stromsystem-II-Regionalisierung-der-erneuerbaren-Stromerzeugung.pdf
 V. Quaschning, N. Orth, J. Weniger, J. Bergner, B. Siegel, and M. Zoll, “Solarstromausbau für den Klimaschutz,” HTW Berlin, 2021. https://solar.htw-berlin.de/wp-content/uploads/HTW-Studie-Solarstromausbau-fuer-den-Klimaschutz.pdf
 Ariadne, “Deutschland auf dem Weg zur Klimaneutralität 2045 – Szenarien und Pfade im Modellvergleich,” 2021, https://ariadneprojekt.de/publikation/deutschland-auf-dem-weg-zur-klimaneutralitat-2045-szenarienreport/
 BCG, “Klimapfade 2.0,” Berlin, 2021. https://www.bcg.com/de-de/klimapfade
 Fraunhofer institute for solar energy systems ISE, “Wege zu einem klimaneutralen Energiesystem, Update November 20: Klimaneutralität 2045,” 2021. https://www.ise.fraunhofer.de/content/dam/ise/de/documents/publications/studies/Fraunhofer-ISE-Studie-Wege-zu-einem-klimaneutralen-Energiesystem-Update-Klimaneutralitaet-2045.pdf
 Wärtsilä Energy, “Front-loading net zero,” 2021. https://www.wartsila.com/front-loading-net-zero
 Agora Energiewende, “Die Energiewende in Deutschland: Stand der Dinge 2021,” 2021. https://www.agora-energiewende.de/veroeffentlichungen/die-energiewende-in-deutschland-stand-der-dinge-2021/
 Institute for energy and climate research (IEK), “Neue Ziele auf alten Wegen? Strategien für eine treibhausgasneutrale Energieversorgung bis zum Jahr 2045,” 2021. https://www.fz-juelich.de/de/iek/iek-3/aktuelles/meldungen/neue-ziele-auf-alten-wegen-strategien-fuer-eine-treibhausgasneutrale-energieversorgung-bis-zum-jahr-2045
 L. Göke, C. Kemfert, M. Kendziorski, and C. von Hirschhausen, “100 Prozent erneuerbare Energien für Deutschland: Koordinierte Ausbauplanung notwendig,” 2021. DIW Berlin. https://doi.org/10.18723/diw_wb:2021-29-1
 Energy Watch Group, “100% Erneuerbare Energien für Deutschland bis 2030,” 2021. https://www.energywatchgroup.org/neue-studie-100-erneuerbare-energien-bis-2030-in-deutschland-moglich/
 S. DaneshvarDehnavi, C. A. Negri, M. G. Giesselmann, S. B. Bayne, and B. Wollenberg, “Can 100% renewable power system be successfully built?,” Renewable Energy 177, 715 (2021). https://doi.org/10.1016/j.renene.2021.06.002
 F. Ram et al., “Global Energy System based on 100% Renewable Energy – Power, Heat, Transport and Desalination Sectors. Study by Lappeenranta University of Technology and Energy Watch Group, Lappeenranta, Berlin,” 2019. http://energywatchgroup.org/new-study-global-energy-system-based-100-renewable-energy
 M. Z. Jacobson, M. A. Delucchi, M. A. Cameron, and B. v. Mathiesen, “Matching demand with supply at low cost in 139 countries among 20 world regions with 100% intermittent wind, water, and sunlight (WWS) for all purposes,” Renewable Energy 123, 236 (2018). https://doi.org/10.1016/j.renene.2018.02.009
 G. Xexakis, R. Hansmann, S. P. Volken, and E. Trutnevyte, “Models on the wrong track: Model-based electricity supply scenarios in Switzerland are not aligned with the perspectives of energy experts and the public,” Renewable and Sustainable Energy Reviews 134, 110297 (2020). https://doi.org/10.1016/j.rser.2020.110297
 C. Berry, “Renewables Scottland 2030,” Common Weal, 2018. https://www.thenational.scot/resources/files/72737
 O. Ruhnau, “How flexible electricity demand stabilizes wind and solar market values: The case of hydrogen electrolyzers,” Applied Energy 307, 118194 (2022). https://doi.org/10.1016/j.apenergy.2021.118194
 C. Chen, Z. Yang, and G. Hu, “Signalling the cost of intermittency: What is the value of curtailed renewable power?,” Journal of Cleaner Production 302, 126998 (2021). https://doi.org/10.1016/j.jclepro.2021.126998
 F. Huneke, C. Perez Linkenheil, and M. Niggemeier, “Kalte Dunkelflaute,” Energy Brainpool, 2017. https://green-planet-energy.de/blog/wp-content/uploads/2017/06/170629_GPE_Studie_Kalte-Dunkelflaute_Energy-Brainpool.pdf
 A. Cherp and J. Jewell, “The concept of energy security: Beyond the four As,” Energy Policy 75, 415 (2014). http://dx.doi.org/10.1016/j.enpol.2014.09.005
 C. Frank, S. Fiedler, and S. Crewell, “Balancing potential of natural variability and extremes in photovoltaic and wind energy production for European countries,” Renewable Energy 163, 674 (2021). https://doi.org/10.1016/j.renene.2020.07.103
 M. Kozlova and A. Lohrmann, “Steering Renewable Energy Investments in Favor of Energy System Reliability: A Call for a Hybrid Model,” Sustainability 13, 13510 (2021). https://doi.org/10.3390/su132413510
 “IWES institute, Kassel,” 2014. https://www.iwes.fraunhofer.de/en.html
 H. D. Behr, C. Jung, J. Trentmann, and D. Schindler, “Using satellite data for assessing spatiotemporal variability and complementarity of solar resources – a case study from Germany,” Meteorologische Zeitschrift 30, no. 6, 515 (2021). https://doi.org/10.1127/metz/2021/1081
 J. Kapica, F. A. Canales, and J. Jurasz, “Global atlas of solar and wind resources temporal complementarity,” Energy Conversion and Management 246, 114692 (2021). https://doi.org/10.1016/j.enconman.2021.114692
 F. Kaspar, M. Borsche, U. Pfeifroth, J. Trentmann, J. Drücke, and P. Becker, “A climatological assessment of balancing effects and shortfall risks of photovoltaics and wind energy in Germany and Europe,” Advances in Science and Research 16, 119 (2019). https://doi.org/10.5194/asr-16-119-2019
 J. Drücke et al., “Climatological analysis of solar and wind energy in Germany using the Grosswetterlagen classification,” Renewable Energy 164, 1254 (2021). https://doi.org/10.1016/j.renene.2020.10.102
 O. Ruhnau and S. Qvist, “Storage requirements in a 100% renewable electricity system: extreme events and inter-annual variability,” Environmental Research Letters 17, 044018 (2022). https://doi.org/10.1088/1748-9326/ac4dc8
 M. Ohba, Y. Kanno, and D. Nohara, “Climatology of dark doldrums in Japan,” Renewable and Sustainable Energy Reviews 155, 111927 (2022). https://doi.org/10.1016/j.rser.2021.111927