The German Grid and the Recent Solar Eclipse

                               (Photo credit

The solar eclipse of March 20th, 2015 and the havoc it threatened to wreak on the German grid generated a lot of portentous web coverage before it happened:

PhysOrg Solar eclipse will test Germany’s green power grid

Will next week’s partial solar eclipse turn off the lights in Germany? Experts say the country’s electricity grid, which relies increasingly on renewable energy, faces a crucial test on the morning of March 20, when the moon will pass in front of the sun and block up to 82 percent of its light across Germany.

Some nail-biting web coverage while it was was happening:

PV Magazine As it happened: Germany’s grid grapples with solar eclipse

10:54: Here comes the big stress test. The greatest change in PV power generation takes place between 11:00am and 11:15am.

And quite a bit of triumphant web coverage after it was over:

Clean Energy Wire Energiewende passes solar eclipse stress test

Grid operators had warned the astronomical event would be “an extreme challenge” that will “stress test the power system” but in the end, all went “wonderfully”. Germany has passed the solar eclipse stress test with flying colours.

But was the eclipse really an “extreme challenge” for Germany’s grid? And what does it tell us, if anything, about the Energiewende? Let us examine the data.

The Fraunhofer Institute publishes an interactive graphic of German grid performance covering the week of the eclipse, and in this post I present some charts developed from this graphic. Figure 1 shows German solar output over the week of the eclipse, which caused the incision in the leading edge of the solar curve on March 20th:

Figure 1: German solar output, week of March 16 – 22, 2015

Figure 2 shows solar output on March 20th in more detail (readings are at 15-minute intervals):

Figure 2:  German solar output, March 20th, 2015

Between 9.45am and the high point of the eclipse at around 10.45am solar output fell by about 7GW in an hour and between 11am and 12am it rose by about 13GW in an hour. The problem wasn’t the size of these swings – the German grid routinely handles larger ones – but the rate of change. Could Germany’s load-following plants ramp down and up this quickly, and what would happen if they couldn’t?

As it turned out they never had to. The bulk of the load-following burden was shouldered not by Germany’s load-following plants but by interconnector flows to neighboring countries. Figure 3 gives details. The red line shows solar generation at 15 minute intervals over the period of the eclipse, the black line shows Germany’s power exports over this period and the blue line shows what was left for the German grid to handle after subtracting exports. By reducing its exports by ~6GW at the height of the eclipse Germany lowered ramp rates to potentially manageable levels before its load-following plants even got involved. (Note that there is a problem with the times given by Fraunhofer, which show the eclipse peaking at around 3.30am. Figure 3 adds 7 hours and 15 minutes to these times to match them to the actual peak of the eclipse, which occurred at around 10.45 am):

Figure 3: Solar generation, exports and remaining requirement after subtracting exports from solar, March 20th solar eclipse

Ramp rates were then lowered further by a 3-4GW reduction in electricity demand during the eclipse, which was caused partly by some industrial consumers agreeing to cut their electricity usage during the eclipse and partly by people turning their TVs off and going outdoors to watch the spectacle. The reduction is illustrated in Figure 4, which plots Germany’s total generation over the period of the eclipse (I assume here that total generation over this period would have matched demand; Fraunhofer gives no demand numbers.)

Figure 4: Total generation and generation from “conventional sources” during March 20th solar eclipse

The green line in Figure 4 plots actual generation from German “conventional” plants, which include everything except solar, wind and exports/imports, during the period of the eclipse. (Wind generation in Germany during the eclipse was negligible, averaging around 0.5GW.) To meet demand these plants had to ramp down at a fairly constant rate of around 3 GW/hour for two hours. The German grid routinely achieves such ramp rates.

Figure 5 is another plot from Fraunhofer showing generation from all sources between March 16 and 22nd (imports/exports are not shown). The eclipse is defined by a small coincident dip in total generation and a small coincident spike in total generation from non-solar sources. Except for these wrinkles March 20th was a weekday much like any other weekday on the German grid:

Figure 5: Generation from all sources, week of March 16 – 22, 2015

The three graphs in Figure 6 compare solar generation during the eclipse with generation from gas, hard coal and pumped hydro, Germany’s main load-following sources. All three were ramped down when solar generation began to recover after the eclipse, but none of them at a rate that was visibly different to the rates observed at the same time on previous weekdays. Note also that most of the ramping was done by coal:

Figure 6: Generation from load-following capacity, week of March 16 – 22, 2015 (note the different Y-scales)

These results show that the solar eclipse of March 20th, 2015 was effectively a non-event on the German grid. It certainly wasn’t a “stress test” of the Energiewende, as has been  claimed (Energiewende 1, Solar Eclipse 0, as Clean Energy Wire put it). The ultimate success or failure of the Energiewende will be judged by stricter and more meaningful standards.

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14 Responses to The German Grid and the Recent Solar Eclipse

  1. Euan Mearns says:

    Roger, if Germany ever claims that it uses all its renewable electrons and exports those dirty brown coal electrons, this data will present a conflicting view.

    • Just before the peak of the eclipse Germany was generating 51% of its electricity from coal and 18% from gas, biomass and oil. Carbon-free sources contributed only 31%, with most of that coming from nuclear and hydro.

      The Energiewende still has a very long way to go.

  2. Willem Post says:


    Thank you for this post, which confirms this was a non event. It would have been more interesting, if it had been a windy day.

    Total generation, minus line losses, minus exports would be demand.

    This could be compared with a typical normal demand to show any “demand management”, in addition to supply management.

  3. roberto says:


    according to the news…

    Energiewende OK…

    Holland kaputt! 🙂


  4. Here is a comment on the impact of the solar eclipse in France received in a private commuication from a colleague:

  5. A C Osborn says:

    Euan & Roger, an excellent post over at No Tricks Zone on the €200Billion invested in 70GW of renewables since 2002, which hasn’t decreased the Fossil Fueled Generation, in fact FF is also increasing.
    So all that investment and all you get is much more expensive energy and a Grid closer to destabalisation.

    • Luís says:

      In 2003 305 TWh of electricity were generated in Germany from lignite and hard coal. In 2004 this figure was down to 250 TWh. In the meantime Nuclear went down from 165 TWh to 92 TWh.

      • Luís says:

        Img tags are being censored; here is a link to a useful image:

        • Luis: That really is an interesting image. It tells me that basically all the Energiewende has done so far is replace nuclear generation with renewables generation, and as a result it’s contributed virtually nothing towards decarbonizing Germany’s electricity sector. And at a cost of how many billions of Euros? I think Willem Post gives a number somewhere.

  6. Luís says:

    A partial eclipse in early March could never be much a problem, especially since the sky was not fully clear that morning. However, at midday in June PV accounts for some 40% of the power supplied to the grid. A total eclipse at that time is largely unmanageable, if it ever happens brown-outs are a likely result.

    • Luís says:

      Or better said: when it happens.

    • It becomes even more unmanageable if Germany has, say, twice as much solar capacity as it has now and can’t cycle its power exports/imports quickly enough to keep up with the fluctuations, as it did on March 20th.

      The Italians shut down their solar completely during the eclipse. Germany, however, doesn’t have this option. “As a transmission system operator, we have no access to the facilities,” a spokeswoman for transmission system operator Tennet explains. “That is handled by the distribution system operators. It is therefore uncertain how many plants we would be able to reach and switch off.” Tennet also doesn’t know how many solar installations there are: Apparently it would not help to “disconnect as many plants as possible from the grid when you don’t know how many they really are in the end,” the Tennet spokeswoman adds.

      • Willem Post says:


        Solar capacity declines at about 0.5%/yr, or about 380 MW during 2014.

        Germany added 1895 MW of new capacity during 2014.

        Effective capacity increase = 1895 – 380 = 1515 MW during 2014.

      • Willem Post says:


        In a pinch, Germany could ground some of that excess energy, if its own system and abroad systems cannot handle it.

        Isn’t excess energy usually “selling” at near-zero wholesale prices?

        Germany built out its subsidized RE systems, but grid adequacy, generating adequacy, demand management adequacy was mostly “left for later”.

        Now there is talk of burying the north-south HVDC lines, that already are about 10 years behind schedule due to NIMBY, at about 2-3 times the overhead cost. Only rich, smug Germany can afford such follies.

        Haste makes waste.

        Other countries are less hystirical about nuclear energy. From:


        Russian has sold a large number of nuclear reactors to various nations during the last quarter of 2014 and the first quarter of 2015, which indicates Russia is doing business as usual, despite sanctions. Is this how “isolating Russia” is meant to work?

        Country………. Qty………Capacity, MW……..Cap. Cost, $billion

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