This article in the journal Nature, claims that:

Covering 10% of the world’s hydropower reservoirs with ‘floatovoltaics’ would install as much electrical capacity as is currently available for fossil-fuel power plants. ... nearly 4,000 GW

Now, certainly, this isn't a turn-key solution to the world's energy problems (reservoirs are not distributed according to world power needs, there is transportation overhead, there are environmental concerns to covering water bodies etc.). But - is this assessment valid? I mean, can humanity get this much production capacity from taking this action? Or are the authors cooking/fudging the numbers somehow?

  • Moved off-topic discussion on plausibility to chat.
    – Oddthinking
    Dec 1, 2022 at 21:19
  • Maybe you could remove your parenthetical remarks, as they just detract from the actual question (this is Skeptics, not Physics, Earth Science, or Sustainable Living; I myself am tempted to comment on the ecological aspect, but this is off-topic).
    – gerrit
    Dec 6, 2022 at 9:04
  • @gerrit: I actually think that without the parenthetical remarks I might get more comments about the energy problems claim. Also, possible answers might actually involve physics, earth science etc by describing why fossil fuel generation is incomparable to solar generation, for example.
    – einpoklum
    Dec 6, 2022 at 11:47

1 Answer 1


Technically possible. Do note, though, that getting a solar cell to produce its nameplate capacity is much harder than doing the same with a fossil fuel generator.

The specific claim ("nearly 4,000 GW of solar capacity") in the article comes with a link to the 2020 article "Hybrid floating solar photovoltaics-hydropower systems: Benefits and global assessment of technical potential" by Nathan Lee et al..

The article states:

Technical potential capacity (MW) is the product of the power density (assumed to be 1 MW per hectare or 100 MW per km2 10 ) and total suitable land area (Equation 1) [14]. To assess the annual generation (GWh per year) we find the product of the capacity, the corresponding solar energy resource capacity factor, and 8,760 hours per year (Equation 2). The corresponding capacity factors were identified from the Global Solar Atlas (downscaled to 100 m x 100 m resolution with a simple nearest neighbor method).

To explain the calculation of technical potential, we present a hypothetical example of a waterbody with an area of 1 km2 . We assume that 10% (for this example) of the total waterbody area is suitable for FPV (Figure 4 and Equation 1) or 0.01 km2 . With an FPV power density of 100 MW/km2 , we find a potential capacity of 1 MW. Next the annual generation (Equation 2) is sensitive to the quality of the local solar resources and associated generation technology assumptions. With 1 MW of capacity (from Equation 1) and a capacity factor of 20%, the resulting annual generation is 1.8 GWh/year; however, assuming a capacity factor of 10% (lower resource quality), we find an annual generation of 0.9 GWh/year. Note that technical potential results are also highly sensitive to the suitable area assumed.

As we can see, authors assume FPV power density of 100 MW/km2. How realistic is that? Going off the Wikipedia photovoltaic power plant list, energy density for the stations with area listed does not seem to exceed 70 MW/km2.

The problem here is also well represented in the linked press release (highlight mine):

“This is really optimistic,” said Nathan Lee, a researcher with NREL’s Integrated Decision Support group and lead author of a new paper published in the journal Renewable Energy. “This does not represent what could be economically feasible or what the markets could actually support. Rather, it is an upper-bound estimate of feasible resources that considers waterbody constraints and generation system performance.”

Basically, these estimates make very generous assumptions of how "floatovoltaic" systems would perform in future. It would require a ~30% increase in nameplate power generation capacity of photovoltaic panels to meet the base power generation figure assumed in the paper. I would not call it unrealistic, but it's very optimistic.

It is also not clear why "Nature" author chose that specific value of capacity. The article by Nathan Lee et al. gives several estimates for different configurations of power plants, ranging from 3,039 to 7,593 GW of installed capacity.

P.S. Note that nameplate generation capacity is not a good metric to compare photovoltaic generators with fossil-fuel plants, as those produce power at a more stable rate. In other words, a fossil fuel plant, when running, produces energy at a relatively constant rate (often over 50% of its nameplate capacity), while solar power plant generation fluctuates based on current insolation (due to time of day/year and weather conditions), which leads to less power generated per day compared to a fossil-fuel plant with the same rating.

  • you can read the full article for free by clicking on "View Open Manuscript" after going to this link: sciencedirect.com/science/article/abs/pii/S0960148120313252 (same link as in the answer). There Table 4 says 10.3% coverage corresponds to 3,899 GW nameplate capacity and 5,441 TWh/year generation.
    – DavePhD
    Dec 2, 2022 at 13:58
  • For comparison world generation of electricity from fossil fuels in 17,000 TWh/year ourworldindata.org/electricity-mix
    – DavePhD
    Dec 2, 2022 at 14:04
  • 2
    The Nathan Lee article also acknowledges that Farfan and Breyer found that it takes 25% coverage to place 4,400GW of nameplate capacity in this article: sciencedirect.com/science/article/pii/S1876610218309858
    – DavePhD
    Dec 2, 2022 at 14:12
  • 2
    You can't just count daylight as the sun isn't always overhead and solar panels are normally not sun-tracking. (And if they are they need to be spread apart more.) Dec 3, 2022 at 3:45
  • 1
    @einpoklum I think I saw data stating that the average capacity factor for solar systems worldwide was ~17% in 2021 and the top output was around 27%, but was unable to find a link to the source at the time I was editing the answer. I'll edit it in when I will have the time to do a proper search. Dec 6, 2022 at 1:53

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