From The Independent, "Why on earth would right-wing people with connections to the fossil fuel industry lie about ‘frozen wind turbines’ in Texas?"

Sure, some wind turbines and solar facilities have also been affected [during Winter Storm Uri], but all through Monday [2021-02-15] while gas plants were freezing, wind turbines and solar panels actually exceeded expected power delivery.

This article seems to make it sound like Wind and Solar outperformed and fossil fuels under performed during the crises. Is this true?

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    "exceeded expected power delivery" could simply mean that these sources had higher output than the (likely conservative) formula-based quotas estimated. – Daniel R Hicks Feb 17 at 13:34
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    @fredsbend See e.g. texastribune.org/2021/02/16/natural-gas-power-storm, it seems a lot of the Texas gas infrastructure simply was not designed to operated in freezing conditions. – mmeent Feb 17 at 14:25
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    @mmeent Thank you, great link. It seems the nat gas issues were entirely short term production issues, not electricity generation per se. Texas doesn't do much storage, thus depends greatly on a continuous flow from the wells. The unexpected cold left them "struggling to bring natural gas to the surface, analysts said, as cold weather and snow close wells or cause power outages that prevent pumping the fossil fuels from the ground." Very interesting, once you cut the politics and get to the nuance. – fredsbend Feb 17 at 18:57
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    There is a difference between wind overperforming the estimate that accounts for bad weather and wind beating its usual performance. Which are you asking about? – Matt Feb 17 at 22:50
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    @fredsbend "nat gat doesn't freeze"? The formation of methane hydrate (a crystalline solid complex of methane and water) is a royal pain for any gas pipeline. These complexes do form in just mildly freezing temperatures, all you need is a bit of moisture in the gas and high pressure (a gas pipeline has both). In colder climates, everything needs to be carefully designed to supress clathrate formation and inhibitors are mixed into the gas. I guess not much of that is typically necessary in Texas. – TooTea Feb 19 at 15:50

Wind production exceeded all forecast metrics, solar production exceeded short-term forecasts but not operational forecasts.

TL;DR: On 2/15 both solar and wind exceeded the short term forecasts which do not account for local operating conditions. The operational wind forecast was about 60% low for the whole day, seemingly due to a conservative application of the icing derate built into the forecast. The operational solar forecast was about 20% high, because ERCOT does not forecast solar for when the grid is under extreme/contingency conditions.

What does "forecast" mean in this context? Obviously solar and wind are weather-dependent resources, which is the most significant factor in the forecast amounts. However, the operational forecast also accounts for technical problems and unexpected shutdowns -- things beyond just wind and sunshine availability (in the short term forecast). Due to it's relatively low capacity statewide, solar forecasts are not included in contingency planning, so was not updated during the event. For wind, since the storm was already factored into the forecast, the fact that it exceeded this forecast indicates that it experienced less technical challenges than expected. Ultimately this says at least as much about the forecast methodology as it does about the capabilities of wind turbines in Texas.

What caused the blackouts? This is still being investigated in detail, but the most significant factors appear to be that load and outages were both under-estimated. Wind out-performed contingency expectations. Outage estimates do not include solar.

ERCOT, the grid operator covering most of Texas, provides a number of detailed generation reports for all resources on the Texas grid, including hourly values for forecast and actual production from wind and solar resources. The data is updated every hour with data for the previous 48 hours. The files give raw data, which I've plotted and explained below:


ERCOT Wind Power Production, Average Actual and Forecast, Feb 15, 2021

Metric Total value (MWh) Actual vs forecast
Actual 73,396 -
COP HSL 46,408 158%
STWPF 48,434 152%


ERCOT Solar Power Production, Average Actual and Forecast, Feb 15, 2021

Metric Total value (MWh) Actual vs forecast
Actual 20,134 -
COP HSL 25,385 79%
STWPF 19,781 102%

Forecasting definitions and methodology

From the ERCOT glossary with some other sources noted.

  • COP HSL: current operating plan high sustained limit. The "maximum sustained energy production capability of the resource." The COP HSL is a seven-day forecast, updated by 2:30pm every day. By definition, the HSL is equal to or lower than the short-term forecasts. It accounts for factors specific to each resource, so would exclude any resource that is offline for repair, maintenance, etc. This is what ERCOT plans for because it accounts for things about each resource that ERCOT would rely on the operator to inform them of.
  • STWPF: short-term wind power forecast. This training presentation from ERCOT provides more detail: "Statistically 'most probable' forecast of production potential for each Wind Generation Resource." By definition, there is a 50% chance that the actual production will exceed this value.
  • STPPF: short-term photovoltaic power forecast. Like the short-term wind forecast, the amount of power that is 'likely' to be produced by PV resources on the grid -- by definition there is a 50% chance that actual production exceeds this value.

The forecasting process documentation indicates that the short term forecasts are generated internally by ERCOT and provided to the QSEs (qualified scheduling entities -- whoever operates and gets paid for each generation resource on the grid) who use it to generate the "operational forecast" -- the COP HSL.

The wind forecast process includes two different derating factors for extreme weather, which specifically accounts for the probability that turbines will be off-line due to icing. The solar forecast accounts for weather, but does not appear to account for snow accumulation on panels.


Wind. Without additional commentary from ERCOT, it would appear that ERCOT forecasters and wind operators were somewhat conservative in their application of the extreme weather derates when they updated their output forecasts, and the actual production ended up greatly exceeding both forecast metrics. There is no comprehensive comparison of actual to forecast values that I could find, but spot-checking a few other days indicates that actual values tend to be within +15% or so of the forecast, which is what we could expect given the definition of the forecast.

Solar. In this case, it looks like ERCOT accounted for snow accumulation in derating their forecast, which came in slightly less than the actual production. However (and I'm speculating here) since the forecast procedure does not appear to account for snow accumulation, the PV operators did not adequately derate their forecasts. This would appear to indicate a shortcoming in the process, as the operating plans, by definition, should not exceed the short term forecast.

So what caused the rolling blackouts?

Obviously there will be a detailed postmortem of this, but we can say that wind and solar were certainly not the most significant factor. The blue line on this real time plot shows available standard capacity (excluding the operating reserves which are dispatched to respond to acute failures) minus actual load -- any time that blue line goes below zero, there are problems. Here's the plot for February 15, showing the value at the low point at 5:13pm:

Capacity Available to SCED, February 15, 2021

ERCOT forecasts load and non-dispatchable generation (wind and solar) on a number of timescales to predict how much energy will be needed on a day-ahead and hour-ahead basis, and then sets prices using the operating reserve demand curve. This curve dictates the price per MWh that is paid on the market, and increases as capacity becomes constrained. The maximum is fixed at $9,000 per MWh, which is considered the VOLL -- value of lost load. At some point generating power is too expensive, and blackouts are cheaper. ERCOT has decided that $9,000 is the price at which this occurs. The intent of the system is to incentivize more capacity to become available as that price goes higher, to ensure that blue line stays at a sufficient height above zero.

On February 15, the prices hit $9,000 throughout the day, and yet rolling blackouts were required to keep the system from collapsing. This means that one of three things went wrong:

1. The load forecast was too low, meaning more capacity was needed than expected?

Energy consulting firm ICF did an analysis of ERCOT's various long term forecasts for the winter and produced this table:

Capacity, GW Expected Forecast Extreme/Contingency Forecast Actual Conditions (8am 2/15)
Peak Load 57.7 67.2 74.5
Resource Outages 8.6 14.0 26.6
Wind Output 7.1 1.8 4.5
Solar Output 0.3 [not forecast] 0.0
Total Generating Capacity 73.1 68.6 53.4
Remaining Reserve Capacity 16.2 1.4 -21.1
Operational Conclusion Normal operations Emergency measures Widespread outages

This shows that the load under extreme conditions was underestimated by 7.3 GW, or about 13% of normal load.

Answer: Yes

2. The solar and wind forecast was too low, meaning less capacity was available than expected?

As discussed above, the wind forecast accounted for the storm, and the solar forecast appeared to correct itself partway through the day. These forecasts ended up being overly conservative -- the system expected to have less wind and solar available than were actually present.

Answer: No

3. The VOLL is too low, and generators didn't have sufficient incentive to stay on-line?

Obviously, generators wouldn't have waited until February 14 to see the price, and then make infrastructure changes so they can produce power the following day despite the cold. The market design is supposed to entice them with the allure of high prices to be prepared well in advance, spending some money up-front so they can be prepared to take advantage of price spikes. Given that there were no outages in far colder parts of the country, it's clear that the technology needed to stay online during such storms is available and cost-effective. So if the VOLL had been higher, perhaps they would have planned ahead and done this? Especially since something similar had happened in 2011?

Answer: It's complicated
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    @RossPresser sure thing -- I linked the EIA definition in the answer. Basically, it's some factor which reduces expected capacity of a generating resource. – LShaver Feb 17 at 20:33
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    This analysis is correct as far as production goes, but it's misleading because it drastically underestimates the capacity shortfall. By physical necessity, the actual load on the grid is always less than the production capacity; blackouts are implemented when necessary, in order to keep it that way. ERCOT has estimated the power demand to be somewhere around 75GW, and production has been about 25-30GW short of that target continuously since Monday. – David Feb 17 at 20:49
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    Maybe you addressed this and Im just not seeing it, but, you are talking about wind exceeding the estimate which accounted for poor performance due to weather, right? How much of a reduction in wind generated power relative to other sources did the weather cause? – Matt Feb 17 at 22:47
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    @LShaver Not really. In theory, if you know that the wind speeds will be low tomorrow evening (or that some turbines may need de-icing) then you can prepare other non-wind generation for dispatch. But only if there is other generation available. If there is no other spare capacity available, as there was not in Texas due to the temperature, then a large reduction in wind generation has much the same effect on rolling blackouts, whether or not it was forecast the day before. – Henry Feb 18 at 15:32
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    @LShaver That is certainly correct. My point was that your chart suggested to me that sharply lower wind generation (even if in the short-term forecast) made things worse but was not the only cause of the problem. That is why I said "That drop in wind generation contributed to the overall supply deficit but was only part of the problem." But with Texas installed wind generation capacity of over 30GW, a production level going below 1 GW did have a significant impact. – Henry Feb 18 at 15:44

Technically yes, but not in the way you may think.

Here's the data for all electricity generation in the ERCOT grid: Generation by source Total generation

The cold front started coming in on the night of the 8th and you can see how wind generation immediately started experiencing major problems and fell drastically, but the other sources started generating more to pick up the slack and also the increased demand from people needing heat. It got steadily colder and colder, until, by the 15th, it was cold enough for water to freeze in pipes and on various sensors and all three other major power sources simultaneously started experiencing major issues. Not as bad as wind, even with the drops, coal was at baseline, gas still 2X above and nuclear only 25% down, but it was not enough for the increased demand.

Now both sides are twisting the facts to their own advantage. One side is saying that wind "outperformed expectations", technically it did outperform the forecast for the day, but only because the forecast had already taken all the off-line capacity into account. It's like having one member of your relay team break a leg during a race, but say: "Look how fast he hopped his part". And the other side is trying to blame it all on wind, when a casual glance at the data shows that the fall in other sources on the 15th was more than total wind generation before the 8th.

  • Good find. I would say that until (and unless) ERCOT releases data on how much the wind generation was affected by ice vs wind availability, it's hard to give a definitive explanation. – LShaver Feb 19 at 16:13
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    nice find! I like the graphs. I like the major hit it shows on the 15th.I see a lot of people talking about how the green power exceeded its forecast, but, even exceeding their forecast the wind and solar were output at about a tenth of their maximum, so more green energy wouldn't have helped as much as people claimed. Still, the failing wasn't on green energy, it preformed better then expected its just the rest of the system that kind of fell apart – Torsten Gang Feb 19 at 16:32
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    @TorstenGang “the failing wasn’t on green energy” - well it at least partially was. The whole system fell apart, not one section or the other. – Tim Feb 20 at 13:46
  • I think this graph also shows that the rest of the system had the capacity to cover a lot of the load and with this cold front being expected they could have prepared better to handle the spike in demand. It appears to show total capacity increasing by over 50% after the blackouts occurred. – Joe W Feb 20 at 17:28

Yes, according to data from the Electric Reliability Council of Texas (ERCOT), wind sources greatly exceeded their forecasted production for Monday, while solar sources slightly exceeded their forecasts. Overall, wind appears to have exceeded the hourly forecasts by an average of about 1000 MW, and solar exceeded the hourly forecasts by an average of about 15 MW.

ERCOT keeps records of forecasts and generation on their website here. On that page, there are links to 'Solar Power Production - Hourly Averaged Actual and Forecasted Values' and 'Wind Power Production - Hourly Averaged Actual and Forecasted Values', which have CSV files of rolling hourly generation/forecast data for the last several days.

I downloaded the last uploaded files for 2/16 for both wind and solar production, and below is the data in there for Monday 2/15. Those files have a bunch of columns, but I only included the relevant ones below:

  • DELIVERY_DATE and HOUR_ENDING are the hour/date the data applies to
  • ACTUAL_SYSTEM_WIDE is the actual production recorded by ERCOT for a given hour, measured in MW
  • STWPF_SYSTEM_WIDE and STPPF_SYSTEM_WIDE is the Short-Term Wind Power Forecast (STWPF) and the Short-Term PhotoVoltaic Power Forecast (STPPF) for a given hour (see ERCOT's glossary for their list of acronyms, they have a lot), measured in MW
  • Difference is a column I calculated, for easier comparison of the actual/forecasted production. It's just the difference between the actual and the forecast, so positive values mean the actual production is exceeding the forecasted production, and negative values mean the actual did not meet the forecast.

Wind, filename cdr.00013028.0000000000000000.20210216.235521.WPPHRLYAVGACTNP4732

02/15/2021          1           5349.97             3815.6              1534.37
02/15/2021          2           5204.78             3734.4              1470.38
02/15/2021          3           5154.51             3697.9              1456.61
02/15/2021          4           5214.14             3666.8              1547.34
02/15/2021          5           4827.79             3238.6              1589.19
02/15/2021          6           4612.36             2910.4              1701.96
02/15/2021          7           4300.11             2620.2              1679.91
02/15/2021          8           4512.09             2722.7              1789.39
02/15/2021          9           4408.57             2682.2              1726.37
02/15/2021          10          4386.19             2725.5              1660.69
02/15/2021          11          3968.91             2413.7              1555.21
02/15/2021          12          3185.68             1932.7              1252.98
02/15/2021          13          2605.37             1602.1              1003.27
02/15/2021          14          2536.77             1655.9               880.87
02/15/2021          15          2322.94             1523.7               799.24
02/15/2021          16          1944.04             1302.4               641.64
02/15/2021          17          1690.28             1206.6               483.68
02/15/2021          18          1185.21              882                 303.21
02/15/2021          19           788.31              604.8               183.51
02/15/2021          20           648.5               410.2               238.3
02/15/2021          21           807.37              467.8               339.57
02/15/2021          22          1022.71              656.6               366.11
02/15/2021          23          1227.61              860.4               367.21
02/15/2021          24          1492.12             1100.9               391.22

Solar, filename cdr.00013483.0000000000000000.20210216.235515.PVGRHRLYAVGACTNP4737

02/15/2021      1              0.01                0                   0.01
02/15/2021      2              0.01                0                   0.01
02/15/2021      3              0.01                0                   0.01
02/15/2021      4              0.01                0                   0.01
02/15/2021      5              0.01                0                   0.01
02/15/2021      6              0.01                0                   0.01
02/15/2021      7              0.01                0                   0.01
02/15/2021      8             43.17               94                 -50.83
02/15/2021      9            558.95              925.3              -366.35
02/15/2021      10          1189.93             2022.2              -832.27
02/15/2021      11          1823.54             1948.1              -124.56
02/15/2021      12          2277.99             2113.9               164.09
02/15/2021      13          2670.41             2374.9               295.51
02/15/2021      14          2829.34             2464.5               364.84
02/15/2021      15          2786.21             2433.8               352.41
02/15/2021      16          2553.12             2380.2               172.92
02/15/2021      17          2307.64             1940.1               367.54
02/15/2021      18           987.29             1005.7               -18.41
02/15/2021      19           106.59               78.7                27.89
02/15/2021      20             0                   0                   0
02/15/2021      21             0                   0                   0
02/15/2021      22             0                   0                   0
02/15/2021      23             0                   0                   0
02/15/2021      24             0                   0                   0

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