AC-DC conundrum: Latest PV power-plant ratings follies put focus on reporting inconsistency (update)


Tom Cheyney
Tom Cheyney
Tom Cheyney, former senior editor of PV-Tech and Photovoltaics International, is now chief curator of and director of Impress Labs’ solar practice.

The solar photovoltaic industry swings both ways when it comes to power ratings. For modules or manufacturing output and capacities, the numbers used are consistently stated as DC. It’s a 250W (DC STC-rated) panel or the production line has a 50MW annual capacity or run rate. On the project side, residential and small commercial systems seem pretty uniformly pegged in DC wattage, but the same can’t be said for larger, utility-scale PV power plants. The heftier the installation, the less uniform seems the reporting, with some companies choosing AC, while others take the direct (current) approach—if they bother to clarify the rating type at all. 

This lack of standardization fosters confusion in the sector (and the media reporting on it) and has led to false bravado about this or that big PV farm being the largest of its kind. Isn’t it time that all the inhabitants of Planet PV get on the same page?

This is not a new problem. I’ve noted the inconsistency since I began covering the sector, but it seems to be getting more troublesome as the size of the largest PV power plants increases, adding to a hype cycle that doesn’t need much nudging to spin out of control. Three recent news stories reveal the latest ratings follies.

Late last month, Good Energies and NIBC Infrastructure Partners bought the second and third portions of the massive Finsterwalde PV project in Germany from Q-Cells, to add to the first phase already purchased. Combined, the three parks were said to “have a capacity of 81MWp, forming the largest operational solar PV plant in the world.”

This claim was duly reported pretty much verbatim by PV-Tech and other media outlets, and the project is now listed (with a size of 80.7MW) on PV Resources “Top 50” list of “large-scale photovoltaics power plants” as number one in the world.

This assertion of maximum bigness raised my suspicions: the recently commissioned Sarnia PV farm in Ontario, built and run by First Solar and owned by Enbridge, had been declared the largest, with 80MW (AC) of installed capacity. Since the Good Energies et al. release didn’t specify whether Finsterwalde’s 81MW were AC or DC, I inquired with two of the companies involved.

First, I contacted Michelle Riley, general counsel for Good Energies in the Americas, who politely refused to deal with my seemingly simple request, replying via email that “we're not taking any media enquiries on the investment, but thanks very much for getting in touch.” (This also raises another pet peeve of mine: when a company puts out a press release, shouldn’t they be prepared to field follow-up questions from the, uh, press?)

I then reached out to Q-Cells, thinking that the solar company would provide a prompt response. The North American unit didn’t have the answer, and pointed me toward people at the company’s HQ in Germany. 

Days passed, and following some gentle pestering on my part, I finally received an answer from Ina von Spies, director of corporate communications, about two weeks after my initial query. (In their defense, most of Ina and her team seemed to be on holiday, on the road, or otherwise unavailable during that time.) 

“We as Q-Cells actually haven’t claimed that Finsterwalde was the biggest project worldwide,” she wrote in her email. “We say that the Finsterwalde project comprises a total of 82MWp—as a DC number.” (Note that she puts the total at 82MW, while the others have it at 81MW.) She pointed out First Solar’s reported 80MW capacity peak at Sarnia, saying, “Whether this is an AC or DC number is a very good question for a journalist to ask.” 

Using my usual quick-and-dirty formula for converting DC to AC for a PV system—MW (DC) divided by 1.2—the AC rating for the Finsterwalde trifecta comes to 68.3MW. Others use the following math: MW (DC) multiplied by 0.86, which in this case puts the German farm’s total at 70.52MW (AC).

Either way, given the estimated AC numbers, Good Energies and NIBC’s claim of Finsterwalde’s pre-eminence was false. In an apples-to-apples comparison, it’s significantly smaller than the Sarnia site in southern Ontario. It also puts into question whether all of Good Energies’ claimed renewables portfolio in Europe—86MW for solar PV, almost 250MW including wind and waste-to-energy assets—is measured consistently in AC or DC.

Speaking of the biggest site in Europe, another soon-to-open PV plant has already laid claim to that title. The following statement was made regarding the sprawling 70MW Rovigo farm in northeastern Italy, which was recently in the newswhen a group led by First Reserve bought it for a big chunk of change from venture partner SunEdison: “When completed in the fourth quarter of 2010, the Rovigo plant is expected to be the largest operating solar power plant in Europe,” according to the press release.

Again, the seemingly simple question was not answered in the announcement: is that 70MW an AC or DC number? 

I reached Brad Oswald, communications manager for SunEdison, who told me that “typically, most releases are [stated] in DC,” when the MEMC project development unit announces the size of a project. Caitlyn MacDonald of First Reserve also confirmed that the Italian job had about a 70 MW (DC) rating.

What this means is that Rovigo will not be the “largest operating solar power plant in Europe,” since the aforementioned Finsterwalde trio of installations tops 80MW (DC). It also means that when we look at SunEdison’s built system assets or pipeline, we can count those numbers as DC (unless otherwise noted).

Two of the bigger downstream players, First Solar and SunPower, take a different approach, choosing to use AC as their metric for larger projects.

“The reason we talk about projects in terms of AC is that’s the form of electricity that utility-scale solar projects deliver to the grid,” explained First Solar’s Alan Bernheimer. “The power purchase agreements with utilities are for AC capacity and AC output. So it just makes sense to talk about our project capacity in the same terms.”

[CORRECTION] The company reports in AC, for systems located in North America, but in MW DC in Europe, per the industry standard, according to the spokesman. (Note: The original version had First Solar stating all systems in AC, regardless of location: a subsequent communication from the company corrected that earlier statement.)

Although it is universally true for North America, in the EU we talk about projects in MW DC, since that is the industry standard there, I am now informed.

SunPower exec VP Julie Blunden explained that her company is “extremely consistent in this regard, and explicit as well. Note that in our earnings release, and for many quarters, we explicitly disclose that wholesale power plant MW are reported in AC terms—see the last line before ‘About SunPower’ in our release: ‘The capacity of power plants in this release is described in approximate megawatts on an alternating current (ac) basis unless otherwise noted.’

“In some cases, we have customers who would like to speak about the MWp rating of the power plant that we have delivered to them; in those cases we offer an editor’s note in our releases to ensure limit confusion.”

She cited the recent release about the sale of Italian PV plants to Etrion, which included the following verbiage. “The capacity of power plants in this release is described in approximate megawatts on an alternating current (AC) basis unless otherwise noted. On a direct current (DC) basis, the first two phases of the Montalto di Castro solar park are approximately 24MW and 9MW, respectively, in capacity. They form part of a multiphase solar park totaling approximately 85 MW (DC) in capacity.”

“The reason we offer AC ratings of power plants is that we operate within the power industry, which is interested in the energy and power delivered to the grid,” Blunden’s email reply continued. “Power plants are rated in delivered energy terms—that is AC—and we communicate in the same manner.”

Even SunPower’s apparent consistency has its limits though.

“Now, for a commercial or residential system, dealers describe systems in DC STC terms because they speak in number of panels of XX W/panel,” she explained. “Thus, we have not tried to convert this segment of our business to speaking in AC lexicon and report net metered/customer-sited systems like Macy’s in DC terms.”  

As Blunden points out, it may not make sense for the industry to adopt a standard installed-capacity reporting practice for all manner of projects, from small residential rooftop systems in the ‘burbs to monster solar farms in the middle of the Mojave.

At a minimum, this reporter would like to see each and every news release about larger-scale PV projects, say >1MW, have the system size clearly delineated and every devco’s pipeline described similarly: is it AC or DC? 

But as the PV sector grows into double- and triple-digit gigawatt-scale proportions and becomes a significant part of the energy-generation portfolio, isn’t it time for the industry to put on its big-boy pants and, taking First Solar and SunPower’s lead, adopt a uniform, simplified rating code that denotes utility-scale system megawattage in AC units?

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