The Wall Street Journal has returned attention to the Ivanpah Solar Electric Generating System, the $2.2 billion Department of Energy-backed concentrating solar power plant whose slow start became the subject of some controversy after a Breaking Energy story last fall.

The Journal reported that “15 months after starting up, the California plant is producing just 40 percent” of the electricity it was expected to generate. The paper said Ivanpah’s operator, NRG, expects the plant to “reach power targets once the kinks are worked out,” but surprisingly didn’t check that claim against the latest data: In the first quarter of this year, Ivanpah generation was up 170 percent over the same quarter in 2014 – 108 gigawatt-hours compared to 40 GWh, according to the Energy Information Administration.

Jennifer Z. Rigney, spokeswoman for Ivanpah’s technology provider, BrightSource Energy, passed along a statement from NRG that said the upward trend continued in April, although the company did not give a precise figure for the month (EIA data for April is not yet available).

“As we expected, our operating crew’s expertise with the technology is increasing all the time and the plant is continuing to increase its generation,” the statement read. “In March, Ivanpah produced the second most power it has ever done in a single month. April exceeded that level of generation within just the first two-thirds of the month and ultimately set a new record for power generated in a single month.”

We reported in January (“Ivanpah Solar Plant Picking Up Steam”) that it’s possible that Ivanpah’s performance has also been aided by an allowance to use more natural gas – 60 percent more than originally planned – which gives the plant a boost in starting up in the morning and avoiding losing steam through brief cloudy periods.

In recommending approval of the request to be  allowed to use more natural gas, which was granted in August, a California Energy Commission staff analysis concluded that the emissions impact would be small and that requirements that “define the allowable amount of natural gas that can be used at a qualifying renewable facility … are sufficient to ensure that the proposed facility would continue to qualify as a renewable facility.”

Information recently obtained from the CEC for calendar year 2014 seems to show natural gas usage at Ivanpah actually trended down after the request was approved. Meanwhile, the amount of energy produced per unit of natural gas used increased. For instance, in March the plant used 50,536 MMBtu of gas and produced 19,954 MWh of electricity, while in October it used 43,094 MMBtu of gas and produced 56,013 MWh of electricity.

The CEC hasn’t published data on the plant’s natural gas usage in the first quarter of this year, and NRG did not respond to requests for that information.

Ivanpah produces energy by bouncing sunlight to the top of 450-foot-tall towers, where water is heated to create steam that drives turbines.

Its performance is being closely watched because power towers are seen as the best bet for concentrating solar power. Although the technology has been hammered by cheap, increasingly productive photovoltaics in the past five years (and has run into environmental issues), its advocates hold out hope that its compatibility with energy storage could make it a winner. Ivanpah wasn’t built with storage, but SolarReserve’s Crescent Dunes plant was. After seemingly endless delays, that plant, in Nevada, supposedly will come online this summer.

BrightSource and NRG, while admitting the plant has had its share of hiccups, have disputed the notion that Ivanpah is falling well short of expected generation, saying that all along they knew it would take up to four years to hit full stride. That claim had seemed a bit dodgy, seemingly coming only after the poor early start was exposed. But in fact the claim is backed up by assertions found in a 2011 federal filing by BrightSource:

As with any plant representing new and innovative technology, there will be an initial shake-out and ramp-up period when the plant is placed into operation. As such, initial performance will be less than full design; however, over time, plant performance will increase towards designed full-rated annual performance (referred to as mature-year performance). This increase is caused by the realization of the operator’s learning curve, procedural optimization, and fine-tuning of equipment and systems for increased plant performance. For the Ivanpah project, BrightSource Energy, Inc. (BSE) has proposed that this ramp-up process may last up to four years.

That said, there are also signs that the first-year performance of the plant was below the levels anticipated in power purchase agreements with the two California utilities buying Ivanpah’s output.

Full PPA details are confidential, but according to state regulatory filings, Pacific Gas & Electric contracted to receive 304 GWh/year from Ivanpah 1 and 336 GWh from Ivanpah 3. Southern California Edison expected 336 GWh from Unit 2.

SEC filings by BrightSource allude to “minimum production standards” in the contracts – 70 percent of the “power expected,” on average, in the first two years of operation, rising to 80 percent at the end of year three and thereafter.

The units declared commercial operations under the power purchase agreements at varying times, on January 10, 2014 for Unit 1, January 31, 2014 for Unit 2, and January 15, 2014 for Unit 3.

According to the EIA, Ivanpah 1 and 3 together produced about 290 GWh by the end of 2014, just a few weeks shy of a full year of operation, equivalent to 45 percent of the annual PG&E contract quantity. To hit 70 percent (895 GWh) for the first two-year measuring period, it appears the units combined will need to generate about 605 GWh this year, or nearly 95 percent of the single-year contract quantity. Ivanpah 2 fared poorer in its first full year, producing 134 GWh (40 percent of the annual contract quantity). That means the unit would have to produce 336 GWh – 100 percent of the single-year target – to meet the contract minimums noted to in the SEC filings.

Mind you, these are broad-stroke calculations and it’s quite likely the power purchase agreements include adjustment mechanisms that take into account the many potential complicating factors in producing and delivering energy. We sent questions to NRG asking about the plant’s performance against its contractual obligations, but never heard back.

Editor’s note: This story was updated shortly after its original publication to add more detail about natural gas usage at the Ivanpah plant.