Just as NSTAR put the finishing touches on a 10-kilowatt demonstration solar power array at the Mass Audubon Center in Mattapan earlier this month, National Grid submitted plans to the city for six megawatts of arrays at locations across Greater Boston, including a highly visible spot around the company's landmark gas tank on Dorchester Bay.

Although National Grid representatives said the I-93 location was chosen purely on a practical basis, for its size, sun exposure, and other factors, the gas tank locale would complement the IBEW windmill on the other side of the expressway and complete a visual triumvirate of energy for thousands of passersby each day, showcasing gas, wind and solar power.

Solar panels have been criticized as being highly energy intensive to manufacture. Decades ago their increased efficiency was negligible, but some new reports say otherwise.

One, published in the March 15 issue of the journal Environmental Science & Technology by Vasilis M. Fthenakis of the Brookhaven National Laboratory in Upton, N.Y., measured the newer, more efficient manufacturing process of photovoltaic cells made today. According to the study, the energy produced by the cells created only 2 to 11 percent of the greenhouse gases as a traditional power plant would generating the same amount of energy. As the cells become less expensive to produce, they are gaining in popularity.

National Grid's array by the gas tank, if built, would be a one megawatt set-up, generating $150,000 to $200,000 worth of energy per year at a construction cost of just under $8 million. The reduction in carbon dioxide production would add to that value.

"The four sites, including the one in Dorchester, were chosen for their size, location and [exposure to the sun.] They're brownfield sites and these locations can help relieve electricity congestion in the Boston area," said Deborah Drew, spokeswoman for National Grid.

The 10 kw system recently installed at the Audubon Nature Center in Mattapan produces just under 10 percent of the center's electricity needs, according to director Julie Brandlen.

"We're getting a 25 year carbon dioxide reduction of 279,739 pounds," she said. "That's the equivalent of planting 1,305 trees or not driving 305,000 miles."

The system's cost - paid for by the George Robert White Fund and the Massachusetts Technology Collaborative - was just over $100,000. That price includes informative plaques and other educational aids that Brandlen said she hopes will convince visitors of the benefits of solar power. The center itself is held up as an example of the heights of green building technology by the Menino administration and others.