In its application to build a solar facility on Gov. Roy Cooper’s Nash County property, Durham-based Strata Solar said its generating capacity would be about 5 megawatts. Enough energy to power continuously about 3,750 homes.
But the plant won’t generate 5 MW of energy 24 hours a day, seven days a week. Much of the time it won’t produce anything. “Solar is an intermittent energy source, and therefore the maximum dependable capacity is 0 MW,” the application notes.
Cooper’s application isn’t unusual. Engineers who’ve worked with electric utilities say solar facilities generate no power most of the day, and seldom reach peak generation, yet they are marketed by how many megawatts of electricity they can produce during the rare times they’re at maximum output.
The ratings are ambiguous at best, and deceptive at worst, raising significant public policy concerns, engineers say.
Herb Eckerlin is a retired engineer who has designed power plants for the electric utility industry and professor emeritus at N.C. State University. Eckerlin has been a proponent of solar energy for more than 40 years and remains a fan. He designed and built the N.C. State Solar House, a nationally recognized education and demonstration center for residential solar power.
But he doesn’t like the way ratings are reported and accounted for.
Eckerlin said county officials who approve zoning and other permits for solar facilities, and state lawmakers who pass laws regulating and rewarding solar plants with subsidies, can be misled by the megawatt ratings assigned to a solar application.
Officials often interpret the MW rating as a constant flow of electricity. In reality the rating reflects only potential — a maximum output that occurs for about one hour around noon on a sunny day. A solar plant generates less than the megawatt rating the other 23 hours, and no power at all the 14 hours the sun is down.
“It misrepresents reality,” Eckerlin said of the megawatt labels. “It gives people the impression that this particular system can do much more than it really can. It misleads the public. It misleads the legislators. It misleads the county commissioners. It misleads everybody.”
As a result, many incorrectly believe solar power is available on demand, solar and conventional power sources are interchangeable, and programs like the Green New Deal would let solar replace conventional power sources, Eckerlin said.
“I don’t think that anything that we’re doing is misleading,” said Sam Watson, general counsel for the N.C. Utilities Commission. Regulations require applicants for a solar facility to inform the commission the potential maximum output so electric utilities can decide how to connect the solar power with the electric grid, he said.
“I think there’s plenty of folks that are educating the public on the pros and cons of solar energy,” Watson said, including its intermittent characteristics.
Gary Rackliffe, vice president of smart grids and grid modernization at ABB Inc. in Raleigh, which deals in solar equipment and power grid operations, said a solar facility can meet the megawatt rating only when all of its panels receive full sunshine.
Electric utilities such as Duke Energy and Dominion Energy must keep redundant fossil fuel-fired electric sources operating constantly to fill in immediately when solar power is disrupted by clouds, rain, and nightfall.
Making matters more costly, the federal Public Utility Regulatory Policies Act requires utilities to buy all commercial solar power generated, even if it’s more expensive than energy from other sources such as nuclear, natural gas, or hydro power.
Eckerlin has expressed his concerns as a consultant for opponents of solar projects in North Carolina and other states. He’s testified before legislatures and other government panels.
And still, he remains a backer of solar energy. He says the industry should be honest about the capability and limitations of a solar power plant, rather than use a fictitious megawatt rating.
Eckerlin captured data from instruments at the N.C. State Solar House to create plotlines graphically illustrating solar power’s intermittent nature.
The plotlines tell a story: On mostly cloudy or rainy days, the solar array powering the house produced less than 10% of its maximum rated capacity. A mostly cloudy day recorded occasional blips of about 45%. The number never exceeded 10% on a rainy day.
Eckerlin said those fluctuations would be the same regardless of a solar facility’s size. They illustrate that assigning a megawatt rating to a solar power plant has no meaning.
Lawrence Watts of Greenville, a retired engineer who worked at Duke Energy’s predecessor Carolina Power & Light, agrees with Eckerlin. He used solar facility data he got from a power company and converted it into a series of plotlines covering several 24-hour cycles in January, June, and August 2016. His diagrams resembled Eckerlin’s — a Bell curve sharply rising and falling during the narrow time band power is generated.
Watts said power companies are stressed with high electricity demand in the morning, late afternoon, and evening. They charge higher rates and make much of their profit in those peak demand hours, the time little or no solar power is produced.
“The hardest information to get in America is the production of a solar farm,” Watts said. “The people that own solar farms don’t want you to know how little they produce before 10 o’clock in the morning.”
Solar enthusiasts misrepresent solar efficacy by omitting peak demand shortcomings, Watts said. That influences public policy when, for example, they push officials to convert farmland into solar installations.
Eckerlin suggests the solar industry created the maximum capacity megawatt rating system to disguise actual power generation, which is much lower and more erratic than industry marketers like to admit.
Rackliffe said solar energy can be stored in batteries on sunny days when there is more supply than demand. Battery storage lets solar energy be used during peak demand when solar installations aren’t generating electricity.
Costs of solar batteries are dropping — from about $1,000 per kilowatt hour in 2010 to less than $200 today. “But that’s not necessarily cheap,” Rackliffe said. Solar batteries account for a small percentage of electricity storage on the grid. About 94 percent of storage comes from pumped hydro power.
To store 1 kilowatt of electricity to produce just one hour of operation would require a storage pod the size of a shipping container, Rackliffe said. By comparison, a typical nuclear plant generates a gigawatt of electricity, he said. One million kilowatts.