Q: Given an appropriate location, how large a solar array would it take to power the entire United States, assuming present efficiency levels? How much CO2 would be released to the atmosphere in producing those panels? What other harmful byproducts would be produced, and how much?

Asked by Paul Hayne, ’03, MS ’05, Los Angeles, Calif.

There’s almost a graduate seminar worth of answers in your questions! Fortunately I’ve already taken it (Civil and Environmental Engineering 301), but even more fortunately, the short answers really are pretty short: Not all that large, not too much, and less all the time.

Still, you raise issues that anyone thinking about going solar should consider: appropriate location, cautious manufacture and proper disposal. While generating electricity using solar panels is environmentally friendly, no energy source is environmentally “free.” For solar, most of the negative trade-offs come at the beginning of the process.

The average solar electric installation emits 16 times less CO2 than coal, 10 times less CO2 than oil, and 7 times less CO2 than natural gas for the same amount of electricity. But what about the energy used to manufacture photovoltaic (PV) panels in the first place?

Eighty percent of the panels on the market are made out of silicon, and mining that element accounts for most of the CO2 emitted during manufacture. While new technologies are emerging that use different materials that require less energy to extract, the toxic and carcinogenic chemicals involved in the mining process are still a big issue. The chemicals, which include cadmium and sulfur hexafluoride, can easily be regulated and contained with appropriate manufacturing procedures, but when it comes to factories in less regulated countries, all bets are off.

On a brighter note, studies show that the efficiency of solar panels has been improving very rapidly. In 2000 the energy payback time—the time required for a device to produce as much energy as was consumed in its production—for solar panels was between 8 and 11 years. Today it’s just 1½ to three years for silicon-based panels, and one to 1½ years for newer, thin-film panels. That means that after just one year of operation, and assuming you didn’t have any chemical spills along the way, a high-efficiency PV panel runs with practically zero negative environmental impact.

Unless, that is, you take land use into account. There’s been some concern that large solar arrays can harm ecosystems and endangered species if placed in sensitive areas. But given an appropriate location—say, in the Great Basin of Nevada and environs—covering less than one percent of the continental United States with solar panels would be plenty to generate all of the country’s electricity needs. That’s a 150-mile by 150-mile piece of land. But here’s the important thing: This doesn’t have to be one piece of land, or one solar array. With a mix of large solar farms with hundreds of arrays each distributed over the country, plus many thousands of smaller installations on homes and buildings, we could reach 22,500 square miles more efficiently, and with a lot less disruption.

So what’s stopping us? In a word, money. It still costs twice as much to generate electricity using solar power as it does using oil, and 1.5 times as much as using natural gas. The good news is, solar generation is getting less expensive all the time, while oil and natural gas are almost certainly going to become more expensive.

Lots of homeowners are adding solar panels to their roofs already. But if you’ve got a little more serious land to spare, it might just be time to start thinking about how much of those 22,500 square miles you can claim.

How much land would we have to cover in solar panels to satisfy America’s demand for electricity? About as much as the three dots on the map above. (Areas in red get the most sunlight.)

Shaker Muasher plans to receive his bachelor’s degree in international relations, with minors in economics and energy resource engineering, in June 2010.