Putting solar power on rooftops is pretty labor-intensive. You need people to design and manufacture the panels. Then people to market the panels to homes and businesses. Then people to come and install them.

The US solar industry employed about 174,000 people in 2014

That's a lot of jobs. Even though solar power provides just a tiny fraction of electricity in the United States — about 0.4 percent — the solar industry now employs roughly 174,000 people, according to a survey from the non-profit Solar Foundation. And the industry is expected to add another 36,000 jobs in 2015, as rooftop installations keep rising at a rapid clip.

To put this in perspective, 174,000 is roughly comparable to the number of workers employed by the US coal industry, if you add up all the workers in coal mining (about 80,000), plus coal transportation and coal power plants.** Yet coal still provides 39 percent of America's electricity — far, far more than solar does.

Now, mind you, this isn't a perfect comparison. Solar is still growing, which means there's a lot of installation work, whereas no one's building new coal plants in the US anymore. (Quite the contrary, many older coal plants have been closing in recent years, thanks to stricter air-pollution rules and cheap natural gas.) Still, it's interesting to look at the two industries side-by-side.

Are all these solar jobs a good thing?

There are a few different ways to look at the boom in solar jobs.

From an employment perspective, it's great. The US has been reeling from a brutal recession, and any source of new jobs is welcome. In 2014, solar companies added positions 20 times faster than the rest of the economy. And these are relatively decent-paying jobs, with average wages around $20 to $24 per hour. About 55 percent were installation jobs, 19 percent were manufacturing, and 12 percent sales.

Solar power is still far more labor-intensive than other sources

A more pessimistic way to look at these numbers, however, is to point out that solar power looks like a relatively inefficient way of creating energy. As one 2012 University of Tennessee study found, it takes far more manpower to generate one megawatt-hour of electricity from solar than it does from any other energy source.

And that's not a good thing. If the world wants to avoid drastic global warming, we'll need to replace dirtier sources of energy, like coal, with cleaner sources — solar, wind, nuclear, say — and fast. And the higher cost of solar is a real impediment to doing so. Here's Michael Liebreich, head of Bloomberg New Energy Finance, making this point on Twitter:

.@scalinggreen Do clean energy boosters understand: boasts about how many jobs a technology "creates" also show how expensive it is. — Michael Liebreich (@MLiebreich) January 14, 2015

If anything, Liebreich added, a solar industry that created fewer jobs could be a positive development:

.@guyshrubsole Automated #PV plants, modern installation systems, self-cleaning glass... That's how you get cost down, not by carrying jobs. — Michael Liebreich (@MLiebreich) January 14, 2015

The more encouraging news is that, in some ways, this is already starting to happen. Analysts expect residential solar systems to keep getting cheaper through 2017, thanks to reductions in "balance of systems" costs — things like installation, financing, engineering, and so on.

One example: As the solar market expands, word of mouth will spread, and it will become easier to find new customers without the need to hire as many people in sales. Solar power, in essence, is contagious. And that makes the industry more productive:

If power plants had to internalize all their social costs, clean energy would fare better

There's another angle here: Solar is far more expensive than coal if you only look at the direct costs of producing electricity. But there are indirect social costs, too. Coal plants produce lots of pollution — soot, mercury, coal ash — that foul the air and water and make people sick. These costs don't show up on electricity bills. Instead, they're dumped on the broader public, in the former of shorter lives or higher hospital bills. Coal plants are also a major source of carbon-dioxide emissions, causing damage through global warming.

If power plants had to internalize all these social costs, then sources like wind, solar, or even nuclear power would be far more competitive — as this 2012 paper by economists Michael Greenstone and Adam Looney explained. (One way to do that might be through a carbon tax on fossil fuels.)

In other words: It would be a major boon for clean energy if solar power keeps getting cheaper and less labor-intensive over time. But getting fossil-fuel users to pay for all the external costs they impose would also make a huge difference.

As the solar industry expands, will it have more clout?

For years, many US politicians have opposed action on climate change or stricter regulations on air pollution. Often this opposition comes in the form of jobs talk: Obama is waging a "war on coal" and killing coal jobs. That worry isn't wholly unfounded. The combination of cheap natural gas and new air pollution regulations really is shrinking the coal industry. And, in areas like West Virginia or eastern Kentucky, that really does mean fewer jobs.

As the coal industry shrinks, its defenders cite jobs as a reason to protect it

But as the Solar Foundation's report points to, measures to bolster cleaner energy are also creating jobs elsewhere. Over time, that may end up creating a bigger constituency for further clean-energy policies. A US solar industry that employs more people as the still-influential coal industry could have a lot more sway — in Washington and elsewhere.

But it's still unclear how that will all play out. Right now, the solar industry is still growing rapidly thanks to a 30 percent federal tax credit for rooftop solar systems. But that tax credit is set to expire in 2016 unless Congress extends it in some shape or form. And if the credit does lapse, the solar boom is likely to slow (though it probably won't stop entirely in states that are sunny and have high electricity costs, like California).

It'll be worth watching how the solar industry deals with that over the next few years — and whether all those jobs translate into extra political clout or not.

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** Note on numbers: Over at Climate Desk, Tim McDonnell argued that solar employs twice as many people as coal, although he's only counting coal miners. I've also folded in 2006 estimates of people employed in transporting coal and working at coal power plants, which brings coal roughly even with solar. (It might even put solar ahead, since a number of coal plants have closed since 2006.)

But even this estimate isn't entirely perfect. As Jonathan Adler pointed out on Twitter, my coal jobs numbers may be an undercount, since they don't include people working on things like regulatory compliance. Fair enough. The broader point still holds: The solar industry and coal industry employ a roughly comparable number of people despite the fact that the coal industry generates a much, much bigger fraction of the country's electricity.

Further reading

Solar power is now growing so fast that older energy companies are trying to stop it

Solar power keeps getting cheaper — though not for the reasons you'd expect

Solar power is contagious: Installing panels often means your neighbors will too