I don't like this, but it's been a long time coming:
The duties are lower than the 35 percent rate the U.S. International Trade Commission recommended in October after finding that imported panels were harming American manufacturers. The idea behind the tariffs is to raise the costs of cheap imports, particularly from Asia, and level the playing field for those who manufacture the parts domestically.
For Trump, they may represent a step toward making good on a campaign promise to get tough on the country that produces the most panels — China. Trump’s trade issues took a backseat in 2017 while the White House focused on tax reform, but it’s now coming back into the fore: The solar dispute is among several potential trade decisions that also involve washing machines, consumer electronics and steel.
Further, as the article points out, 1) the first 2.5GW of solar panels imported each year are zero tariff, and 2) they may be opposed by the WTO anyway.
So this has caused a lot of uncertainty in that market.
All that said, the solar industry is probably going to take a hit, but will recover quite a bit. Panel prices have been falling faster than the savings you'd gain. If you bought a system last year, you would have saved money if you waited and bought on this year. Prices are in a bit of a freefall right now (one of the many reasons the US ITC recommends tarrifs).
On top of all that, the federal government subsidy for solar installations (still about 1/3 the total cost to consumers) is set to expire after 2019. So in a year we're going to have a big wrestle in congress about whether to extend or modify them.
All that said, I suspect that the tariff will measurably increase solar costs, and decrease adoption, but it'll go down to maybe 2014 levels, rather than increasing at the furious pace is has been these last few years.
It's something I'm watching carefully as I'd like to be a little more self sufficient in energy consumption (and we eat a lot of energy in the Davis household).
Here's the report from October where the commission made the recommendation, the reasons behind it, and the reaction from the industry:
https://www.nytimes.com/2017/10/31/business/solar-industry-import-tariffs.html