A tariff is always a double-edged weapon in international trade. Countries apply them to protect domestic producers but risk retaliation and penalize their own consumers, who must pay higher prices.

That’s how things are shaping up with the 31 percent tariff the Commerce Department wants to slap on Chinese solar panels later this year. That would undercut one goal of President Barack Obama’s administration — the promotion of green energy.

This dispute began with a complaint from the U.S. unit of a German company, Solar World AG, which was joined by other firms. Commerce officials later concluded the Chinese were “dumping” government-subsidized products, or selling below cost.

And how did Washington determine Chinese production costs? As a proxy, it used data from solar producers in Thailand — a method nobody could describe as precise.

Yet thanks to this convoluted process, expect the price of solar panels to rise in the U.S. market, raising costs for homeowners and businesses seeking lower-cost power. The move also threatens job losses among workers employed as solar-panel installers.

It other words, the tariff will apply not only to the Chinese producers but also, in effect, to the American buyers of those products.