If solar power is ever going to take off—and the world needs it to—photovoltaic cells will have to become a whole lot cheaper to produce.

Making solar cells from silicon, the most common approach, can be expensive and relatively inefficient at turning sunlight into electricity. As semiconductor manufacturer Applied Materials chief technology officer Mark Pinto told me last year: "With solar, it's all about cost."

But there are signs of improvement, writes Richard Swanson of SunPower Corp. in this week's Science. Last year, manufacturers made 5 gigawatts of photovoltaic panels. And some of these panels required just under six grams of silicon per watt of power—down from 15 grams at the turn of the century. And that watt of power now costs around $1.40 to produce compared with $2 or more in the 1990s.

Swanson argues, that cost will fall to $1 per watt within five years—making solar power for the first time cost competitive, without subsidy, with conventional fossil fuel–fired generation.

Of course, solar power represents just 0.375 percent of all installed power generation worldwide and there's the little problem of producing electricity at night. Plus, installing the solar panels can more than triple that cost per watt to more than $4.

But if photovoltaic technology improves as Swanson predicts, solar panels’ day as a practical source of power may finally be dawning.