Many in solar power focus on efficiency. For every watt of sunlight, how much of it can we turn into electricity? 10%? 20%? Billions have gone into researching more efficient solar cells, with substantial progress over the last thirty years.

But efficiency is the wrong thing to focus on. Who cares about watts per square meter? Square meters are free. We have a bazillion of them – we can use as many as we want. The key metric is watts per dollar. Electricity buyers care almost exclusively about cost. Watts per dollar determines whether they use solar or coal. If a new panel is 10% more efficient, but double the price, it’s useless.

For a cheap solution, how about this: Take a bunch of desert. Cover it in a layer of insulator, then (on top) metal, black paint, water, and glass. When the Sun comes up, it will heat the paint very quickly (~20 – 100 C / hr.), causing the water to become high-pressure steam. With double-paned glass and good insulation, the heat will be trapped well enough for steam to reach many hundred degrees C. That steam can then run an old-fashioned steam turbine.

At 250 C, maximum (Carnot) efficiency would be about 40%. In practice, steam turbines aren’t very good, so you might only get 10%. But even at 10%, time-averaged power output would be ~25 W/m^2, or $100/m^2 at current $4/W prices. And there’s no way bulk insulation and low-quality glass panels cost $100/m^2.