Author Sarah Zielinski discusses the major problem with solar and wind-generated electricity: relying on it puts you at the mercy of weather and daylight.

The major problem is not the unreliable timing of wind and sun. The major problem is that renewable energy sources are so diffuse.

David MacKay made this point in an excellent free book I recommended to WanderingEng: Sustainable Energy – Without the Hot Air.

To make a difference, renewable facilities have to be country-sized.

Figure 25.5. The celebrated little square. This map shows a square of size 600 km by 600 km in Africa, and another in Saudi Arabia, Jordan, and Iraq. Concentrating solar power facilities completely filling one such square would provide enough power to give 1 billion people the average European’s consumption of 125 kWh/d. The area of one square is the same as the area of Germany, and 16 times the area of Wales. Within each big square is a smaller 145 km by 145 km square showing the area required in the Sahara – one Wales – to supply all British power consumption.

Some time ago I complained privately to b_b about a comment observing that a square solar panel 100 miles on a side could power the U.S. It wasn't meant as a feasible proposal, but it didn't sound utterly unfeasible either, and a math error in a followup comment reduced the area needed by 99%.

I told b_b that we don't even have a 10,000 square mile patch of empty land in Nevada.

The feasibility challenges are formidable, even in the empty Sahara. How do you rinse dust off the panels, in the desert? How do you maintain the installation, when would take hours just to drive past it?

MacKay is supportive of renewables, but he is insistent on making the numbers add up. I got the impression that photovoltaic panels will not practical for more than a small fraction of consumption anytime soon.