I’ve written recently on the need to do the math regarding rooftop solar’s promise, not just to consumers but to policymakers, too.

Writing in the LA Times, Robert Bryce of Manhattan Institute does the math on the idea that California can become 100 percent powered by renewable energy sources. Advocates estimate it could be done with “about 124,608 megawatts of onshore wind-power capacity, 32,869 megawatts of offshore wind capacity, and 236,243 megawatts of solar-energy capacity.”

Those are enormous numbers, but especially in terms of the land usage they would require. A few snippets:

Last year, global solar capacity totaled about 219,000 megawatts. That means an all-renewable California would need more solar capacity in the state than currently exists on the entire planet. … [It would] require nearly 33,000 megawatts of concentrated solar plants, or roughly 87 facilities as large as the 377-megawatt Ivanpah solar complex now operating in the Mojave Desert. Ivanpah, which covers 5.4 square miles, met fierce opposition from conservationists due to its impact on the desert tortoise, which is listed as a threatened species under the federal and California endangered species acts. …

California would need 41.5 billion square meters, or about 16,023 square miles, of turbines. To put that into perspective, the land area of Los Angeles County is slightly more than 4,000 square miles — California would have to cover a land area roughly four times the size of L.A. County with nothing but the massive windmills. Turning over even a fraction of that much territory to wind energy is unlikely. …

Don’t count on offshore wind either. Given the years-long battle that finally scuttled the proposed 468-megawatt Cape Wind project — which called for dozens of turbines to be located offshore Massachusetts — it’s difficult to imagine that Californians would willingly accept offshore wind capacity that’s 70 times as large as what was proposed in the Northeast.

To expand renewables to the extent that they could approach the amount of energy needed to run our entire economy would require wrecking vast onshore and offshore territories with forests of wind turbines and sprawling solar projects. … The grim land-use numbers behind all-renewable proposals aren’t speculation. Arriving at them requires only a bit of investigation, and yes, that we do the math.

Astute readers will note that Bryce isn’t doing the math with respect to effect on electricity costs, even though cost is the No. 1 issue to electricity consumers, especially poor consumers.

Some math, gentle readers, is best left described by Dante. Abandon all hope, ye who enter here.