My new job is in a building with a revolving door, and spinning that thing around a few times a day will get you thinking. We already know that revolving doors have the potential to save a not-insignificant amount of energy through climate control efficiency. But I wanted to know the answer to a more ambitious question – if we made every door in the United States a revolving door, *and attached a turbine to them*, how much energy could we generate?

I’m not such a science whiz, but fortunately I had a little bit of help – the inestimable Randall Munroe had already tacked a similar question, and his calculations were a good guide to the information I needed to know.

Fortunately, the MIT study cited in both the above links had already done a decent chunk of work, calculating that an average revolving door requires, in foot-pounds, fourteen times the length of the door, which is also the radius of the circle which incloses it. Browsing this brochure, a decent estimate for the average diameter of a revolving door is 2700mm, and from there we can do a little simple math and deduce that on average it takes 0.023 watt hours to open a revolving door (and let’s assume the turbine can perfectly convert work to electricity for now).

Next, I had to guesstimate how many times Americans would enter or egress through revolving doors, assuming revolving doors were the only ways in and out of all buildings except single-family houses. Using some data from the BLS time use survey (augmented by NMHC data) I came up with a few coefficients using the venerable technique of “meh that seems rightish” and came up with a daily aggregate average of 5,221,919,613.23. This works out to about 21 spins per person per day, which seems more than close enough to estimate. Then you just need to do some multiplication and then attach a price to electricity (Randall used $0.15, but I’m going to go with a dime based on this EIA data). Once you’ve done that, you get a grand total of….$4,451,115.65.

If this sounds underwhelming, it is. Revolving doors start at $2,500, and therefore the Fermiest of estimates suggests that retrofitting every building in America with one might be a $20 billion job, meaning your ROI here is, generously, two-and-a-half basis points.

The reasons for this, fundamentally, is that it just doesn’t take very much work to push a revolving door relative to the energy needed to power modern society. A kilowatt hour is enough energy that, if you hit a golf ball with that much force at St. Andrews, you could sink a hole-in-one at Pebble Beach. Going through a revolving door only generates one fifty-thousandth or so of that much energy; but your average American house uses thirty times that much energy *every day*.

What does this all add up to? Our whole social, economic, and political order is built, on a very fundamental level, on massive production and consumption of energy beyond that humans and animals could supply directly with their body. Finding sources for that energy is hard, and even if you could capture all the adult humans in the United States exert, you’d probably only be generating at most $20 billion a year, which is still not even 10% of the energy residences alone consume annually. This, semi-coincidentally, is also why *The Matrix* was pretty dumb.

Anyway, to really meet our energy needs as we wonder whether fossil fuels will run out before or after we’ve trashed the planet beyond the point of no return, we need to turn to much vaster sources of energy. You know, like the sun.

Oh, and:

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