Tip-tap-tip-tap-tippety-tap. We all spend our days bashing keys at our computerâ€”so how much energy could we create if we could recover just a little of it?

First, it's worth noting that not all energy recovery systems work: some, like regenerative braking, do; Solar Freaking Roadways, not so much. But Randall Munroe of XKCD has run some numbers to find out if typing could, you know, power your laptop just through pressing its keys.

One study reckons that it requires about 1.5 millijoules to press a letter key and 2.5 to depress a big key, like the enter key or spacebar. Great! But how much is a millijoule? Munroe explains:

It's enough to heat a drop of water by 1% of a degree. It's also enough to lift a squirrel 300 micronsâ€”all the way from the ground to the top of a stack of four sheets of paper!

Hmm. But what if you type a lot of words? Well, an average novel contains half a million to a million characters; type all of them, and you generate one whole kilojoule. And if you rewrite a lot, then that number could be more like 2 to 3 kilojoules. Now we're getting somewhere, right? Umm, no. Munroe puts into context those figures:

Writing one full novel would provide enough energy to run a laptop for a total of about 15 seconds. If each novel takes you six months, you'd spend one second out of every million running off keyboard power. This would save a fraction of a penny of electricity... To keep a laptop running from keypress power alone, you'd need to write a novel every ten seconds.