Steam power sets massive, clunky turbines into motion. Horsepower involves explosions of work, with engines hurtling cars down a freeway. Solar power taps the awesome might of the sun itself and may someday light entire cities.

But willpower seems different. For one thing, it more often describes disciplined inactivity, such as resisting a temptation, than some observable action. While metaphorical horses produce horsepower and the sun solar power, it’s not easy to frame “wills” in an equivalent way. It won’t work to say,...