Why the watt is the most important spec in battery-powered devices

Every time I see “mAh” on a consumer product, I cringe. Megahertz are starting to bruise, too. That’s because the most critical components of portable gadgets — batteries and processors — should really be measured in watts: it’s the one common spec that gives you a real idea of what your device can do.

With watts, you can tell how much literal power is under the hood — how much energy is provided by your battery and how much is coursing through your chip. Watts determine whether...