"W" on a light bulb

WATT

This unit measures the power consumption of your household appliances, named after a Scottish inventor who improved the steam engine.

The letter "W" here isn’t just the letter—it’s shorthand for "watt," the unit measuring electrical power. Light bulbs are rated in watts to show how much energy they use, so the clue plays on the double meaning of "W" as both a letter and an abbreviation. It’s a straightforward example of how crosswords use abbreviations as wordplay.