A 100W device left on all month costs about $10

100 W running continuously for a month is roughly 72 kWh — about $10 at a typical $0.15/kWh rate.

Why it works

Watts × hours ÷ 1000 = kWh. It’s the fast mental shortcut for "is this appliance actually worth unplugging?"

Do it exactly

Estimate with the rule, then check it against the calculator that models it properly.

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