100 W running continuously for a month is roughly 72 kWh — about $10 at a typical $0.15/kWh rate.
Watts × hours ÷ 1000 = kWh. It’s the fast mental shortcut for "is this appliance actually worth unplugging?"
Only if the device really draws 100 W continuously. A fridge cycles, so a 100 W compressor might run a third of the time; a PC idles far below its rating. And a standing charge plus a tiered tariff can make the marginal unit cost double the average rate you divided by.
At 0.25 per kWh the rule says 10 and the exact answer is 18.25 — 45% low. At the point most people use it the shortcut is already outside its own 15% tolerance. It only holds between 0.12 per kWh and 0.16 per kWh, which is not where it gets used.
The rule encodes a price of about 0.137 a unit, which is roughly what electricity cost when the rule was coined. At present-day prices it understates the bill by close to half - a rule of thumb with an expiry date nobody printed on it.
The rule against the exact answer, computed across the range. Inside the shaded band the shortcut is close enough to use; outside it, reach for the calculator.
Estimate with the rule, then check it against the calculator that models it properly.
Open Energy Cost Calculator →100 W running continuously for a month is roughly 72 kWh — about $10 at a typical $0.15/kWh rate. Watts × hours ÷ 1000 = kWh.