In short: Your power bill arrives as one unexplained total, but it is really the sum of every device multiplied by its hours. A 1,000-watt space heater and a 5-watt LED bulb both plug into the same wall; one of them costs a hundred times more per hour.
Updated August 28, 2026, after a fresh review, with updated visuals and links.
Enter watts, hours per day, and your rate per kWh; the tool returns the cost per day, month, and year, which is usually the moment behavior changes.
The two numbers that run your bill
Power x time = energy: a 1,000 W heater on for 4 hours eats 4 kWh. At $0.16/kWh that is $0.64 per day, $19.50 per month, about $234 per season. The same device at 2 hours halves everything; watts and hours carry equal weight.
Heaters, kettles, dryers and ovens dominate because anything making heat draws 1,000-3,000 W. Electronics barely register: a 65 W laptop 8 hours a day costs about 8 cents daily, under $2.50 a month.
Find your real rate on the bill (energy charge plus variable fees), because quoted averages hide local reality. Rates near $0.12 feel fine while $0.30 areas, like much of Europe, make the same heater $438 a season.
Worth remembering
- The label 'watts' on a device is its maximum; real draw is often lower. A cheap plug-in watt-meter gives the honest figure in one afternoon.
- Standby loads add up to 5-10% of annual bills. A strip switch for the TV corner is the laziest real saving available.
Frequently asked
How do I find my rate per kWh?
On the bill, divide the total variable charges by the kWh consumed. Utilities split the number into generation, distribution and riders; combining them gives the true marginal cost.
Is air conditioning really the biggest cost?
In hot climates, yes: a 3,500 W central AC running 6 hours a day is 21 kWh daily, near $100/month at $0.16. Each degree of thermostat reduction cuts roughly 6-8 percent of that.
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