kWh explained: what a kilowatt-hour actually measures
Your electricity bill charges you per kilowatt-hour (kWh), but appliances are usually labeled in watts. Converting between the two is the entire trick to estimating what something actually costs to run.
Watts measure rate, kWh measures total
A watt is a rate — how fast a device uses energy at any given moment, the same way "miles per hour" is a rate. A kilowatt-hour is a total — how much energy was actually used over time, the same way "miles" is a total distance. A 1,000-watt appliance running for one hour uses exactly 1 kWh. The same appliance running for 30 minutes uses 0.5 kWh.
Energy (kWh) = Power (kW) × Hours run Cost = Energy (kWh) × Rate ($/kWh)
A worked example
A 1,000-watt space heater (1 kW) running 2 hours a day for 30 days, at $0.20 per kWh:
- Energy = 1 kW × 2 hours = 2 kWh per day
- Total = 2 kWh × 30 days = 60 kWh
- Cost = 60 × $0.20 = $12 for the month
The devices that surprise people
High-wattage appliances that only run briefly — a hair dryer, a microwave, a kettle — often cost less per month than people expect, because the hours-run number is small. Lower-wattage devices that run constantly — a refrigerator, a router, an aquarium pump — often cost more than expected over a full month, because the hours-run number is large even though the wattage looks modest. Multiplying both numbers together, not just eyeballing the watt figure on the label, is what actually predicts the bill.
Standby power
Many electronics draw a small amount of power even switched off, as long as they're plugged in — often called phantom or standby load. It's usually a few watts per device, which is negligible on its own but can add up meaningfully across a household of always-plugged-in electronics over a full year.