Your electricity bill is measured in kilowatt-hours. One kilowatt-hour is the energy used by a 1,000-watt appliance running for one hour. A space heater on high for an hour consumes about 1.5 kilowatt-hours. A phone charge takes a tiny fraction of one.

The unit exists because power and energy are different things. Power is the rate of use. Energy is the total amount. A 100-watt bulb and a 1,000-watt heater both run on the same voltage, but the heater drains energy ten times faster. The kilowatt-hour captures that total.

Prices vary widely. In parts of Europe, electricity can cost 30 to 40 cents per kilowatt-hour. In the United States, the average is closer to 15 cents. In some oil-rich countries, it is a few cents. These differences shape behavior. High prices push households toward efficiency and solar. Low prices encourage waste.

Typical consumption

• LED bulb: about 0.01 kWh per hour
• Laptop: about 0.05 kWh per hour
• Refrigerator: 1 to 2 kWh per day
• Electric vehicle: 0.2 to 0.3 kWh per mile
• US household average: about 30 kWh per day