The calorie (cal) is a small energy unit defined as the amount of heat needed to raise one gram of water by one degree Celsius, primarily used in physics and chemistry. In everyday language, 'calorie' usually refers to the kilocalorie (kcal), but the true calorie is 1/1,000th of that amount. One calorie equals exactly 4.184 joules, providing a direct bridge between thermal and mechanical energy.

The kilowatt-hour (kWh) equals 3,600,000 joules and is the standard billing unit for residential and commercial electricity consumption worldwide. Utility companies charge by the kilowatt-hour, and understanding this unit helps consumers evaluate appliance running costs and solar panel output. One kilowatt-hour represents the energy used by a 1,000-watt appliance running for one hour.