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 watt-hour (Wh) equals 3,600 joules, representing the energy consumed by a one-watt device operating for one hour. It is commonly used for small battery capacities, solar panel output ratings, and electric vehicle charging metrics where kilowatt-hours would be too large a unit. One watt-hour equals exactly 0.001 kilowatt-hours or about 3.412 BTU.