Energy is a quantitative property that must be transferred to a body or physical system to perform work on the body, or to heat it. The standard unit is the Joule (J), but daily life uses Calories (for food) and kWh (for electricity).
It is crucial to distinguish between a small calorie (cal) and a food calorie (Cal or kcal). One small calorie is roughly the energy needed to heat 1 gram of water by 1°C. One Kilocalorie (kcal) equals 1,000 small calories and is the standard used on nutrition labels.
The kilowatt-hour is a unit of energy equivalent to one kilowatt (1 kW) of power expended for one hour. It is the standard billing unit for electrical energy. For context, a 100-watt lightbulb running for 10 hours consumes exactly 1 kWh.