What is the difference between electrical demand and energy?

Demand (measured in kW or MW) is the rate of energy consumption at a specific instant or averaged over a short period (typically 15-30 minutes). Energy (measured in kWh or MWh) is the total power consumed over time. Utilities bill for both: demand charges cover infrastructure costs to meet peak requirements, while energy charges cover fuel and operating costs. High demand with low energy use still requires significant infrastructure investment.