When reviewing electricity bills or receiving energy efficiency proposals, building owners need to understand building Service Classification (SC). Service Classification 2 (General Small) is for small users of energy and demand. Service Classification 9 (General Large) is for high users of energy and demand. Demand (kW) charges represent the greatest consecutive half hour of energy usage for a billing period. For commercial buildings peak demand happens during the workday. Residential buildings peak demand is during the evening. Buildings can become a SC 9 when a building registers two consecutive months of over 10 kW in demand. SC 9 accounts are billed differently than SC 2 accounts. SC 2 are charged for usage only while SC 9 are charged for demand and usage. Demand charges can be costly if not controlled. For SC 9, kWh are charged about two thirds less than a SC 2 because of demand charges. When looking at energy efficiency measures, for SC 2 accounts, look for the greatest reduction in kWh. For SC 9 accounts, look for the largest reduction of kW of demand. Plan building systems to start at different times to help keep your kW low. Use data loggers to verify usage and demand. Understanding demand can also give you insights in building efficiency. Mark Bourbeau is the owner of GreenCents Solutions, Pelham, N.Y.