Efficiency is a measure of the conversion of inputs to outputs. The higher the conversion ration the higher the efficiency. In terms of energy efficiency within a server room or datacentre environment the input for electrical equipment is electricity. Efficiency measurements (in percentage % terms) measure how much of the input power (Watts) is used to generate the output. The difference between this figure and 100% is wasted energy (normally in the form of heat and noise). Heat loss in a datacentre adds to the overall requirements for air conditioning. Energy efficiency in a server room or datacentre also affects electricity usage (measured on kilo-Watt-hours kWh). The higher the kWh of electricity used the higher the electricity bill for the period.