A number of terms are used to characterize the magnitude and intensity of
loads. Several such terms are defined and uses outlined below.
Energy—Energy use (over a specified period of time)
is a key identifying parameter for power system loads. Energy use is often
recorded for various portions of the power system (e.g., homes, businesses,
feeders, substations, districts).
Utilities report aggregate system energy use over a variety
of time frames (daily, weekly, monthly, and annually). System energy use is
tied directly to sales and thus is often used as a measure of the utility or
system performance from one period to another.
Demand—Loads require specific amounts of energy over
short periods of time. Demand is a measure of this energy and is expressed in
terms of power (kilowatts or Megawatts).
Instantaneous demand is the peak instantaneous power use of
a device, facility, or system. Demand, as commonly referred to in utility
discussions, is an integrated demand value, most often integrated over 10, 15,
or 30 min.
Integrated demand values are determined by dividing the
energy used by the time interval of measurement or the demand interval.
Demand = Energy Use Over Demand Interval/ Demand Interval
Integrated demand values can be much lower than peak
instantaneous demand values for a load
or facility.
Demand Factor—Demand factor is a ratio of the maximum
demand to the total connected load of a system or the part of the system under
consideration. Demand factor is often used to express the expected diversity of
individual loads within a facility prior to construction.
Use of demand factors allows facility power system equipment
to be sized appropriately for the expected loads.
Demand Factor = Maximum Demand/ Total Connected Load
Load Factor—Load factor is similar to demand factor
and is calculated from the energy use, the
demand, and the period of time associated with the
measurement.
Load Factor = Energy Use/ Demand x Time
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