• What is Power Factor?
• Results of Power Factor Correction
• Impact on Utility
• Power Factor Surcharges Encourage Higher Efficiency
What is Power Factor?
Power Factor reflects how efficiently a facility uses electricity. Of course there are many
other measures of energy efficiency. Power factor compares the amount of useful work that is
extracted from the total amount of electrical energy supplied.
Power factor is defined as the ratio of "useful" power, which is resistive in nature and measured
in kilowatts (kW), divided by the total electric power demand, often referred to as "apparent" power
measured in kilovolt-amperes (kVA).
useful electric power (kW) 1000 x watts
Power Factor = ---------------------------------- = ---------------------------------
apparent power (kVA) 1000 x voltage x current
A power factor of 1.00 of unity, means that all of the power consumed by a facility goes to produce useful
work. , such as resistive heating strips and incandescent lighting, have power factors close to unity.
On the other hand, reactive devices -- those devices that use inductive coils or capacitors (such as electric
motors, transformers, etc.) -- a significant portion of the electric power required to operate these devices
goes to create an electro-magnetic field and not to resistive work. This portion of electric power is generally
referred to as reactive power.
The relationship between apparent power (kVA), resistive or useful power (kW), and reactive power (kVAr) generally
follows this equation. The effective power factor may be lower in the presence of non-linear devices (such as
solid-state or switched power supplies, variable speed drives, DC drives) and the harmonic distortion that these
devices generate. Harmonic distortion essentially converts a portion of the useful energy into high frequency energy
that is no longer useful to most devices and is ultimately lost as heat. In this way, the presence of harmonic
distortion further reduces the effective power factor.
kVA = sqrt [ ( kW x kW ) + ( kVAr x kVAr ) ]
Generally motors that are operated at or near full nameplate loading (e.g., a 100-Hp motor delivering 100-Hp of work)
will have a power factor of 0.90 to 0.95. Whereas, the same motor running completely unloaded may exhibit a power
factor in the range of 0.30 or less. For example, motors in large hydraulic machines, such as plastic molding machines,
operate much of the time in a lightly-loaded condition contributing to an overall power factor of 0.60.
A power factor of 0.60 means that to do 100 kW of useful work requires 167 kVA of total or apparent electric power.
At 480V, the electric current demand would be 347 amps. Installing power factor correction equipment to raise the power
factor to 0.95, would reduce the amount of apparent power to 105 kVA, and reduce the required current to 219 amps -- a
37% reduction in current.
Results of Power Factor Correction
Line and transmission losses are directly related to current, so line losses
would be reduced proportionate to current. Reducing the amount of current required by the facility will generally
improve line voltage, especially where a distribution system is operating close to maximum capacity. Improvement
in equipment efficiency is well documented. Efficiency gains of between 1% and 6% have been observed. The higher
gains are typically associated with lower voltage systems, such as 220V or 240V systems, and those facilities with
very low power factors, in the range of 0.60 to 0.65. Motors generally are more efficient as the power factor approaches
unity and also when operated at higher voltages -- there's less slip and the motors run cooler. Cooler operating
temperatures reduces HVAC loads and contributes to longer run lives.
Thus improving a facility's power factor not only reduces utility power factor surcharges, but also can result in
reduced power consumption.
Impact on Utility
Nearly everything the power company builds is geared to the supplying of electric current.
The utility must build sufficient generating capacity to meet demand for electric current. The utility must also
have adequate lines, transformers, and substations to distribute and deliver electric current to each customer.
Low power factor results in congested distribution systems and poor capital utilization. The utility must make
unnecessary investments in equipment. Unnecessary current leads to extra line loss and wasted fuel. All contribute
to higher costs to consumers and lower profitability for the utility
Power Factor Surcharges Encourage Higher Efficiency
To encourage more efficient use of electricity and to
allow the utility to recoup the higher costs of providing service to customers with low power factor that demand
an inordinate amount of current, many utilities provide incentives to their consumers in the form of power factor
surcharges. Most utilities only assess power factor penalties on their larger commercial and industrial customers.
Often these penalties are assessed only when power factors fall below 0.90 or 0.95, some even as low as 0.80.
Without these surcharges there would be no motivation for consumers to install power factor correction.
The incentive can take the form of power factor penalty (e.g., adjusted demand charges or an overall adjustment to
the bill) or a rate structure where the demand charges or power rates are based on current or apparent power, kVA.
Customers of utilities that charge demand based on current or kVA, in effect pay more whenever their power factor is
anything less than unity. The difficulty consumers face in areas with kVA-based demand charges is that there is no
specific indication, no separate line item, on their utility bill that identifies the 'power factor penalty', yet it
is there. The only way to tell is to record the power factor at the facility's peak kVA demand.
More and more utilities are introducing power factor penalties (or kVA-based demand charges) into their rate
structures, in part, to comply with provisions of the Clean Air Act and deregulation and to some extent in response
to growing competition in a newly-deregulated power market. By adding charges related to low power factor to their
rate structure, power companies can effectively lower their basic power prices and unbundle the service of providing
reactive power. These surcharges provide tangible incentives for consumers to improve their power factor. The result
is overall lower costs to consumers and the utility.