LED lighting technology used for general lighting purposes can offer many advantages over more traditional light sources such as incandescent and fluorescent including better color rendering, longer life, and better control of the light casting. One of the main advantages is the better efficiency and sizable reduction in electric power demand for the same amount of light, which results in dramatic savings over the life of the lamp.
The traditional method of calculating the power consumption of an electric device is to simply multiply the voltage and current used by the device or
“load” as it will be called for this Case Study. However, this method does not work for LED lighting products, and there is really only one way to accurately determine the true power consumption of a lamp, especially an LED lamp.
To begin the explanation, first note that there are two types of loads:
Resistive Loads – include incandescent bulbs and most heating devices such as toasters and electric heaters (without fans).
Inductive Loads – include devices that use motors, transformers, or switching power supply like most modern electronics use (i.e. TV, computer, microwave, CFLs, air conditioners, LED lighting products, etc.). These devices actually return a portion of the energy used back to the source (the power company).
Next, consider the ways that power must be measured in Alternating Current
(AC) circuits. Since the current in AC circuits is continuously varying in magnitude and polarity, in certain AC loads, a part of the energy delivered to the load is momentarily stored and then is returned back to the source (the power company). This cycling typically occurs 120 times per second for a 120
VAC, 60 Hertz service.
The increase of electrical current on each wave makes it appear that more power is being used than actually is being consumed by the device as a whole if you simply multiply the service voltage by the current draw. The product of multiplying the current and the voltage is the “apparent power” (with Volt
Amps being the units), because it appears that this is the power that is being consumed by the load.
Seesmart
® – 4139 Guardian Street – Simi Valley, CA 93063
Tel: 877.578.2536 – Fax: 888.239.0959 – Web: www.seesmartled.com
© 2010 Seesmart, Inc. All rights reserved.
File: CS003_True Power vs Apparent Power Measurement
POWER COMPARISON
True Power (W) – actual power consumed.
Apparent Power (VA) – calculated power consumption.
OTHER TERMS FOR
“TRUE POWER”
Real Power
Active Power
True RMS Power
PROPER TRUE POWER
CONSUMPTION
FORMULA
TP = AP x PF
TP = (V x A) x PF
POWER MEASUREMENT
Obtain measurements using equipment that can sample/monitor voltage and current simultaneously.
RESULTS
Only be concerned about the
True Power since the utility company only charges for the actual power consumed
(True Power). Use the proper calculations and/or equipment to determine the
True Power.

So to calculate the true energy consumption by the load, True Power, the total energy consumed by the load must be measured, ignoring the amount of energy stored and returned to the source. The ratio of the True Power (W) to the
Apparent Power (VA) is called the Power Factor (PF). If the Power Factor is known (either from the manufacturer or other source), then it can be used to calculate the True Power after determining the Apparent Power by multiplying the voltage and the current draw by the load.
Power Factor (PF) =
True Power (Watt)
Apparent Power (VA)
OR
Power (Watt) = Apparent Power (VA) x Power Factor (PF)
See the following example Apparent Power and True Power calculations for an LED T8 Tube Light:
V
= 120V
A
= 0.2A*
PF
= 0.62*
EXAMPLE – LED T8 Tube Light Electrical Measurements
Apparent Power True Power
Volts x Amps = AP(VA) Volts x Amps x PF = TP(W)
120V x 0.2A = 24VA 120V x 0.2A* x 0.62* = 14.9W
* Based on ‘low power factor’ LED T8 Tube Light sample.
If some of the information cannot be obtained through calculations, then a special Additional information can be gathered using equipment that is able to monitor and sample voltage and current at the same time so that it can determine how much energy is returned to the source. These monitors can also typically figure out other information for your load including operating performance and cost calculations. Some example meters that may be used for this purpose are provided in the table below.
Example Energy Consumption Monitoring Equipment
Company P3 International Electronic Educational Devices Brultech Research
Product Kill-A-Watt Meter Watts Up? PRO
Contact Seesmart with any questions and for additional information.
ECM-1220
Seesmart
® – 4139 Guardian Street – Simi Valley, CA 93063
Tel: 877.578.2536 – Fax: 888.239.0959 – Web: www.seesmartled.com
© 2010 Seesmart, Inc. All rights reserved.
File: CS003_True Power vs Apparent Power Measurement