About Power Factor Charges and Commercial Electric Bills

Understanding Power Factor in Alternating Current Systems

North America uses AC power grids, where the voltage switches from positive to negative at 60 Hz, or 60 times per second. If you connect a power quality analyzer to the voltage supply, you can see a sine wave graph, where voltage and current are constantly switching between positive and negative. Voltage and current switch at the same time in the following example, and when this happens they are considered to be “in phase”:

If you measure power consumption, you will notice it increases and decreases along with voltage and current, but all the power is consumed (100% power factor). In the graph below, you will notice that no electric power is sent back to the grid – it reaches zero watts when voltage and current are zero, but it never drops to negative values.

In this example, the AC voltage is 120V, the AC current is 10A, and the average power is 1,200W. The power factor is 100%, since no power is sent back from the load to the source at any time. 

The following graph shows the same voltage and current when the power factor drops to 60%. Note that voltage and current now reach their maximum and zero values at different times, which means they are “out of phase”.

The device consumes power when voltage and current have the same sign, but it sends back power when they have opposite signs. This can be visualized more easily in the following graph:

The apparent power is still 1,200 VA, but the average power consumption is no longer 1,200W. At 60% power factor, it has dropped to 720W. However, you need a circuit that is suitable for 120V and 10A, even when the real power consumption is not 1,200W.

Power factor penalties are used to achieve a higher PF in buildings. Since power lines and transformers must be sized for the apparent power, a low PF makes the grid more expensive. While a single building may not have a considerable impact, a large group of buildings with low PF can be a major burden for the grid.

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