Power Factor and Compact Fluorescent Bulbs (CFL) Christian Donner, January 21, 2009November 2, 2009 What is the power factor? What is the power factor of compact fluorecent bulbs? Do manufacturers of compact fluorecent bulbs cheat with their promised energy savings? Continue reading for answers to these questions. The power factor (PF) is the ratio of real power vs. apparent power of a system. Electric appliances use Real power (measured in Watt, W) to produce work, for instance in the form of heat and light produced by a light bulb. Apparent power (measured in Volt Ampere, VA) is the product of voltage and current and is a rather abstract thing for most people (except for electrical engineers). Apparent power is stored by inductors and capacitors in an electric circuit during each cycle, for instance in the electromagnetic field of a vaccum cleaner motor, and returned to the source in the same cycle (the generator in the power plant). The AC frequency in the United States is 60 Hz, so this cycle happens 60 times per second. Apparent power is not ‘consumed’ by appliances, and luckily, because it swings in and out of your house in each cycle, the net result on your residential electric meter is zero. Utilities do not charge residential customers for apparent power. However, apparent power requires additional current flowing across the grid, and thus creates distribution losses in transformers and power lines in the form of heat. Therefore, utilities have great interest in minimizing apparent power in the grid. They require industrial plants and other large consumers of electric power to take measures for compensating their power factor, or if that is not possible, will charge a penalty. While incandescent bulbs have a power factor close to 1, and can be considered an almost ideal load, the electric circuit in a compact fluorecent bulb (also called ballast) causes apparent power and lowers the power factor. Current (top) and voltage (bottom) curves of a CFL bulb Ideally, voltage and current are perfect sine waves. When you observe voltage and current on a resistor, both sine waves will appear undistorted and in phase (i.e. both curves have their maximum at the same time). When you observe voltage and current on a CFL (as in the graph above), you can see that the curve of the current is no longer a sine, and that both waves are out of phase. The “out-of-phase-ness” indicates that the load causes apparent power. The odd shape is caused by non-linear characteristics of the electronics in the ballast and contributes to appearent power as well. For an 11W CFL purchased at Ikea (a 40W-equivalent bulb), I measured a real power of 9W, an apparent power of 14VA, and a power factor of 0.65. So what does this mean? It means that almost 50% more current is drawn from the grid to light up this bulb than it can convert into light and heat. But you don’t pay for this extra power, because it is not consumed. It oscillates back and forth between the generator and the (inductive) load. The apparent power causes distribution losses, but because there are so many different loads on the electric grid at any given time, the effect gets lost in the grand scheme of things, and even for a single residence the overall power factor is likely to be close to 1, despite a few CFL bulbs. If every residential customer in the country replaced all incandescent bulbs with CFLs at once, this situation could change. Electric utilities, sensing an opportunity for a new revenue streams, may begin to charge residential customers for apparent power at some point in the future. The fact that CFLs have a power factor that is substantially smaller than 1 does not take away from the fact that they are a better choice than incandescent bulbs. Despite the fact that many a cradle-to-crave environmental impact analysis for CFLs on the web is based on assumptions of dubious origin, there is no doubt that their overall energy balance is superior to incandenscent light. See this very good summary on Wikipedia for an in-depth explanation of the power factor. Related Posts:The Great Cat Litter Poop OffTyreWiz not working after battery changeEnphase Envoy Local AccessAmazon threatens customer of 26 years Environment Geek's Home CFLCompact FluorecentEnergyGreenPower Factor
A question. I am not an engineer and would like to know if using a CFL will allow me to use a bulb of greater wattage since I understand the CFL draws less power. For example, a lamp fixture has a label that says not to use more than a 60W bulb. I assume that means an incandescent, which draws a certain voltage and generates a certain amount of heat. If I use a CFL, can I increase the size (wattage) of it since I understand it draws fewer volts? If so, is there a table that compares incandescent with CFL bulbs as it pertains to volts drawn for specific wattage values? Thanks.
Bruce, a 60W incandescent bulb is typically replaced with a 15W CFL bulb, and the Wattage limit is a non-issue. CFL bulbs usually have an incandescent-equivalent Wattage rating printed on the package – based on the light output. You can certainly replace a 75W incandescent bulb with a 42W CFL that has a 150W equiv. rating. The purpose of Wattage limits on fixtures is to limit the current (Amps, not Volts) on the wiring. Higher currents cause heat in wires due to resistance and are a fire hazard. As long as the actual Wattage (not the equiv-Wattage, which compares light output) of the bulb is below the rated Wattage of the fixture, you are fine (but factor in a 50% safety margin for the power factor).
Very good points on the power factor issue with CFL’s. It’s just one of many problems with them. But before we stock up on incandescent bulbs, we should concider this. Currently there are LED lamps on the market with power factors well above .95. Not all LED’s, you have to look for them. On average, LED’s use 1/6 the power of incandescent bulbs & 1/2 the power of fluorescent bulbs. They outlive incandescents by 25 to 50 times & fluorescents 4 to 8 times. Also LED’s contain no mercury, and emit zero infrared radiation or harmful EMF.
Power factor is an important consideration for the good of the power infrastructure, and is one of the many reasons why CFLs are simply a ‘bridge technology’. LED is improving rapidly, and I encourage anyone interested to check out Vu1’s ESL technology (.99 power factor). Hopefully in two years well have cost effective pleasent lighting options from both LED and ESL, and hopefully some other disruptive technology.
RE The fact that CFLs have a power factor that is substantially smaller than 1 does not take away from the fact that they are a better choice than incandescent bulbs. …there is no doubt that their overall energy balance is superior to incandenscent light. Hello Christian? Energy savings is not the only reason for choosing lighting, there is something called light quality, environmental safety etc too… Besides – and using official DOE and EU institutional etc references = hardly “dubious sources”, the overall switchover energy savings are seen to be small anyway, not just for power factor reasons.
CFLs don’t seem to be a good alternative when considering the sheer volume of environmentally un-friendly waste they can produce in comparison to a standard incandescent bulb (glass and a bit of tungsten are not likely to be too very detrimental to the environment), not to mention the mercury that may be present in CFLs (and all other waste products related to electronics (lead, plastics, etc.)). The LED bulbs of the future might be a bit less polluting (no mercury). As for power factor, is it not true that the reactance produced by CFLs is of a capacitive nature, thus helping to offset the total amount of inductive reactance that industry exerts on “the grid” (Hydro transmission lines)?
CFL reactive quality seems to destroy wall switches faster than incandescent bulbs. Five CFL bulbs on a bathroom vanity make a 15 amp wall switch arc internally while producing a sizzle sound.
Good explaintion. However, I see one mistake in the paragraphs below,the author wrote “Apparent Power”. However, it should be “Reactive Power”. “Apparent power is stored by inductors and capacitors in an electric circuit during each cycle, for instance in the electromagnetic field of a vaccum cleaner motor, and returned to the source in the same cycle (the generator in the power plant). The AC frequency in the United States is 60 Hz, so this cycle happens 60 times per second. Apparent power is not ‘consumed’ by appliances, and luckily, because it swings in and out of your house in each cycle, the net result on your residential electric meter is zero. Utilities do not charge residential customers for apparent power. However, apparent power requires additional current flowing across the grid, and thus creates distribution losses in transformers and power lines in the form of heat. Therefore, utilities have great interest in minimizing apparent power in the grid. They require industrial plants and other large consumers of electric power to take measures for compensating their power factor, or if that is not possible, will charge a penalty. While incandescent bulbs have a power factor close to 1, and can be considered an almost ideal load, the electric circuit in a compact fluorecent bulb (also called ballast) causes apparent power and lowers the power factor.”