I’m not an electrical engineer so my analysis of this topic might have some slight misconceptions. Anyone who is more knowledgeable on the subject of power factors and alternating current is welcomed to submit information via the comments.
If you are an energy conscious consumer, you likely take note of the key parameters printed on the packaging of the light bulbs you buy. Lumens and Watts are probably the two biggest factors that most people pay attention to with lumens per Watt being a helpful guide of the overall efficiency of the bulb. But are lumens and Watts the whole story? Is there a hidden factor that makes the bulb use more energy than the packaging would have you believe? You are not alone if you think that an LED bulb that says it uses 13 Watts, uses 13 Watts. There is another factor that has to be considered, power factor.
Power factor (to paraphrase Wikipedia and other sources) is the ratio between real power and apparent power. Real power is how much power the load is actually using and apparent power is the product of how much current is flowing and the Voltage. Real power is measured in Watts, and apparent power is measured in Volt Amps (VA). If you think back to when you learned about electricity fundamentals, you might remember that current (Amps) times Voltage equals Watts. This is true for a direct current system and it is also true for an alternating current system with a power factor of 1.0. An old fashioned incandescent light bulb, due to being a purely resistive load, has a power factor of 1.0. However, many electrical devices including LED and CFL light bulbs have power factors less than 1.0. What this means is that while they might be rated at 13 Watts real power, they will draw more current than you would suspect. Assuming a 120V system, 13 Watts should require approximately .11 Amps (120V*.11A = ~13W). In reality, a typical LED light bulb has a power factor of .6 (and as low as .2 or less!). This means that the apparent power of the bulb is more like ~22VA (13Watt/.6 power factor). Remembering that apparent power is current * volts you can find that with a .6 power factor this bulb is using ~.18 Amps.
The good news is that residential power meters typically measure real power and not apparent power…for now. This means that you are charged by the kWh and not Volt Amp hour. However, utilities are now beginning to install “smart meters” that are capable of measuring your home’s overall power factor (the average power factor of all connected loads) and charging you accordingly. For now, it seems like you are in the clear and you can trust the numbers printed on your bulb’s packaging.
Okay, so this means that LED bulbs are good for your wallet, but are they good for the environment? The short answer is yes. They consume up to 85% less energy than an equivalent incandescent light bulb and contain no hazardous mercury like their compact fluorescent ancestors. The long answer is that power factors less than 1.0 do cause a greater demand on the power grid which results in the power plant having to burn more fuel to supply the load. To use an exaggerated example, if a 1 kilowatt load has a power factor of .2, then it will take 5 kilovolt-amps (1/.2) to supply the needed power. This means that five times the current must flow through distribution lines, generators, and transformers. If a transformer is 98% efficient, it is now 98% efficient with five times the current…in other words, more wasted energy.
So that is the back story on power factors. I decided to sit down with my LED bulbs and see how efficient they really were. I built a test setup with a dimmer so that I could test the bulbs at varying brightness levels. Unsurprisingly, the power factor of LED bulbs is less than 1.0. At full brightness, my two Utilitech 60W equivalent bulbs have a power factor of approximately 0.6. My Philips Ambient 75W equivalent also has a full brightness power factor of about 0.6. Winning the power factor competition, the Ecosmart 60W equivalent LED has a full brightness power factor of about 0.9.
Notice that I stated the full brightness power factor of these bulbs. As I began to dim them, the power factor drops lower and lower as the knob is turned toward dim. At full dim, most of the bulbs have power factors of around 0.2. Now here is the interesting part; the bulbs do progressively consume fewer Watts (real power) as they are dimmed, however, the apparent power actually begins to go UP as the bulbs are dimmed. In my tests, I found that dimming the bulbs to about 2/3 brightness actually resulted in a higher overall current flow. This is due to the power factor decreasing at a greater rate than the decrease in current flow. Very interesting! Once again though, consumers are charged real power and therefore you will save money by dimming your LED bulbs. If your utility company ever switches to apparent power billing, then it will actually cost less to run your lights at full brightness than somewhat dimmed.
The below video is of me experimenting with several of my LED bulbs. It is about 12 minutes long, but I think that you will learn quite a bit about how these bulbs behave while on a dimmer. Click here for 1080P version.