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Calculation of LED energy use versus incandescent Christmas lights



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I remember the Christmas tree when I was younger. I had these big colored bulbs, they were nice, but you could not touch them. Yes, these were the incandescent lights of the old style, and they could get very bad. They were essentially smaller versions for the bulbs that you would put in your lamp.

Today, many people still use a type of incandescent Christmas light. They are much smaller, but also much more efficient, and they do not get so hot. But now we have another option of lighting, the light emitting diode or LED. The LED is different from an incandescent in that it does not use a high filament temperature to create light. Instead, there is a diode that produces light when the electrons make an energy transition within the device.

Are LED lights better than incandescent lights? Let's see three comparisons between LED and incandescent lights.

What uses more power?

Energy is not free. In the United States, the average price of electricity is 1

3.3 cents per kilowatt-hour. But be careful, do not confuse power with energy. When you pay for your electricity, you are paying for energy. Power is a measure of how fast you use energy. It can be calculated as:

If you use energy in units of Joules and time in seconds, you will get power in units of Watts. But what about the kilowatt-hour unit? That's the energy you get from using 1,000 watts for 1 hour (or 3,600 seconds). Using the previous power equation, I can solve the energy as a power multiplied by time. With 1,000 Watts for 3600 seconds, that would be 3.6 million Joules.

Now to measure the power. I'm going to use some kind of power meter; The basic idea is to connect a device and report the power it consumes. Here are the two sets of lights that I will try with the LED lights on the left.

Okay, there are two differences between these lights (other than LED vs. incandescent). One set of lights has a white wire and the other is green, but, of course, that does not matter. However, the incandescent is a chain of 100 lights and the LED is only 50. Therefore, I can not simply compare the power of the two filaments. Instead I will get the power by light.

For incandescents, I get a power of 20.0 watts or 0.2 watts per light. The LED cable is 2.4 watts or 0.048 watts per light. Clearly, incandescents use more power. But is it enough to make a difference in your wallet?

Let's say you have a Christmas tree with 500 lights (or maybe even more if you live in my house). If you run these lights for 5 hours at 13.3 cents per kilowatt hour, the LED lights would cost 1.6 cents and the incandescent ones would cost 6.6 cents. So, that's not so bad just for a tree.

Just for fun, here's a nice picture that shows why LED uses less energy. This is an infrared image so you can see which set of lights gets hot.

Notice that the lights on the right (the LEDs) are much colder. Actually, there's a hot spot there: I'm not exactly sure what it is, but it's not an LED bulb.

Which one is brighter?

Well, the LED uses less energy. But are they as bright as incandescents? You could make a visual comparison, but I like to take it to the next level. The brightness of a light can be measured with a photocell, but I will use this light sensor that can register the intensity as a function of time. I also included a rotation sensor to measure the intensity of light at different angles with each light. Here is my configuration.

The idea is to rotate the light so that the sensor obtains data from the top and side of the bulb. Here is the output of the incandescent bulb.

Note that for this bulb, it is actually brighter on the sides than on the top (0 radians is the top of the bulb). You can probably calculate the total light output, but you would have to do some mathematical things.

Now for LED light.

The LED is somehow brighter. In the upper part of the LED, it exceeds 70 lux, where the incandescent peaks are approximately 15 lux. Once you get away from the top of the LED, you get very little light. But wait! There is another very important difference. The LED is only brighter for part of the time. This is because the LED light is not always on. In fact, the LED light turns on and off. Here is a graph of brightness versus time for both lights in a time interval of 0.1.

In this time scale, you can clearly see that the LED lights (the blue curve) turn off and on again. This is due to the "D" LED, the diode. A diode only allows the current to move in one direction through the light, but the electrical outlet produces alternating current (AC). Of course, you can solve this by placing an AC to DC converter in the light sequence, but that will increase both the cost of the lights and the power they use.

(If you have epilepsy or are sensitive to vision stimuli, stop moving now to avoid an intermittent GIF.)

Here is a simple experiment you can try yourself. Take out your phone and make a slow motion video of some Christmas LED lights. You will probably see something like this.

Yes, that's super annoying in slow motion, but it looks like you can still detect the flicker without the camera. I think I'll just keep the incandescent bulbs for now. I could say that LED lights are better, but we can all agree that incandescents are better than putting candles on your tree.

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