LED Light Bulbs: Part 3

The discrepancy with the current and power that I showed in the first LED bulb post has been bugging me some so I decided to try to figure out what was causing it.  To help I purchased a Kill A Watt and did a lot of reading on the internet.  The Kill A Watt is a device that you plug into the wall and then plug something else into it.  The Kill A Watt will display electrical values such as voltage, current, power, power factor, and ,on the EZ model, the amount to run the item for a set time.  So, using the Kill A Watt I was able to get values for the current and power factor as well as figure out how much it would cost to run the various light bulbs constantly for a year and month. In the table below, V is Voltage, A is current in Amps, W is real power in Watts, VA is apparent power in Volt Amps, and PF is the power factor.   The power factor was about where I thought it’s be for the CFLs but not for the LED bulb.  This may be because the LED bulb isn’t a brand name bulb and why it was cheaper.  My power rate is 11 cents per kilowatt hour which is entered into the Kill A Watt to calculate cost.

$/Year $/Month V A W VA PF
LED $6.74 $0.55 123.0 0.1 7.8 11.2 0.69
CFL (13W) $11.56 $0.95 123.4 0.2 12.2 20.7 0.58
CFL (26W) $22.16 $1.82 122.2 0.3 23.8 41.0 0.58
40W Incan $36.61 $3.00 122.7 0.3 39.0 39.0 1.00
60W Incan $50.10 $4.11 122.2 0.4 54.5 54.5 1.00
100W Incan $95.39 $7.84 123.1 0.9 106.0 106.0 1.00

As you can see by the cost to run numbers the LED has a significant saving over the CFLs and Incandescents.  Of course you’re not going to run a light bulb constantly so the savings wouldn’t be less per year or month.   So, at the current price points based on the energy cost for lighting alone, the LED isn’t going to pay for itself against the CFLs until a couple of years later but will against the incandescents.  Of course the LED is rated to last 25K hours so you’ll get a lot of years out of it.

If you want to skip out on some finer points of multimeters stop reading here.  So, why were my current measurements wrong on the non incandescents?  It turns out that the LED and CFL bulbs have non-sinusoidal current plots unlike the incandescents.  This affects my multimeter’s ability to calculate current.   In a linear circuit such as the incandescent bulb, a plot of the current vs time looks like a sine wave.  For the CFL, and probably LED, the plot looks like a series of spikes.  Go here is you’d like to see some plots of the current of a CFL.  The problem is my multimeter.  I have a cheaper meter which assumes you’re measuring a sinusoidal wave. It takes an average of the current or peak value (it depends on the model) and multiplies it by a factor to get the RMS current.  Since the current is non-sinusoidal this assumption is incorrect and you end up with the wrong value.  There are more expensive multimeters out there that correctly measure the current.  They’re typically labeled “true RMS” and take numerous readings of the current vs time to correctly calculate the RMS current.  Hopefully, this will be the last light bulb post.  It probably won’t be the last time the Kill A Watt makes an appearance though.

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3 Responses to LED Light Bulbs: Part 3

  1. Tim Graydon says:

    Looks like I’ll need to loan you my Fluke “true RMS” meter. Always wondered what that really meant. Thanks for the lesson!

    • davidjbod says:

      Sweet! I wouldn’t mind using to see the difference. What model is it? I was looking at Flukes (that I can’t afford) yesterday. I promise not to drool on it too much.

      • Tim Graydon says:

        It’s a Fluke 335. Not sure if it will measure Ma. I use it for measuring AC voltage, amperage, and inrush current.

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