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[bkenobi]

I have a wireless meter reader that I'm installing on my outdoor meter to automatically report electric usage.  In order to use it, I have to apply a Kh value to multiply the rotation rate of the spinning wheel such that I record the actual energy used.  The calculation is based off a guide published on a grad student's old web page:

http://staff.washington.edu/corey/power.html

Long story short, I have a Kill-A-Watt energy meter and a 1000W halogen shop lamp that I used to calculate the Kh factor. The lamp is 2 500W halogen tube bulbs but when I use the Kill-A-Watt it indicates that the draw is actually 824W.  I didn't expect the reading to be exactly 1000W, but I didn't expect that I'd see 20% less than the bulb's rating.

Does anyone know if that seems reasonable?  I know that bulbs have a large inrush/surge current when initially turned on, but I don't know if that's what the bulb's indicated wattage is based on or if it's steady state.  I need to get an accurate measurement of some kind of load so I can calculate the Kh for my purposes.

The other issue is that the utility company is using a multiplier of 40 to convert the number wheels to electric usage.  However, when I run my calculations, I find that the actual multiplier is closer to 29.  I don't know enough to be able to intelligently ask them if I'm being ripped off yet, but that would be something I want to confirm.   

[dhouston]

When the Kill-A-Watt was introduced, I bought one and gave it a very favorable review on comp.home.automation. Back then, I was in communication with an engineer involved with the company that makes it. However, it has been a long time and my memory for details has dimmed as much as has your halogen bulb (perhaps?).

Are you measuring watts or kWh? I believe the watts reading is instantaneous so inrush should not affect later readings.

I don't know whether halogen bulbs dim with age or how, if they do dim, that may affect measured wattage. I'm afraid that's beyond my expertise - perhaps others will know.

[dave w]

The lamp is 2 500W halogen tube bulbs but when I use the Kill-A-Watt it indicates that the draw is actually 824W.  I didn't expect the reading to be exactly 1000W, but I didn't expect that I'd see 20% less than the bulb's rating.

Wattage is voltage times amps.  Likely the manufacturer rated the 500W at 120 volts. The original packaging should give a voltage for the bulb, and that voltage should be what the manufacturer rated the wattage at. The lamps wattage actually drawn is controlled by voltage applied. So with the light on, measure voltage at the socket. If less than 120V, the wattage actually drawn will be less. Also factor in some slop for the Kill-A-Watt and a slop factor from the manufacturer. I.E. "Well we designed the filament for X resistance but sometimes we get nichrome wire with a little more" etc.  Some manufacturers will pass and sell the product figuring who will know.

[bkenobi]

I'll check the voltage and see if I have a split extension cord so I can measure current with my clamp meter.  That way I can have a second reading to back up or dispute the Kill-A-Watt.  The lamp is a few years old, but I've only used it for a handful of hours so I wouldn't expect the bulb to have dimmed yet.  I'm still on the original bulbs, so I don't have a package to say whether they should be marked 110/120/130/etc volts.

[bkenobi]

I checked wattage and voltage of the shop light again.  There were no markings on the light or bulbs, so I simply know it's supposed to be 500W T3 halogen bulbs.  The Kill-A-Watt indicated around 915W. The voltage at my house was 119V at that time.  The current was 7.68A with the KAW and close to the same with the clamp meter.  So, it appears that the light is really using less than 1000W.   I wonder if the bulbs are 130V since that would be 998W.  Not sure, but sounds possible.

[dhouston]

I wonder if the bulbs are 130V since that would be 998W.

That's likely. The 130V rating should give a very long life. I bought a case of 130V/100W incandescents before they were banned. I only use one incandescent these days (on a dimmer module) and it's now about 5-1/2 years old so my case will likely last many, many years longer that I will.

[bkenobi]

I'm satisfied with that.  But, that does bring up a different question.  I recall at least a couple threads on this forum asking about halogen bulb life when used on X10 dimmers and how it can be dramatically shortened due to buzzing caused by the module.  Is there a difference between x10 dimming (triac), a standard dimmer, and lower voltage?  If I remember correctly, the triac cuts the waveform off.  A lower supply voltage would obviously have a perfect sine wave.  Does a normal dimmer cut off the wave form or scale it somehow?

[dave w]

I'm satisfied with that.  But, that does bring up a different question.  I recall at least a couple threads on this forum asking about halogen bulb life when used on X10 dimmers and how it can be dramatically shortened due to buzzing caused by the module.  Is there a difference between x10 dimming (triac), a standard dimmer, and lower voltage?  If I remember correctly, the triac cuts the waveform off.  A lower supply voltage would obviously have a perfect sine wave.  Does a normal dimmer cut off the wave form or scale it somehow?

FWIW
All consumer or "home" (wall box size) dimmers are triac based and truncate the sine wave to dim the bulb. A rheostat dimmer actually reduces the amplitude of the sine without distorting the sine. They act like a series variable resistor and dissipate the power not going to the bulb as heat. I know there are applications where rheostats or "variacts" (auto-transformers) are used in light dimming, but it is industrial and Hollywood applications.

As far as dimmed halogen bulb life I can only repeat what a GE lighting engineer, who worked at the big lighting division in Ohio, told me, and that was: because dimming reduces the temperature of the filament and reducing the temperature extends filament life, that dimming a halogen will not adversely effect halogen bulb life. But his assumption was that the amount of dimming was enough to be very visible and not a tiny bit.