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calculating cost of operation

Posted by w0lley32 (My Page) on
Wed, May 29, 13 at 12:01

Hi! last Christmas, I installed 8430 watts (yes, 8.43 kW) of Christmas lights at my mother's house, and would like to pay her for the power used. She pays $0.0751 per kWh, the lights were on 8 hours/night for 50 nights, so I calculated that I owe her $253.24. She claims that I miscalculated, she thinks it can't be that high.

Can someone please tell me, what would the correct price be?


Follow-Up Postings:

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RE: calculating cost of operation

If all those numbers are right, the rate is right as well. Are you sure you calculated the wattage right? Even the old style large incandescent that would be over 1600 bulbs.


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RE: calculating cost of operation

Thanks for answering. Yes those numbers are right. There is a 50-amp double pole breaker labeled "Christmas lights" feeding an 8-circuit subpanel with 3887 bulbs of various sizes. All incandescent.


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RE: calculating cost of operation

Of course if they aren't all on at the same time, you'll use less power. The size of the breaker has nothing to do with the power used.


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RE: calculating cost of operation

For a resistive load like incandescent bulbs you can use a clamp on ammeter.

You are left with the problem of how to compute varying loads though.

You might be able to get your hands on an actual watt-hour meter and wire your lights through it.

Even a watt meter will have the same issue as an ammeter.

It only tells instantaneous usage, (watts) and not cumulative usage (watt-hours).


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RE: calculating cost of operation

The lights are all on at the same time, from 5 PM to 1 AM, November 20th to January 6.
Here are the wattages of each tree, measured with a Fluke clamp meter:
First tree, 1078.5 Watts + 1018 Watts, on a "canadian kitchen countertop" outlet
Second tree, 1187.5 watts
3rd tree, 1150 Watts
4th tree, 1180 Watts
5) arrangement of mini-lights, 1376 Watts
6) another arrangement of C7 and mini lights, 1439.8 W

The measured watts are consistent with the watts calculated by adding up the nominal wattage of the bulbs written on the box within 8-10%, so I am pretty confident that my ammeter reads right.

And no, the strings are not all plugged end to end. I have an array of 16-gauge and 14-gauge extension cords made specially for that purpose by a now deceased electrician friend of my late father, all plugged in heavier 12-gauge SJOW extension cords going back to an array of receptacles in the garden shed. No section of Christmas string carries more than 4 amps, 2 in the case of mini-strings. All strings are 18 AWG PXWT wire.


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RE: calculating cost of operation

Why don't you just show your mom the bills for the months leading up to and after the period where you had the lights on so that she can see the jump in her electrical bill during that time?
I imagine that should be enough to convince her that your calculations are correct.

Out of curiousity, is this a huge house or are you a Clark Griswold-type person?


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RE: calculating cost of operation

She is on an equal-installment plan, so that she pays the same amount year-round based on her consumption of the previous year, and she has a bi-energy heat pump/oil heating system and the Christmas period overlaps the peak heating period, making it hard to evaluate Christmas lights consumption.

"Out of curiousity, is this a huge house or are you a Clark Griswold-type person?"
It's a standard 1970's ranch style house on a big lot with lots of mature trees. And I am a Clark Griswold-type person.

I started doing this setup in the early 90's when I was a teenager, and kept adding up every year until around the early 00's.

Unfortunately, except for a 2400-watt fir tree in my backyard, I don't have enough mature trees on my property to match my mother's setup, which is why I keep doing this at my mother's house, mostly for my own enjoyment, but also for the benefits of others. Cars on the street stop by and many people ring her bell asking if they can take pictures.


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RE: calculating cost of operation

"Here are the wattages of each tree, measured with a Fluke clamp meter: "

Did you already multiply 120 V times amps?
Simple meters cannot measure watts.

They measure voltage or with a clamp on current.

Someone (or something) multiplied amps and volts to get watts.


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RE: calculating cost of operation

I indeed measured currents, then multiplied them by 122.4 which is the voltage I measured between the two slots of one of the receptacles.


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RE: calculating cost of operation

So divide by 1,000 to get kW-hours and multiply by her rate.

She is already paying a connection charge, and using the typically higher priced first kW-hrs.


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RE: calculating cost of operation

Actually, divide the total by 1000 to get kW, then multiply by the number of hours active to get kW-hours. If you have a tiered rate, or one that changes by winter/summer, then take that into account.


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RE: calculating cost of operation

"a 2400-watt fir tree in my backyard"

Griswold, your the only person I know who measures trees in watts!

Brickeyee, I'm surprised you aren't picking him apart for code violations in the garden shed. :-)


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RE: calculating cost of operation

Using cords for temporary displays is allowed.

If you want a general idea of the requirements look at the carnival rules (though some of that covers generators).

As long as the whole thing is limited in duration and uses the correct size conductors and cordage it is unlikely an AHJ is going to say much.


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