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03-17-2011, 09:40 AM   #1
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## For the math wizards

This is in regards to lighting, bulbs, and costs to run.

Is there a formula that you can use to calculate the cost of energy consumed for different bulbs?

The reason I ask is that I made another post about my recessed lights. Im installing 30 3" pot lights that use 55w halogen lights. Is there a way to figure the amount used per hour, and the cost of using them for a month. To make things easy, lets say they are all running for 1hr a day. If they are all turned on, that 1650 watts right? Ok so 1650 x what? Or am I totally wrong.

I wanted to see the difference in cost of using these bulbs versus something more energy efficient, and how long it would take to recover the initial cost of replacing the bulbs.

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03-17-2011, 09:47 AM   #2
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Divide by 1000 to convert units to kilowatts and then multiply by the days of the month they are running.

Mark

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03-17-2011, 09:55 AM   #3
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Quote:
 Originally Posted by Jackofall1 Divide by 1000 to convert units to kilowatts and then multiply by the days of the month they are running. Mark

So 1650/1000 =1.65 Then multiply that by the cost of 1 kilowatt hr, and then 30 for the number of days. Is this correct?
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 03-17-2011, 09:59 AM #4 Experienced     Join Date: Dec 2010 Location: Southern Michigan Posts: 2,822 Rewards Points: 2,000 You are correct for your example, as the untility company invoices power at rate per kilowatt. I would multiply by hours not days, that way if you change your utilization the units will be correct. __________________ When its all said and done there is usually more said than done
03-17-2011, 10:27 AM   #5
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Quote:
 Originally Posted by Jackofall1 You are correct for your example, as the untility company invoices power at rate per kilowatt. I would multiply by hours not days, that way if you change your utilization the units will be correct.
Right, thanks. I was just trying to come up with a per hour cost. So I could figure 1hr per day, and find the savings of standard lights VS energy efficient ones.

Here goes on my math

1650W total for 30 55w halogen bulbs /1000 = 1.65 x cost of 1 hr running .10 x 30 days = \$4.95 for 30hrs.

30x4W LED bulbs = 120W 120W/1000 = .12 x cost of 1hr running .10 = .012 x 30 days = \$.36 for 30hrs

That cant be right
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 03-17-2011, 10:40 AM #6 Member   Join Date: Feb 2011 Location: Southern California Posts: 71 Rewards Points: 75 Let me give you an example. I installed 12 PAR30 recessed cans for my kitchen/dinning area. The one window and sliding glass door lead to a enclosed patio, so there is not a lot of natural light in this area. As a result, these lights are pretty much on from when we get up to when we go to bed, subtract times when we are not home. I figure an good number is 10hrs a day. Normal bulb for these cans is a 65W PAR30 reflector flood light. The CFL replacement is 15W 12x65=780W or 0.78kW x 10hrs/day = 7.8kWh/day x 30days = 234kWh/month 12x15=180W or 0.18kW x 10hrs/day = 1.8kWh/day x 30days = 54kWh/month So with the CFLs I save around 234-54 = 180kWh/month I live is "deregulated" SoCal Edison land. During most of the year, my electricity costs around 0.23-0.24 per kWh. So that 180kWh per month saves me about \$42 a month. At \$7 each x 12 = \$84 (when I bought them) the CFLs paid for themselves in 84/42 = two months. For me is was a no brainer. I did not like the color of the "Soft white" so I went with the 3500K lamps. Originally I had the GE Incandecent bulbs, but after realizing how much I used the lights, and the first incandecent bulbs started burning out after 7 months, I replaced 4, and 6 months later replaced the rest. I have had them for over 2 years now and they all work. My only complaint is the warm up time. It take 30 seconds to reach a usable brightness and about 60-90 to reach full brightness. But it doesn't really bother me.
 The Following User Says Thank You to FixitDragon For This Useful Post: NCpaint1 (03-17-2011)
03-17-2011, 10:46 AM   #7
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Quote:
 Originally Posted by NCpaint1 Right, thanks. I was just trying to come up with a per hour cost. So I could figure 1hr per day, and find the savings of standard lights VS energy efficient ones. Here goes on my math 1650W total for 30 55w halogen bulbs /1000 = 1.65 x cost of 1 hr running .10 x 30 days = \$4.95 for 30hrs. 30x4W LED bulbs = 120W 120W/1000 = .12 x cost of 1hr running .10 = .012 x 30 days = \$.36 for 30hrs That cant be right
Oh yes it can! The LED you are looking at uses only just over 7% of the power the halogens do. You may not like the color of the LED bulbs though. Check the CFL options, they have gotten pretty good in the last few years. The bright white is around 3500K, which is similar to the color temperature of halogens. If you don't like the big box store offerings, check a lighting store or online. Even if you get a 12 or 13W CFL, you will be only using about 22% of the power

 03-17-2011, 10:47 AM #8 Experienced     Join Date: Dec 2010 Location: Southern Michigan Posts: 2,822 Rewards Points: 2,000 Simply put NCpaint1 you are correct and yes there is quite a bit of savings using the LED's you are talking about. Mark __________________ When its all said and done there is usually more said than done
03-17-2011, 10:51 AM   #9
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Quote:
 Originally Posted by NCpaint1 Right, thanks. I was just trying to come up with a per hour cost. So I could figure 1hr per day, and find the savings of standard lights VS energy efficient ones. Here goes on my math 1650W total for 30 55w halogen bulbs /1000 = 1.65 x cost of 1 hr running .10 x 30 days = \$4.95 for 30hrs. 30x4W LED bulbs = 120W 120W/1000 = .12 x cost of 1hr running .10 = .012 x 30 days = \$.36 for 30hrs That cant be right
Given your example of a savings of \$4.59 / mnth now you have to figure what the pay back time is differencial cost divided by differential cost of operating.

Mark
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 03-17-2011, 10:54 AM #10 Coatings Nerd   Join Date: Aug 2009 Location: Michigan Posts: 364 Rewards Points: 250 Thanks Fixit Being that this is a basement I dont think they will be used 10hrs a day. Im all about being efficient and saving money, but only when it makes sense. My whole house has CFL's, with the exception of my basement now. Im just trying to justify the cost of the LED's at almost \$30 each. Unfortunately, in this style bulb LED's are about the only option. Crappy ones run around \$10/each. Good ones closer to \$30/each \$30x30 lights = \$900 +/- Even if I ran the lights for 3 hours a day, to recoup the cost would take years 55W x 30=1650 or 1.65kW x 3hrs a day = 4.95 x 30 days =148.5kW per month @ \$.10 = \$14.85 4W x 30 =120 or .12kW x 3hrs a day = .36 x 30 days = 10.8kW per month @ \$.10 = \$1.08 for a savings of \$13.77 To recoup my investment will take 65 months or 5 years and 5 months. I dont think that makes sense, at least in my case __________________ www.northcromwellpaint.com Benjamin Moore, Sikkens, TWP, PPG, Airless sales and service Last edited by NCpaint1; 03-17-2011 at 10:56 AM.
 03-17-2011, 12:11 PM #11 Member   Join Date: Feb 2011 Location: Southern California Posts: 71 Rewards Points: 75 You are right, in your case it does not make sense. There are areas of my home I don't use CFLs (like my bathrooms), even with the high cost of electricity, because the lights are not on very much and are frequently turned on and off which shortens the life of CFLs.
 03-17-2011, 12:44 PM #12 the Musigician     Join Date: Jul 2008 Location: I'm right here! Posts: 10,404 Rewards Points: 2,000 Just the title of this one leaves me out.... 1+1=4 I can't even figure out how much electric (approx.) it costs to heat a 50 gallon tank of water! Glad someone could help though! DM __________________ [SIGPIC][/SIGPIC]Click here to see some of my original magic tricks and trick boxes!
 03-17-2011, 01:04 PM #13 Coatings Nerd   Join Date: Aug 2009 Location: Michigan Posts: 364 Rewards Points: 250 I re-read the thread and I sure sound like a cheap a\$\$. __________________ www.northcromwellpaint.com Benjamin Moore, Sikkens, TWP, PPG, Airless sales and service
03-17-2011, 01:57 PM   #14
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Quote:
 Originally Posted by NCpaint1 I re-read the thread and I sure sound like a cheap a\$\$.
Nothing wrong with first, knowing what something costs to run and then second, trying to figure out a way to save some money. It's when you spend \$5 to save \$3 that gets me and a lot of energy saving systems do just that.

 The Following User Says Thank You to nap For This Useful Post: DangerMouse (03-17-2011)
03-17-2011, 04:21 PM   #15
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Quote:
 Originally Posted by nap Nothing wrong with first, knowing what something costs to run and then second, trying to figure out a way to save some money. It's when you spend \$5 to save \$3 that gets me and a lot of energy saving systems do just that.
In this case I think so. The only real alternative to the standard halogen in this case is LED. At \$30 a piece, its too much, and the numbers prove it. I will use them for my landscape flood lights. There will only be a few, and they'll be turned on for 8 hours or so each night.

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