Brinson
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#31 id:37781 Posted Nov 18, 2009, 8:55 pm
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quixadhal said:I just wish the daylight bulbs would come down in price. I like the clean white light (as opposed to the pink "soft white" or the yellowish "normal" ones). Now, if they could get LED bulbs to output a decent light level, that would be ideal. So far, they're pretty expensive and only useful for things like flashlights or night lights.
Another thing CFL's have going for them is reduced air conditioning costs in warmer areas (or summertime). I don't have any scientific way to study it, but consider how much heat a 60W bulb puts out compared to the 15W CFL equivalent. Now, count how many lights you have in your house. Nice in the winter around here, but not so good in the summer! :)
quixadhal said:David Haley said:
quixadhal said:However, I can buy a 6 pack of 60W light bulbs for $1, whereas finding CFL's for less than $2 each is almost impossible.
Here's an 8-pack for $12.50, coming out to $1.56 per bulb.
quixadhal said:So, does the power savings from using 15W vs. 60W offset the cost of replacing the bulbs? Especialy since they do NOT last 12 times longer, at least not in my experience.
Where does this 12 figure come from? Using your figures of $1 vs. $2, you only need it to last at least twice as long... The link I gave above claims it lasts up to 8 times longer, so even if you're very conservative and change that to 2 times longer the two bulb types are equivalent when it comes to dollars spent on the bulb per hour of usage.
That's $1 for a 6-pack of normal bulbs (at the dollar store) vs. $2 PER CFL bulb.
You found them for $1.60, which is better... but until I can get them for about $0.50 each, I probably won't feel compelled to switch everything. I do like the "daylight" CFL bulbs, but those are even more expensive, clocking in at around $3 each unless they're on sale. I would guess my CFL's have lasted about twice as long as incandescents.
Even if you pay $20 a bulb you'll stil see savings. The bulbs are a 75% drop in power consumption. 23 watt CFL for 100 watt normal. If you run 2 bulbs 8 hours a day you are using 48 kwh. Over $5 a month in power. After a year, that's over $60.
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David Haley
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#32 id:37782 Posted Nov 18, 2009, 9:41 pm
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No, bulb price is very important, and you certainly aren't saving if you are spending $20 per bulb. You have to take into account how often you're replacing the bulbs -- you might be saving $60 a year in power, but if you're spending $20 per bulb and replacing it three times a year, you've just spent $60 on bulbs.
To do the math right, you need to compute the total price per hour, which includes the price of the bulb itself per hour -- you can get this by dividing the bulb's price by the average lifetime of the bulb in hours.
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Lobotomy
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#33 id:37783 Posted Nov 18, 2009, 10:13 pm
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David Haley said:No, bulb price is very important, and you certainly aren't saving if you are spending $20 per bulb. You have to take into account how often you're replacing the bulbs -- you might be saving $60 a year in power, but if you're spending $20 per bulb and replacing it three times a year, you've just spent $60 on bulbs.
To do the math right, you need to compute the total price per hour, which includes the price of the bulb itself per hour -- you can get this by dividing the bulb's price by the average lifetime of the bulb in hours.
The CFL's I use in my apartment I bought almost three years ago now. They cost $1 each, put out 100W light for ~23-25W of energy, get used quite often, and still have yet to burn out or even noticably diminish in light. If someone out there is somehow burning out CFL's three times a year then they're either incredibly unlucky, or they just flat out didn't read and/or follow the usage instructions.
From the Wikipedia page for CFL's:
Wikipedia said:The life of a CFL is significantly shorter if it is only turned on for a few minutes at a time: In the case of a 5-minute on/off cycle the lifespan of a CFL can be up to 85% shorter, reducing its lifespan to "close to that of incandescent light bulbs". The US Energy Star program suggests that fluorescent lamps be left on when leaving a room for less than 15 minutes to mitigate this problem.
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Scandum
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#35 id:37785 Posted Nov 18, 2009, 11:24 pm
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Tyche said:
I was always afraid of using laptops for any extended period of time.
Good point, in my experience they run less hot if you take out the battery. Not sure if notebooks using an atomic processor have heat issues.
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Runter
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#36 id:37788 Posted Nov 19, 2009, 1:11 am
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David Haley said:There's something somewhat amusing when you recommend leaving an energy-saving device on in order to make it cost less.
Yeah, I was going to comment that I've had some CFL's for about that long as well. I also don't turn off lights if I intend on returning to the room within a reasonable amount of time for that reason. (Usually within 15-30 minutes.) In fact, from my research, if you're constantly turning them on and off as you leave rooms (provided you're getting up for rest room or a drink often) then it can lower the life of the light to the point where it's rather pointless from a cost saving perspective.
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Brinson
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#37 id:37789 Posted Nov 19, 2009, 1:52 am
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David Haley said:No, bulb price is very important, and you certainly aren't saving if you are spending $20 per bulb. You have to take into account how often you're replacing the bulbs -- you might be saving $60 a year in power, but if you're spending $20 per bulb and replacing it three times a year, you've just spent $60 on bulbs.
To do the math right, you need to compute the total price per hour, which includes the price of the bulb itself per hour -- you can get this by dividing the bulb's price by the average lifetime of the bulb in hours.
Even running 24/7 no CFL should fail in under a year.
If you pay $20 per bulb and replace them every year, which is abserd given most bulbs have 5+ year warranties, you still save money. I did the math prior to making that post.
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Mudder
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#38 id:37791 Posted Nov 19, 2009, 2:39 am
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We should all be clear: Turning light bulbs on and off limiting the life of a bulb is not limited to CFL. In fact I didn't actually know it applied to CFLs until now. It definitely shortens the life of incandescent bulbs though. It's actually what causes them to wear out in the first place.
Quix: Not to sound like a jerk... But $3 a bulb is nothing, especially when it will actually save you money. I've never had a CFL wear out yet, I replaced all of mine two years ago.
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David Haley
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#39 id:37802 Posted Nov 19, 2009, 7:59 am
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Brinson said:Even running 24/7 no CFL should fail in under a year.
If you pay $20 per bulb and replace them every year, which is abserd given most bulbs have 5+ year warranties, you still save money. I did the math prior to making that post.
I think we're agreeing... I said that bulb price is a very important factor, but obviously so is relative lifetime. As I said, correct math must take into account the total price per hour, which includes the lifetime of the bulb and its price. As a side note, it would generally be helpful to show your math if we're talking precise numbers, if anything so that we're all on the same page.
That said, your experience obviously isn't the same as Quix's when it comes to bulb lifetime, so he certainly would not be saving any money at all at $20/bulb. As others have said, though, it's quite possible that he's doing something wrong with them.
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Brinson
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#40 id:37804 Posted Nov 19, 2009, 9:40 am
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I've taken 23 watt CFL and compared them to 100 watt standard bulbs and plugged it all into a spreadsheet. I created cells for bulb price, power cost, number of bulbs bought per year, and average hours they are on per day. Before making the post I set bulb cost to $20, and most reasonable values for everything else still returned a savings each year. The savings becomes less as bulb number in house increase, and as hours of use go down. At the crazy $20 price point, one or two bulbs on 6 hours a day is still a profit at the average US electric cost. If you have three bulbs you'd need to use them at least 6 hours a day to see profit, on average.
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David Haley
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#41 id:37805 Posted Nov 19, 2009, 9:49 am
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Brinson said:The savings becomes less as bulb number in house increase
Why? Presumably the savings are per-bulb, not per-household, assuming equal usage of all bulbs (and that you would use CFLs and incandescent bulbs the same way).
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Brinson
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#44 id:37808 Posted Nov 19, 2009, 11:56 am
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Both formulas are linear, for CFL and for normal.
The linear formulas have different slopes, therefore the difference in any two arbitrary values does not correlate in a linear fashion.
Basically, while they both are linear, the plot of one minus the other would not be linear, because the difference changes.
y = Cost of Normal Per Year - Cost of CFL per Year
And each one is Cost of Electricity + Cost of Bulb.
Basically (((365xNORMALWATTSxHOURS)/1000 * NORMALBULBS * ELECTRICCOST) + (NORMALPRICE * NORMALBULBS)) - (((365*CFLWATTSxHOURS)/1000 * CFLBULBS * ELECTRICCOST) + (CFLPRICE * CFLBULBS)) is how its entered into excell, with my #### in caps as cells. That's not a copy and paste, might be a mistake somewhere in my typing, but its how I'm calculating.
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Last edited Nov 19, 2009, 11:59 am by Brinson
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David Haley
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#45 id:37809 Posted Nov 19, 2009, 12:10 pm
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Brinson said:Basically, while they both are linear, the plot of one minus the other would not be linear, because the difference changes.
This is, well, mathematically incorrect.
Let E1 and E2 be two linear equations for lines in the plane. Therefore,
E1 is of the form: y = m1 * x + c1
and
E2 is of the form: y = m2 * x + c2
If we take the difference, E2 - E1, we end up with:
y = (m2*x + c2) - (m1*x + c1)
which is:
y = (m2 - m1) * x + (c2 - c1)
which is the form of a linear equation.
When you have two lines, even with different slopes, you can easily compute the (linear) slope of their difference: it is given above as m2 - m1 (in other words, the slope of the difference is the difference of the slopes).
The point of all this is simply to state that the savings per bulb will not diminish as you add more bulbs, because the price per hour remains constant per bulb and therefore the difference in price per bulb will also remain constant.
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