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I'm redoing all the electrical in my entire house, and I was running some numbers just for fun on the energy efficiency gains by just having LED, vs incandescent bulbs. I know the basic LEDs use much less power for the same number of lumens, but I noticed that almost no one takes in to account the lower cooling costs. So I ran some quick numbers and wanted to make sure my math was correct.

I'm in a hot climate so that's why I'm interested in the numbers for cooling.

Say my baseline is 1000 watts of incandescent. I move to LED lighting and we'll say the LED lighting uses only 15% of the energy for the same lumens. So now I'm at 150 watts. So I saved 850 watts of heat, and from reading online comes out to ~3.4 BTUs/watt, so I should have saved ~2,890 BTUs which is close to a 1/4 of a ton of cooling capacity that I saved. Assuming I have a SEER 16 AC I saved roughly another ~180 watts in cooling as well.

So it would appear that the total energy savings would be roughly 850+180=1030 watts.

I realize the actual energy usage has many more variables, but I just wanted to make sure that my calculations for cooling energy requirements were generally correct, and that I wasn't missing anything major.

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    Calculations like this are a fool's errand. First off, you're working with "imaginary" numbers. Second, these numbers are based on all lights being on, all the time. Third, it doesn't take into account any heat generated by current flowing through the wiring. Etc. There's far too many variables to get an accurate number. In the end, it's easier to simply say. "By switching to LED, you could reduce your lighting load by about 85%".
    – Tester101
    Commented Jul 19, 2015 at 13:35
  • I think you've forgotten to take the efficiency of the heat pump (air conditioner) into account -- your A/C uses less than 1 watt of power to move 3.4 BTUs of heat. Based on 1000W of lights turned on for one hour, an A/C with a SEER of 10 will need 100Wh to remove that energy, versus 15Wh with LED's. So your savings is (1100Wh - 165Wh) = 935W. Also, don't forget to take into account heating - if you heat with (relatively inexpensive gas), your 1000W bulbs are using more expensive electricity to heat the room than LED's would.
    – Johnny
    Commented Jul 20, 2015 at 5:20

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That is fundamentally correct for the time the lights are on, yes. It will depend on the specific LEDs and ballasts chosen (the ballast may or may not be part of the fixture, and some cheesy outfits try to give the lumens per watt INTO the LED itself, ignoring the watts wasted in the ballast converting from AC to DC.)

@Tester101's "wire heating" strawman (in comment) would still be affected in the desirable direction (less amperage draw for same light = less wire heating), and is insignificant for any properly sized and constructed wiring regardless of light fixture choice.

Lumens per watt of LED fixtures vary widely, still, and there are definitely still fixtures for sale which seem to be built on the belief that buyers will see "LED" and think "very efficient" without actually delivering good efficiency (inefficient LEDs presumably being cheaper than efficient ones).

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