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nowwhatnapster 03-20-2013 10:52 AM

How many watts does this system consume?
I am trying to determine the watts/hr this HVAC system uses when operating in cooling mode. The end goal here is to get a ballpark number on potential energy savings if we reduce our cooling load by upgrading our halogen lights to LED.

Air handler label
HZ 60
HP 3/4
FLA 7.00

Compressor label

York Stellar
Model No. H1DB042506B
COMPRESSOR 208/230V 1PH 60HZ 20.5RLA 120LRA
FAN MOTOR 208/230V 1PH 60HZ 1.6FLA 3.8LRA

mpoulton 03-20-2013 11:20 AM

The actual power consumption depends on the efficiency of the unit (SEER, plus some other factors like condenser shading and age) and the temperature differential between evaporator and condenser. It is not a constant number, but varies with the weather and heat load. You would need to use a meter to check the actual power consumption of the system in operation. Going by the ratings gives you only a very rough result that will not be accurate enough for your needs. And the SEER rating doesn't seem to be in the list of specs up there. For an extremely rough estimate, start with the SEER rating, apply about an 80% factor to it, and then calculate how the change in thermal loading due to your reduction in lighting translates into a change in AC power consumption:

al_smelter 03-20-2013 01:13 PM

Have you googled anything about this yet? I have never really though about it, but I'll bet a dollar to a dime that there have been studies done for this exact scenario. Won't surprise me if the LED manufacturers commissioned one in order to sell their products.

On the surface it sounds like it would take a very long time to re-coup the initial outlay for the switch. Interesting question though.

k_buz 03-20-2013 03:45 PM

My guess is you will see zero savings. Any savings you do see will be attributed to the change of lamps, not that the AC runs less.

nowwhatnapster 03-20-2013 03:56 PM

k_buz I think you underestimate the amount of heat halogen lights produce. If we don't keep the door cracked open when its 50F outside the compressor will kick on. No joke. I work in the basement next to the air handler so I know when its on. Right now the high for the day is 45F. The heat was on for maybe 20 minutes this morning now its 5PM. They set the thermostat to 70F.

al_smelter I have tried searching unfortunately I have not found anything yet that is simplistic enough for me to understand.

Basically I know upgrading the lighting to LED will save $2500+/year in raw electricity cost @ 12cents/kwh but that is without factoring in the greatly the greatly reduced thermal load.

mpoulton I did some searching on the compressor and found this pdf which seems to indicate the unit has a SEER of 10.0 at the very least.

The lights we are installing will reduce enegry consumption from 81.48 kwh/day to 14.73 for a savings of 66.75 kwh/day. For simplicity's sake, lets say every watt gets converted to heat. So 66.75kwh*(3.41btu/watt*1000) = 227,617 BTU/day we will be saving.

Since I can't calculate actual usage maybe a good question to ask is, what is the theoretical maximum watts the HVAC system can consume?

SquishyBall 03-20-2013 04:15 PM

I found this topic interesting, purely from the physics of it. My instinct leans toward buz' take. There will be no measurable savings due to running the AC less.

Keep in mind one of the challenges still facing LED is the buildup of heat and large heat syncs they have. So I think they handle heat differently. Halogen lights, you stand under them and you feel immediately warmer. LEDs not so much, tho they are pooling up heat up in your can or around the fixture. So that heat is going to transfer to the house, albeit slowly.

You could do an experiment.

Run the AC to whatever temperature. Shut it off for an hour. Measure the temp increase after an hour. Now turn the AC on, and measure the time it takes to cool it back to the original temp.

At an identical temperature time the next day, repeat the process w the halogens on. This time, you should see after the hour with no AC, a higher high temp. Then a longer time to re-cool.

That difference in time to re-cool is going to be exactly the amount of extra time you have to run your AC specifically due to the halogens being on. Once you figure the watts, it's easy to turn that into KWH and see how much power it would actually take.

However, I think this experiment is going to be so laden with inaccuracy and variable outside temp that the results, if you repeat it several times would simply indicate "inconclusive / no consistent savings". And that's only comparing halogens on to halogens off. It's not comparing halogens on to LEDs on.

Oso954 03-20-2013 04:27 PM


The lights we are installing will reduce enegry consumption from 81.48 kwh/day to 14.73....
What the heck are you doing ?

That is not a normal amount of lighting for a residence. And halogens are not really good for the one use that I can think of that would drive usage that high.

zircon 03-20-2013 05:01 PM

rule of thumb
It takes 1.5 watts of AC to get rid of one watt of heat from the lights.

mpoulton 03-20-2013 06:25 PM


Originally Posted by zircon (Post 1141716)
rule of thumb
It takes 1.5 watts of AC to get rid of one watt of heat from the lights.

That doesn't seem right. A SEER of 10 is equal to a coefficient of performance of about 3 (typical COP for air conditioners is 2-4 according to some sources, so the calculation seems about right). That equates to 0.3 watts of AC power to remove 1W of heat load.

Michaelangelo83 03-21-2013 12:57 AM


Originally Posted by Oso954 (Post 1141679)
What the heck are you doing ?

That is not a normal amount of lighting for a residence. And halogens are not really good for the one use that I can think of that would drive usage that high.

:laughing: Are you in the Emerald?

Maybe he's prepping for the nation wide legalization?

al_smelter 03-21-2013 05:35 AM

Now that I see your numbers, I too am interested in what type of building you are talking about. If this is your house, you need to whip up on the wife and kids to turn the lights off when they leave. I also cannot see where a home would seem very "comfortable" with blazing halogens all around.

Just curious. Your calculations indicate a butttload of fixtures to consume that much power.

dmxtothemax 03-21-2013 06:06 AM

You will feel the difference
due to less heat.
when changing over from incandesant to led.
But to qualify it exactily is difficult
as there are too many variables.

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