Hybrid Car – More Fun with Less Gas

converting heat watts to airconditioner watts

register ::  Login Password  :: Lost Password?
Posted by captain bob on July 18, 2004, 9:46 pm
I was just trying to figure out what the additional cost to run an AC
is to cool a room with a Compact flurescent bulb than an incandescent
light bulb

the 75 watt equivalent CF bulb uses 18 watts
how much of this is heat and how much is light?

assuming there is 2 watts of light, then the cf bulb is making 16
watts of heat and a standard incandescent makes 73 ( 75-2 )
this is a difference of 57 watts

I read that a btu = 3.41214 watts of heat.
since the AC has an EER of 10.7  (btus per watt to run the AC)

then it moves 3.14 watts of heat with 1 watt of power for the AC?
this is more effecient than I expected.

so the standard bulb uses 57 watts more, plus another 18 for the AC to
move the heat.

is this right? or where have I gone wrong?

Posted by captain bob on July 18, 2004, 9:49 pm

oops I mean the additional cost for incandescent over CFL

Posted by danny burstein on July 18, 2004, 9:59 pm

except for the amount of light going out the window, every single watt
being used up by the lamp, whether as "light" or as "heat", eventually
turns to heat inside teh room.

nope. it's 18 watts versus 75 watts. again, it doesn't matter what the
watts get used for in the room. it can be for light, it can run a fan,
operate a tv, run a refrigerator. It all turns to heat.

eyup. that's why, an air conditioner in "heat pump" mode is more efficient
than straight electric heat, and often better than a gas or oil furnace.

eyup, again, except for the bit of calculating in ALL the wattage.

You're pretty much right except for that sidetrack...

Knowledge may be power, but communications is the key
[to foil spammers, my address has been double rot-13 encoded]

Posted by captain bob on July 18, 2004, 10:15 pm
 On Sun, 18 Jul 2004 21:59:33 +0000 (UTC), danny burstein

ok thanks, I forgot about that
the light is reflected and absorbed by everything in the room until it
all turns to heat

the difference between 75 and 18 is still 57, so my calculations are
the same

does anyone know how efficient a household light bulb is? 1%? 2%?
I know the higher wattage ones are better
if memory serves one 100 watt bulb produces more than twice the light
of 2 60s

with the price of CFLs dropping (.50 per 18 watt bulb)  they are
becoming a better deal



Posted by nicksanspam on July 19, 2004, 12:29 am

No. Btus are energy. Watts are power. Please learn the difference.

No... 10.7 = Btu/Wh, in these bastardized units. If it moves
10.7 Btu/h with 3.41 Btu/h of electrical power, the COP is 3.14.

Yes. Removing 18 watts of heat requires 18/3.14 = 5.7 W of electrical power.


This Thread
Bookmark this thread:
  • Subject
  • Author
  • Date
please rate this thread