Calculating heat output from lighting?

[BioHazard]

I'm trying to calculate just how many BTUs of heat the thousands of watts of lighting I have in my shop adds to the room working against my A/C. I understand the basic 3.4 BTU/watt calculation for electrical resistive loads, but obviously, some lights are more efficient than others. If I have, lets say, a 100 watt light, does a standard indandescent bulb make 340 BTUs? What about fluorescent and HID lighting, do they put out less heat per watt of lighting due to the fact they make more light from the same amount of energy?

[DanG]

http://www.mhi-inc.com/Converter/watt_calculator.htm

I'd consider lighting to be a closed system - 100 watts in is 100 watts of heat even if its from CFL's as the light will be absorbed and converted back to heat in the conditioned space. When they calculate cooling energy savings by retrofitting lighting systems it is kilowatts straight off the top and not fractional, ie: a cooling season of 32 weeks, and remove 22,000kWh from lighting load.

[Carlb]

Don't confuse watts with lumens,

A 100 watt incandescent light bulb produces 1750 lumens and  341 btu's

A 26 watt CFL will produce 1800 lumens and 89 btu's

So using cfl's will reduce both your energy consumption and your heat production by approximately  3/4

[BioHazard]

A 26 watt CFL will produce 1800 lumens and 89 btu's

How did you come up with this number? Is there a formula?

[bschwartz]

Carl, I think what Dan was saying is that 100W of lighting will produce the same amount of heat, regardless of the type of bulb.  100W of incandescent bulbs will throw off as much heat as 100W worth of CFLs.  The difference comes in only that you can use less watts worth of CFLs to produce the same amount of light.

[Carlb]

Carl, I think what Dan was saying is that 100W of lighting will produce the same amount of heat, regardless of the type of bulb.  100W of incandescent bulbs will throw off as much heat as 100W worth of CFLs.  The difference comes in only that you can use less watts worth of CFLs to produce the same amount of light.

I agree with Dan that 100w of electricity will produce the same amount of BTU's no matter what the light output, but you can light 4 times as much area using CFL's for the same amount of energy and heat. So for real world applications using CFL's will reduce you energy use and heat output by 75%

[Carlb]

A 26 watt CFL will produce 1800 lumens and 89 btu's

How did you come up with this number? Is there a formula?

I don't have a formula for the light output for a 26watt CFL but published information for a typical 26 watt CFL indicates that it produces 1800 lumens, about the same amount of light as a 100 watt standard bulb.  As far as the heat goes 26Watts of electricity produces 89BTU's

[BioHazard]

So to summarize...

The 3,000 watt fluorescent light switch in my shop is also a 10,200 BTU heater?

I'm hoping to cut that nearly in half, right now they're old 8' T12 HO tubes but I'm going to replace them with 4' T8s.

[DanG]

Inefficiencies sneak in on that one - 40 watt labeled bulbs have an additional 5 to 25% of each lamps wattage consumed by the ballast...

Electronic Ballasts on T12 bulbs can save up to ~20%, going to T8 bulbs can increase that to 40% (supposedly)

So those 72 T12 40W bulbs could be eating  ~3.75 kW/h and generously providing 12800 BTU/hr.

[BioHazard]

Yeah...right now I have a ton of T12HOs so thats 60w + ballast losses for every 4 foot of tube. (240w+ for each fixture!) I'm tired of the noisy ballasts too. The T8s I'm replacing them with only use 25w for every 4 foot of tube. It won't be quite as bright as it is now but I don't need it to be, my shop used to be an auto body shop so they had lights everywhere!

The electronic ballasts I have actually drive a 32 watt tube at around 25 watts...