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I am on a light forum and there are debates about watt heat output of lights.

I'm using my common sense for the debate but can't find the data online to support my ideas.

People are thinking that a 400 watt heater produces the same heat as a 30% efficient 400watt light. And further same as 400 watt of 60% efficient lights.

Now my logic says that heat being the main by product of a heater will of course cause the 400 watt heater to produce the most heat

A light that's 60% efficient at light production like new LEDs cannot produce the same amount of heat because its byproduct is photons or light. But still produces heat as its not 100% efficient.

Can any of you geniuses explain to me smartly what heat production wold be produced from lights?

HID lighting is about 30% efficient

New led is getting to 60% and more

But people think that 400 watts translated directly to heat

update: I think I figured it out. The room is much cooler because: the 60% energy as light from LEDs is projected to concrete floor which goes for many meters outside the room and connected to the earth thus dissipating it effectively outside of my room. The remaining 40% heats the rooms air Where as a less efficient light uses 70% of its energy to heat the air and only 30% is sent to the concrete and this dissipated to earth and other rooms. Am I correct here? –

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  • \$\begingroup\$ You've been drinking it seems. \$\endgroup\$ – Andy aka Oct 15 '15 at 23:14
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    \$\begingroup\$ A 400W heater produces the same heat as a 400W light, because light is a form of heat. Light which falls on a surface and is absorbed will heat that surface. A 30% efficient light should dissipate 30% of its heat (120W worth) in the form of light. \$\endgroup\$ – Kaz Oct 15 '15 at 23:18
  • \$\begingroup\$ So a 60% efficient LED light at 400 watts would heat a room the exact same as a 400 watt heater? This does not appear to be true in my experience as 400 watt 30% efficient light heated my room to 80+ and 400 watts of led heated my room to 76 \$\endgroup\$ – Bm9ags1 Oct 15 '15 at 23:23
  • \$\begingroup\$ If light is escaping your room, then it's not heating your room, but the outdoors. 400W in is 400W out, one way or another. Also, are you sure both sources are producing "exactly 400W"? Just looking at the rated power isn't good enough. \$\endgroup\$ – helloworld922 Oct 15 '15 at 23:46
  • \$\begingroup\$ Using a kill-o-watt meter so it's accurate. \$\endgroup\$ – Bm9ags1 Oct 15 '15 at 23:47
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Ok, so as a rule: $$Watts_{in} = Watts_{out}$$ This is always true unless you're storing energy somewhere.

A 400W heater will produce 400W of thermal energy.*

A 400W incandescent will produce 400W of thermal energy and light energy combined. Unfortunately if you measure the amount of light energy being produced, it is rather small compared to the total energy consumption of 400W.

A 400W LED fixture will produce 400W of thermal energy and light energy combined, but the amount of light energy is going to be a more significant figure in the process. Now you will be able to tell that it's not 400W being lost to the heat sink, but some amount less than that. (This ratio is the efficiency). The rest is being emitted as mostly-visible light.

However, if you put the LED in a well-insulated black box, you would not be able to tell the difference between a 400W lightbulb and a 400W LED by the temperature of the box.

In general, the actual efficiency of a light source (in Watts emitted as light) is not published.

Efficacy is not the same as efficiency. It takes into account the human eye response that determines how bright the light is, as produced from the source. This is not in units of Watts/Watt like efficiency, but in Lumens/Watt.

For example, comparing a white LED to a UV or deep blue LED, the white LED would have much higher efficacy compared with the deep blue LED, even if the amount of energy released in photons is the same, because the human eye does not respond to deep blue very well.

*As an aside, a 400W heat pump can present more than 400W of thermal power at its output, but it does that by moving heat rather than purely producing it.

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  • \$\begingroup\$ I think I figured it out. The room is much cooler because: the 60% energy as light from LEDs is projected to concrete floor which goes for many meters outside the room and connected to the earth this dissipating it effectively outside of my room. The remaining 40% heats the rooms air Where as a less efficient light uses 70% of its energy to heat the air and only 30% is sent to the concrete and this dissipated to earth and other rooms. Am I correct here? \$\endgroup\$ – Bm9ags1 Oct 15 '15 at 23:47
  • \$\begingroup\$ It could be. As I said elsewhere, both the tungsten and HID lights given off a lot of infrared which is not visible, but appears as radiated heat. LEDs are going to be a lot brighter compared to the heat they both produce at the source and as radiation. As for the particular mechanisms of why your room is cooler, that depends A LOT on the configuration of your room and what factors are most important. I couldn't even say for sure without at least looking around or something. \$\endgroup\$ – Daniel Oct 18 '15 at 4:17
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A 400W heater and 400W, 60% efficient bulb will heat a room roughly the same amount. Most of the energy that starts off as light will become heat once it's absorbed by items in the room. The only losses would be out the window or by other energy conversions that somehow make it out of the room (like a solar panel).

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As far as I understand it the theoretical maximum efficiency converting light with reasonable color spectrum is something like 400 lumens/watt. If you're satisfied with monochromatic green light it's more like 700lumens/watt. I don't think commercially available lights are much better than 100lumens/watt (though claims of higher efficiency for prototypes and for chips ignoring the power conversion circuitry float around). Russell may have more to add on this. So actual efficiencies are more in the 20% range and less when you take into account circuitry, ballasts, covers and so on.

It sounds like you are looking at actual bulbs. There are a number of reasons why you may be seeing a discrepancy. One is that electronic bulbs do not draw current evenly, they may have a lot of harmonic content so your wattmeter may not be accurate. If there is actually light escaping through a window then there would be a slight effect, but you'd have to do an accurate well-controlled test to be sure about this- outside solar radiation and temperature (and adjacent room temperatures) would figure into it. I think you'd need a fairly carefully controlled experiment to see the difference.

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Thanks for your replies.

There's no windows. It's mostly concrete

If you search Phillips is producing a 200 lumens per watt LED T8 light to replace the tubes.

Cree produces a cob led that when ran low power it is up to 70%efficient at 6500k at 3500k they're about 60% efficient with 25 watts. Producing 200-240 lumens per watt depending on color of course.

I understand lumens and how it's really only for our eyes and different color lights effect the reading.

I was trying to understand why a 400w HID light at about 30% efficient heats my room so damn much when 400 watts from very efficient led isn't close.

As discussed here it appears that because the efficient LEDs transfer that electricity to light more it is thus hitting my cool concrete where the less efficient light turns more of its electricity in to heat directly not light.

So I believe that's the answer.

I was thinking from knowledge of refrigeration aspects and energy needed to make gas cooler and the delta. So it made me assume that the higher efficiency lights are less restrictive and cause lower heat and more light. Wasn't thinking how light itself is heat or energy. Still kind of confused.

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  • \$\begingroup\$ The HID probably makes you hotter because it throws off a lot more infrared radiation, which heats things via radiation rather than thermal transfer. But that's not the question you asked. \$\endgroup\$ – Daniel Oct 16 '15 at 5:59
  • \$\begingroup\$ I'm just trying to understand why my room is hotter with same wattage pulled from wall with a HID light vs LED \$\endgroup\$ – Bm9ags1 Oct 16 '15 at 16:36

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