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RandomFact314 12-25-2010 10:33 PM

Lightbulb watts vs Heat emitted
So I know this is a odd question but I cannot find anything on it online, I have 11 hermit crabs in a tank and the temps have to stay pretty high for them to be happy.

I have been using a small light bulb that is 40w 120v to keep it warm in the tank, well with the cold weather its been getting to not be hot enough and I already have a heating pad on the tank too

So I was wondering if a 60 watt bulb or a 100 watt bulb would increase the temperature in there or is it dependent on the volts?

Ryland 12-26-2010 12:11 AM

Most forms of energy can easily be measured in watts and around 95% of the energy going in to your light bulb is being turned in to heat, so a 40 watt bulb is going to put out around 38 watts worth of heat, a 60 watt bulb is going to put out about 57 watts worth of heat and a 100 watt bulb is going to put out around 95 watts worth of heat.
Thing is, where is that heat going? doing something as simple as a chunk of foam between you tank and the outside wall can help alot, same with putting a chunk of space blanket under the tank or an insulated lid if you can still keep it vented so you don't kill them.

Ford Man 12-26-2010 01:39 AM

A small wattage bulb will heat a large area if it's insulated good. When I was living in NC I had my well house very well insulated and usually didn't need any heat to keep the pipes from freezing, but when I did need heat such as when the temperature was 0 or single digits for a long term all I used for heat was a 75 watt flood light bulb to heat approximately 40 cubic feet and it would get quite toasty in there.

NeilBlanchard 12-26-2010 04:53 AM

Incandescent bulbs are only about 2.3 - 2.7% efficient, I think? So a higher wattage bulb will definitely put out more heat. The higher wattage bulbs are slightly less inefficient...

Piwoslaw 12-26-2010 10:38 AM


Originally Posted by NeilBlanchard (Post 211270)
Incandescent bulbs are only about 2.3 - 2.7% efficient, I think?

"Efficient" depends on what you are measuring: 5% efficient when used for lighting, but 95% efficient when used for heating;)
Good for a laugh: Heat Balls.

Random: Look for heatbulbs, which emit radiation at an even higher heat-to-light ratio (making them more "efficient" for heating).

RobertSmalls 12-26-2010 10:42 AM

Power in * (1- Luminous Efficiency) = heat emitted. For the purposes of your crabs, power in roughly equals heat emitted, no matter which lighting technology you're using.

Also check out Heatball :: Uses

Edit: Hah, Piwoslaw beat me to it!

NeilBlanchard 12-26-2010 04:19 PM

The 40W that the OP asks about is 98% efficient as a heater...

Incandescent light bulb - Wikipedia, the free encyclopedia

ShadeTreeMech 12-26-2010 04:37 PM

I'd actually like to replace the light bulbs in my bathrooms with infared bulbs in the winter and then not heat the bathroom with the central heating. Infared bulbs will warm you nearly instantly, basically as soon as the light hits you.

Ryland 12-26-2010 04:42 PM

Either way, your light bulb is working mostly as a heater and to answer your question directly, a 100 watt light bulb will put out roughly 2.5 times as much heat as the 40 watt bulb.
If you go with a hotter bulb you might think about getting a thermostat as well as insulating, that way you don't cook the crabs.

Piwoslaw 12-27-2010 03:12 AM


Originally Posted by Ryland (Post 211326)
If you go with a hotter bulb you might think about getting a thermostat as well as insulating, that way you don't cook the crabs.

Then what's the point of getting hotter bulbs???

ShadeTreeMech 12-27-2010 05:14 PM

mmmmm.....cooked craaabbb......Of course I think his are a bit too small to eat easily :D

I think the OP said the bulbs he had weren't putting out quite enough heat.

Ryland 12-27-2010 07:22 PM


Originally Posted by Piwoslaw (Post 211409)
Then what's the point of getting hotter bulbs???

The point is, if the 40 watt bulb was working most of the time and it's only really cold days and nights that he needs more heat then why risk over heating them? or waste the energy? a thermostat set at 110 degrees or whatever would keep them from cooking, otherwise you need to keep a stock of light bulbs on hand and change them every few days/weeks in the spring and fall.

RobertSmalls 12-28-2010 07:00 AM

Just bury the aquarium like so and let the crabs heat themselves:

ShadeTreeMech 12-28-2010 10:07 AM

want a bit of water heater for the insulation????

Wow, that is nicely done!!!! Why didn't the top get covered? It looks to be electric, so no harm in it. Or was it removed for the sake of the photo?

IsaacCarlson 12-28-2010 10:56 AM

I think that is a flue on top...might be gas? Might be insulated good enough that it uses only the pilot light and would not need venting.:eek:

RobertSmalls 12-28-2010 05:57 PM Super insulate your hot water tank

If this heater weren't already converted to electric, you'd need to leave both the top and bottom open to airflow. However, he did finish the job with insulation right up to the top of the plywood box.

Remember, anything worth insulating is worth superinsulating.

IWillTry 12-29-2010 04:43 AM

Hi. Crazy hot water tank insulating guy here. You may be interested to know that the above hot water tank only requires about 65W on average to keep the water at 45 degrees C for showers each morning. It could literally be heated with a light bulb but I use a 520W element and just run it 3 hours each morning on a timer.

A couple points of interest related to the crab tank discussion:
1. The efficiency of any kind of light bulb as a heater can be considered very nearly 100%. With an incandescent bulb you might get around 95% heat and 5% light "at the bulb" but the light will be converted to heat when it is absorbed by a surface. A 20W fluorescent bulb or a 20W incandescent bulb or a 20W anything will all put exactly 20W of heat into a closed container if you don't let any of the light escape. That's the law of conservation of energy.
2. Temperature rise (difference between internal temperature and ambient temperature) is proportional to power (Watts). So if your tank gets 10 degrees warmer than the room when you use a 40W bulb, it should get 15 degrees warmer than the room with a 60W bulb or 25 degrees warmer than the room with a 100W bulb.
3. If you're starting from zero insulation, adding a little insulation will go a long way. Even just some cardboard taped to the sides of the tank could raise the temperature enough.

Good luck.

Piwoslaw 12-29-2010 07:27 AM

Instead of replacing the light bulb with a more powerful one, what about adding a second low-watt bulb at the other end of the tank? That way the heat is distributed much better, instead of a single hot heat source which could potentially be hazardous.

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