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?

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.

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.

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...

"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).

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!

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

Incandescent light bulb - Wikipedia, the free encyclopedia

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.

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.

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