I want to know how much degree celsius hot will a 100W electric blanket get? Thanks for the information.
To add to the other posts here...
There is an equation called the "heat equation" that was found many years ago by Fourier if i remember right. It was a breakthrough at the time and amazingly some of the concepts developed that long ago started being used to detect underground nuclear bomb. For the problem of heat flow it gets a little involved with a partial differential equation that many dont want to get involved with. If you understand a little about electrical current flow there is a simpler way of looking at it though.
It starts with the heat producing element. This can be a resistor, transistor, etc. For conduction the heat flows through a conductor just like an electrical current flow through a wire. If it encounters a resistance, just like electrical current flow it will not be able to flow as well so the temperature at the end of the heat conductor will be reduced. Then there is radiation. This is when the heat dissipates into the air round the conductor.
For an electric blanket a simplified view would be the heat element, followed by a very short conductor, followed by a decent insulator, followed by conduction on one side and heat dissipation into the air on the other. If we consider the conduction side to be non dissipative, then we are left with the top side where the heat dissipates into the air.
To calculate the maximum temperature you would have to know the same things you would for any other heat vs temperature problem such as a heat sink on a transistor. The main parts would be how much wattage the heat element puts out as a max (which you seem to know) and the insulation on the top side. You have a spec for the heat element, 100 watts, and next you would need a spec for the insulation, which would be a coefficient that acts to reduce the temperature effect of the heat element on the very top of the blanket. To get that, you would have to turn the control all the way up and measure the top side temperature. You could then come up with a factor that tells you how much the temperature drops from the heat element to the top surface. This would vary depending on the type of material and thickness.
Once you figure all this out, you can then figure out the effective heating on the lower side, which will be hotter than the top side. That's probably the most important because you want to know how hot the animal underneath gets with a given wattage setting. You can get more information about this by looking at how a heat sink for a transistor works, then apply it to the blanket. It's a little hard to explain it all right here.
Since it is hard to gather all this information, you could just stick a thermometer under the cover and turn it on full blast. You'll get a temperature reading and then you have the answer. If there is to be a person underneath then they should be there under the blanket. If it is something else like a fluid or solid then that should be under the blanket instead. That's probably the most reliable way to determine the max temperature.
I can tell you from experience that they get pretty warm underneath but not really hot. You do end up turning them down. It depends on the temperature above too though as that takes some of the heat away.
If you have a specific application for this you could mention that too.