How To Calculate Wattage Requirements Quickly

When you need to determine the wattage required to heat an application, you can delve into some nitty-gritty details when it comes to calculating that number. If you have a good temperature controller, you're safe to overestimate that number and still be okay, resulting in a system that reaches the desired temperature without the need for lengthy calculations.

Presented here are a few quick ways to determine the wattage requirement for different scenarios. You don't have to worry about heat loss via convection and radiation or concern yourself with other aspects of the calculation.

To heat something by a certain amount, you just need to know the type of metal you're heating, its mass, the change in temperature required, and how quickly you want to get it to temperature. Based on the type of material, you'll look up a coefficient for the material being heated that goes into a simple equation, and you're off!

The equation looks like this:
Wattage = Material Coefficient x Lbs of Material x (Desired Temp - Starting Temp [in °F]) / Hours to Get to Temperature

Here's the table of coefficients for some common materials:

The equation above, using the numbers from the table, calculates the wattage requirement for heating the mass to temperature and then adds about 40% on top of that. The additional wattage is enough to account for the heat losses to the environment that would occur. The Material Coefficient number is just a modified Specific Heat of the material to give the wattage calculation that extra boost. 

Here are a few examples: 

The equation above, using the numbers from the table, calculates the wattage requirement for heating the mass to temperature and then adds about 40% on top of that. The additional wattage is enough to account for the heat losses to the environment that would occur. The Material Coefficient number is just a modified Specific Heat of the material to give the wattage calculation that extra boost.

Here are a few examples:

1) Let's say you have a sealing platen weighing 24 lbs that you need to get to 300°F from room temperature (72°F) in 30 minutes. It's made of stainless steel.

The solution to the equation above is: Wattage = .05 x 24 x (300-72) / 0.5 = 547.2 watts. Call it 550 or 600 watts to play it really safe. That's how many watts you need (presumably in electric heaters) to heat your platen from room temperature to 300°F in about half an hour. You can drill a few holes parallel to the face of the platen and insert cartridge heaters that will produce 600 watts.

2) You have a large outdoor circular aluminum trough weighing 50 lbs filled with 80 lbs of water that you need to get to 70°F from a cold condition of 40°F, in an hour.

This solution requires using the equation twice; one for the aluminum trough and one for the water. For the trough itself the equation is 0.100 x 50 x (70-40) / 1 = 150 watts. For the water the equation is 0.420 x 80 x (70-40) / 1 = 1008 watts. Add those two together and you get 1158 watts in total. Round that up to 1200 watts and you should be good to go. The heat can be applied either by wrapping the trough with a large band heater or by heating the base with inserted cartridge heaters. Either way, get 1200 watts into the system and you should be able to get it to temperature in about an hour.

That takes care of heating solids and liquids with an easy estimation that will get you working well enough. When heating air, a couple of other equations are used, depending on whether the air is compressed or not. That’s a story for another Ask Ian. If you have a complex application that needs more attention to determine the wattage requirements, please contact Tutco and we’ll be glad to help.