Power [Watts] = Current [Amps] x Voltage [Volts]
Based on our project consisting of a 12-volt battery and a current of 4-amps, the power consumption would be (4-amps x 12-volts = ) 48-watts. The heat output of 48-watts, can be expressed in British Thermal Units (BTU) per hour by performing the following conversion:
BTU / Hour = Watts x 3.413
Put simply, one British Thermal Unit (BTU) is the amount of heat required to raise the temperature of 1-pound ( lb) of water 1-degree Fahrenheit. Thus the vest in our project would output ( 48-watts x 3.413 = ) approximately 164 BTUs per hour. If this value isn't intuitive to you, consider that 48-watts is similar to a 50-watt light bulb, and imagine the amount of heat that a 50-watt light bulb outputs. That may not sound like much, but when that amount of heat is contained under a jacket, you will find that it is plenty warm.
Also, if you were to look at the power output of Gerbing's heated pants, you'll see that they put out 44 watts of energy, so our pants will be warmer than theirs. If you want to go even hotter than that, just use a shorter length of wire. Just remember ohm's law and you'll be all set.