BTU stands for British Thermal Units, a means of measuring heat and energy. One BTU equals the amount of energy required to heat one pound of water by 1 degree Fahrenheit. The BTU output measures how much heat or other energy a given appliance is generating --- very useful when determining if a unit can heat a given space.
Locate the voltage (in volts), current (in amps) and/or wattage (in watts) of the device you wish to measure. The instruction manual of the unit or a label on the device itself should list these measurements.
Calculate the wattage of the device by multiplying the volts by the amps. For example, if the device has a voltage of 160 volts and a current of 2 amps, its wattage would be 320 watts (160 times 2).
Divide the result by 3.413 --- a constant figure --- to determine the BTUs per hour. To cite the example in Step 2, divide the 320 watts by 3.413 to get 93.76 BTUs per hour.
Multiply the BTUs per hour by the number of hours the device runs to determine its total BTU output. To continue the example, if the device runs for 4 hours, it will generate 375.04 BTUs in that period of time (93.76 BTU/hour times 4 hours).
Inefficient heaters may generate less wattage than the paperwork suggests. If you know the device is not generating the amount of BTUs you calculate that it should, take it to a repair shop or purchase a new one.