Transformers raise or lower electric voltage, but leave the power transmission roughly constant. To calculate a transformer's energy output, you therefore needn't consider details such as the device's coil ratio. Instead, just take into account the transformer's efficiency. This produces the power output in watts. The British thermal unit (BTU) is a unit of energy. To calculate energy output from power output, you also need to know how long the transformer runs.

Multiply the transformer's input current and voltage. If its input current is 2 amps and its input voltage is 120 volts, 2 * 120 = 240 watts.

Multiply your answer by the transformer's efficiency. If it operates at 95 percent efficiency, 240 * 0.95 = 228 watts.

Divide your answer by 3.413, a conversion constant: 228/3.413 = 66.8 BTUs per hour.

Multiply your answer by how long the transformer runs. If it runs for 3 hours, 66.8 * 3 = 200.4 BTUs.
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 Photo Credit Transformer image by Maxim Glazunov from Fotolia.com