A transformer is essentially two electrically-separate coils of wire wrapped around a magnet. The source voltage passes through the primary coil, producing a magnetic field which induces a voltage in the secondary coil. The ratio of the number of windings in each coil determines the output voltage. For example, if a transformer has twice as many windings in the secondary coil as in the primary, the output voltage will be twice the input voltage. The power of the input and output currents, however, is always equal. If you calculate the power consumption of the load on the output, you'll know how much power is passing through the transformer.
Find the voltage and current ratings of the device, or "load," connected to the output of the transformer. For example, the output of the transformer may be connected to a washing machine that operates on 240 volts and 7 amps. The voltage and current ratings should be printed somewhere on the device. If there is more than one device connected to the output of the transformer, add them all together so you have figures for total voltage and total current.
Multiply voltage and current to get the power draw of the load in watts. The transformer itself does not dissipate power, aside from a negligible amount due to resistance in the wire used in the transformer, so the total power input and output of the transformer is the same as the power of the load.
Compare the power draw of the load with the power rating of the transformer, which is often printed on the device. The power rating of a transformer is the maximum amount of power it can handle before it overheats and breaks down. Never use a transformer with a load that's greater than 90 per cent of the transformer's power rating. For example, it's unsafe to use any load greater than 180 watts with a 200 watt transformer.