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A transformer's load capacity is given as apparent power (S) and its unit is voltampere (VA).
If the transformer's feeding load is purely resistive, the effective output of the transformer is equal to its apparent power.
Effective output (P): P = S cos φ
If the load has reactive components such as inductance (motors, inductors, discharge lamps), the transformer must be chosen taking into account the load's power factor (cos φ).
Example:
In all the below cases the transformer's apparent power S_{n }should be 500 VA, although the load's effective output P_{n} differs.

Cos φ 
P_{n} 
S_{n } 
Heating resistor 
1 
500 W 
500 / 1 = 500 VA 
Electric motor 
0.8 
400 W 
400 / 0.8 = 500 VA 
Discharge lamp (uncompensated) 
0.5 
250 W 
250 / 0.5 = 500 VA 
The transformer's power is stated as apparent nominal power in nominal ambient temperature. Due to this, the installation site's temperature should always be taken into account when choosing a transformer. When the ambient temperature increases, the load capacity of the transformer decreases. The transformer's load capacity decreases almost exponentially relative to the temperature increase.