A transformer does not make energy. In the ideal case, a transformer with 100% efficiency transfers all of the input power to the output, so the power in equals the power out. Electrical power is potential difference multiplied by current, which gives the power equation for a transformer: Vp × Ip = Vs × Is.
V is the potential difference in volts (V) and I is the current in amperes (A). The p means the primary coil and the s means the secondary coil. The power is in watts (W). So the primary p.d. multiplied by the primary current equals the secondary p.d. multiplied by the secondary current.
This means that when the voltage goes up, the current goes down. A step-up transformer gives a smaller current in the secondary coil than in the primary coil. Worked example 1: a step-up transformer has 12 V and 5 A in the primary coil. The power is 12 × 5 = 60 W. With 60 V across the secondary coil, Is = 60 ÷ 60 = 1 A.
Worked example 2: a step-down transformer has 230 V and 2 A in the primary coil. The power is 230 × 2 = 460 W. With 23 V across the secondary coil, Is = 460 ÷ 23 = 20 A. The voltage goes down, so the current goes up.