To calculate watts, you need to know both the voltage (V) and the current (I) in the circuit. The formula to calculate watts is:

Watts = Volts x Amps

Where:

- Watts (W) is the unit of power
- Volts (V) is the measure of electrical potential difference
- Amps (A) is the measure of electric current flowing in the circuit

By multiplying the voltage (V) by the current (I), you can determine the power consumption in watts. This calculation is commonly used in electrical engineering to determine the power usage of a device or circuit.