Define one watt hour.

One watt hour is defined as the amount of electrical energy consumed by an electrical appliance of power $1$ watt when it operates for a duration of $1$ hour.
Mathematically:
$1 \text{ watt hour} = 1 \text{ watt} \times 1 \text{ hour}$
Since $1 \text{ watt} = 1 \text{ J s}^{-1}$ and $1 \text{ hour} = 3600 \text{ s}$,
$1 \text{ watt hour} = (1 \text{ J s}^{-1}) \times (3600 \text{ s}) = 3600 \text{ J}$.