What is the commercial unit of electric energy?

$1 \; kWh = \ldots \ldots \ldots \; \text{watt-seconds}$

The applied potential difference across a given resistor is altered so that the heat produced per second increases by a factor of $16.$ By what factor does the applied potential difference change?

$(a)$ Derive the formula for the calculation of work done when current flows through a resistor.
$(b)$ One electric bulb is rated $40 \ W$ and $240 \ V$ and another is $25 \ W$ and $240 \ V$. Which bulb has higher resistance and how many times?

What is the other name of the commercial unit of electrical energy?

If three bulbs of $100 \; W$, $60 \; W$, and $40 \; W$ are used for $2 \; \text{hours}$ daily in a house, how many units of electrical energy are consumed in $30 \; \text{days}$?