Convert 15 minute kW readings to a monthly kWh total
Assuming the power is constant for every 15 minute interval. You get
$$E[n] = P[n] * 0.25~\text{h}$$ where \$ P[n] \$ is in kW and \$ E[n] \$ in kWh
$$Total Energy = \sum_{i=1}^n {E[n]} $$
Where n is the number of 15-minute readings in a month (n will vary from month to month)