There can be a number of reasons why this might be the case. The ESB Networks Online Account smart meter data file presents 30 minute interval smart meter data in kilowatts (kW). Kilowatts is a unit of measure of “power”. Every one of these data points represents the average power value for the preceding 30 minutes. Your electricity supplier data is in kilowatt-hours (kWh) which is the unit of measure of “energy”.
The equation to convert power to energy is: kWh = kW x h
kWh is the energy
kW is the power
h is the time in hours
So, a power value of 0.25kW (the data presented to you on your ESB Networks Online Account) equates to 0.25kW x 0.5 hours = 0.125kWh of energy (energy is the data presented to you by your electricity supplier).
A second reason that customers may see a difference is that your electricity supplier could aggregate the ESB Networks smart meter data and present their customers with, say, hourly interval data.
As an example, your ESB Networks smart meter data shows 30 minute interval power data of 0.25kW at 10:30am and 0.75kW at 11:00am respectively, then to convert this to a single data point of energy for the hour simply use the formula:
(0.25kW x 0.5) + (0.75kW x 0.5) = 0.5kWh of energy @ 11:00am