The power to the house is from the National Grid and our solar panels.
240V AC mains power
In the UK mains power is supplied from the National Grid. Alternating Current (AC) as voltage and current input is supplied as a nominal sinusoidal voltage of 240v RMS at 50 Hz with a peak to peak of about 338v. In reality this varies between 216 and 253 RMS. The voltage from the Grid goes from 0 volts to +338 volts down to -338 volts in 20 milliseconds. It repeats this 50 times a second (1000ms).
On the very simplest level a house is a resistive load. The voltage inside your house will follow a similar sinewave voltage at a slightly lower voltage because of the resistance in the wires of the Grid and inside your house.
The voltage at the substation (on the Grid) will be more than that at the resistive load of the house and therefore the current will flow into the house from the Grid, through the electricity meter.
Grid tie inverter
Solar panels on the house generate electricity as Direct Current (DC). DC is fed in to a Grid tie inverter. The inverter inspects the sine wave voltage that is generated by the Grid as seen at the resistive load of the house and creates a matching sine wave voltage slightly above that of the house. This will cause current to flow from the Grid tie inverter through to the house and if there is enough power from the solar panels, out onto the Grid.
So the power consumption of the house will approximate to 240Volts RMS multiplied by the current into the house from the Grid plus that from the Grid tie inverter from the solar panels. Because the loads used in the house are not purely resistive this is not the whole store but it is a fairly good approximation.
Power management and current flow
To truly calculate the power usage, we would need to measure the voltage and the current profile throughout each cycle. In power management our aim is to manage the current supplied to the house so that we may use locally generated current rather than draw from the Grid. Power management rather than accurate power monitoring. Knowing the current flow is sufficient to achieve this if we also know the direction of flow. Both the Grid tie inverter and the National Grid will determine the voltage dynamically but use of the 240V RMS value will suffice to estimate power usage for our power management needs. Some electricity meters offer an LED that becomes solid lit when current is flowing out of the house and pulses when current is consumed by the house.