Nowadays there are a lot of posts and articles about renewable energies, smart grids, electric vehicles, energy efficient products and other environmental friendly solutions.
Their usage are increasing day-by-day in everyday life.
Starting from a simpler example, a flat’s electric distribution panel.

There is a main circuit breaker near the power consumption meter (20A rating in the picture), which protects the whole flat.
The power infeed in then divided into 3 lines, each protected with its own circuit breaker.
As more and more electrical appliance in used, there can be certain intervals when the instantenous power need is more than the circuit breakers’s ratings.
Let’s assume that the mians is nominally 230V, there’s a waching machine operating in the bathroom, an electric oven in the kitchen and one want to do some hoovering in the living room.
Normally the waching machine is rated at 2200W (~9.6Amps @ 230V) but this much power is only needed in the water heating phase, otherwise the power need is only 150-200W.
The electric oven has a pure resistive heating element which is connected in series with a bimetal temperature switch. It switches on and off when it reaches the desired temperature.
If this happens at the same time, the 20A breaker trips.

To avoid these situations the current consumption must be measured. The voltage also could be measured but the breaker only senses current, so basically only current measurement is needed.
There are quite a few techniques to measure current, the simplest of them is using AC current transformers (CT). These provide AC current output which is in direct relation to the measured primary current.

The idea is to provide information about the available current on a certain line.
In case of a washing machine, the available current information could be used to decide if the water heating phase can be started or not. If there is not enough power, then the waching machine (so called smart device) waits until the desired available power level comes.
The breaker’s rating is known, from which the measured current must be substracted, the result is the available current.
The smart device gets this information and since it knows its own power need, the decision on switch ON can be done.

Looking back to the starting situation: having a 10 Amps breaker supplying the bathroom is enough for the washing machine, but the rest of the flat uses power as well, so the overall power usage trips the main breaker at the infeed line.
This problem can be solved if there is information about the “parental” supply line’s available current.
If there is less available current from the infeed line, this must be taken into account.

Staying with the flat example, the infeed line and the flat’s individual supply lines’ current consumption must be measured, and a calculation unit is to be used to generate the available current outputs for each line.
Practically this is a microcontroller based unit, which measures the instantenous current usage, calculates the RMS current consumption and computes the available current output.

As stated above there must be an additional input for measuring the available current from the supplying line. In case of a block of flats, this comes from the main distribution box.

The example can be ridiculous, but looking from bigger perspective, this method is very useful in urban environment, where the power need is concentrated, and the power consupmtion is to be controlled.
Electric vehicles need much power for charging, but this power need can be delayed or reduced (resulting in longer charging time) in peak power usage intervals.

On the other hand, if there are a lot of solar panels installed and electric vehicles are charging at the same time, the charging power can be dinamically adjusted depending of the solar irradiation, thus some kind of grid power peak shaving can be obtained (besides the narrower grid voltage regulation).