To explain what it means to be "in phase" we will use the example of the sled pulled by two dogs.
If the dogs pull both in the same direction, the slide will go fast if a dog pulls in a different direction from his partner will slow down.
The greater the direction difference and the lower the sledge speed.
Similarly, if current strong> and voltage are "in phase", ie they pull in the same direction, the useful power strong> will be greater.
In order to calculate the useful power the formula is:
Power (watt) = Voltage (volt) x Current
But if they are not in phase, the useful power also depends on the width of the corner that comes up: