Power is the rate (energy amount per time period) at which work is done or energy converted. The scientific unit of power is the watt (W), which is equal to one joule (energy amount) per second (time period). The PV Data Display shows electrical power units as kW, which is equal to one thousand watts.

Energy and power are often confused and used interchangeably. Quite often people will refer to the "power" generated by their PV panels today when they really mean the electrical energy produced during this time period.

The difference between energy and power is that the former is an amount and the second is a rate. Consider two full 1000L tanks of water on stands of equal height. Both tanks contain the same amount of potential energy. One has a large diameter discharge pipe connected to a water turbine. This tank will be emptied more quickly than its neighbor with a small discharge pipe. The turbine connected to the large diameter pipe will generate energy at a faster rate and will be more "powerful" than the other turbine. It will of course generate electricity for a shorter time than the turbine connected to the small diameter pipe. Ignoring the slight differences in efficiencies, both systems will generate a similar amount of energy by the time each tank is empty.  

The term watt is named after James Watt, a Scottish engineer. Watt greatly improved the performance of the steam engines that existed at the time. He also measured the power output of draft horses and ponies and refined the unit "horsepower". This unit was originally used to compare the output of steam engines with the power of draft horses and is still in use today, especially to describe the power output of vehicles, boat engines, etc.