Power

Watt is the unit of electric power.

• One watt of power is equals to the work done in one second via one volt of potential difference in moving one coulomb of charge.
• One coulomb per second is an ampere. So power in watts equals the product of volt time's amperes.
• Power in watts = volts x amperes

                                  P = V X I

        Dimensionally, in this equation the right side is the product of joules per coulomb and coulombs per second, which generate the expected dimension of joule per second or watt. 

• The sketch indicate that if one of the terminal of the element is v volts positive with respect to the other terminal, and current i is entering in the element through the terminal then the power is absorbed by the element.
• It is also right to say that a power   p =vi is being convey to the element.

Passive Sign Conventions

1. If the current arrow is indicated into the + marked terminal of an element, then p=vi refer the absorbed power.
2. A negative value show the power is in fact being produced by the element, it may have been better to described a current flowing out of the + terminal.