Current are classified into 2 types: Direct Current (DC) Alternative Current (AC)

DC power source DC power source is electrical power source that does not change direction of current flow during its supply period. Samples of DC power supply are Battery, Solar Cell, etc. AC power source AC power source is electrical power source that changes direction of current flow periodically during its supply period. Practically AC current is sine wave with frequency 50 Hz or 60 Hz. A sample of AC power supply is Utility power from wall outlet. Electrical power consumption Electrical power consumed by DC electrical appliances can be determined by the voltage that they work on and the current that they consumed. Electrical power consumed by AC electrical appliances is more complicated than DC electrical appliances because the direction of current is changed periodically then the value of voltage and current should be measure in term of RMS (Root Mean Square) to eliminate changing of current direction. There are two types of AC loads, which are Linear load such as incandescent lamp Non-linear load such as fluorescent lamp, electronic ballast of fluorescent lamp, PC, PC monitor, TV, etc. Each load types have difference consuming current characteristic as the following The pictures below show the current characteristic of linear load and non-linear load with same input voltage at same power rating of load Current characteristic of incandescent light bulb Power factor = 1 Current characteristic of a set of PC Power factor = 0.52 What are Watt and VA? Watt   is unit of measuring power. VA is unit of measuring electrical power consumed by non-linear load current electrical appliances. Example: A Computer is consumed power as a non-linear load then VA should be the measuring unit that indicates power consumed by PC. A UPS is used to supply power to PC once the utility power is failed then a UPS should be declare its power in VA as well. AC power (Watt) measuring To measure real AC power of electrical appliances, the measuring equipment called "Power meter" is requested. This equipment will measure voltage and current at the same time and make calculation to get power in "Watt". The following pictures show measuring of a PC with 17" monitor power consumption by using Power meter. AC appearance power (VA) calculating We can measure the power of the same set of a PC with 17" monitor by measuring voltage (RMS) and current (RMS) after calculation power by multiply working voltage with consumed current. The relation of AC power and AC appearance power Power factor (pf) is a number between 0 to 1 indicating the variation of load current shape varied from sine wave. Power factor of linear load equal one. (=1) Power factor of non-linear load is less than one. (<1) From a sample of a PC with 17" monitor, we can calculate power factor by Watts   = VA X pf 132 = 252.23 X pf pf = 132 / 252.23 pf = 0.523