Whr, Watts, Volts and Amperes are basic units of measure for a DC (Direct current) power supply. A battery, for example, is a direct current power supply and the combined measure Volts x Amps = Watts.
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Watts are important because watts represent the electrical energy spent by a battery (power generator) and used by an electrical device. Watts in effect is the measure of the amount of work done by a certain amperage (amount) of electric current at a certain pressure or voltage.