https://cdn.pixabay.com/photo/2016/06/01/00/08/button-1428090_960_720.png Power Power is essentially energy per second. This can be derived from voltage time amperage which equates to joules per second. James Watt is owed the credit for developing the watt unit and horsepower unit. For simplicity, power is computed by volts times amps or amperage squared times resistance. It is interesting to note that power in a resistor can only yield a positive value; meaning that a resistor can only consume power. This makes sense because of  P = I^2 x R or P = V^2 / R. If the current is positive or negative or if the voltage drop across the resistor is positive or negative, the wattage will always be positive since they are both squared. Consequently, this is not the case for a battery. Since P = V x A, the power in a battery could be either positive or negative. Positive for consuming power such as if the battery is taking a charge. Negative for giving power such as a battery powering a light bulb. Since energy is always conserved, the power produced is equal to the power consumed. From the previous series and parallel circuit examples, the power consumed can be contrasted. In the parallel circuit, the two lamps are noticeable brighter and the total power consumption is 96 watts. In the series circuit, the two lamps are dim and the total power consumption is 24 watts. With the parallel circuit, its easy to see that more power is utilized, but this would result in the battery not lasting as long when compared to the series circuit with the same components. For this reason, batteries include two ratings: Voltage and Ampere hours (Ah). This helps identify how long a circuit can be powered with a battery.