Efficiency compares the actual output of a process with the potential output that could occur under ideal circumstances. For example, to calculate the efficiency of your car, you could multiply a given distance travelled times the amount of force required to get it there. This then would be compared to the maximum amount of work that could be accomplished by the amount of gasoline consumed. However, actually performing such a calculation would be difficult, because there are a lot of factors at work. A much easier calculation of efficiency can be carried out using your digital camera's battery charger.
Things You'll Need
 Digital camera battery charger
 Calculator

Look on the bottom of the charger to find the input voltage (volts) and current (amps). Many chargers operate on a variable amount of voltage, so use the value that your home is wired to. In many cases this will be 120 volts.

Use your calculator to find the input power (watts) by multiplying the voltage times the current according to the formula V x I = P, where V is voltage, I is current and P is power. For example: 120V x 0.09A = 10.8W. If the current is given as milliamps, divide the result by 1000. This amount of power represents the maximum potential output of the charger.

Multiply the output voltage by the output current to find the actual output. You will also find these values on the bottom of the charger. Remember to divide by 1000 if the current is given in milliamps. For example: 4.2V x 700mA / 1000 = 2.94W.

Divide the output power by the input power and multiply by 100%. (Multiplying by 100% in effect amounts to moving the decimal place to the right two places and adding a percentage sign.) The result is the efficiency of the charger. For example: (2.94W / 10.8W) x 100% = 27.2%. This number will always be less than 100%, because all processes expend a certain amount of wasted energy. Battery chargers become fairly warm while they're charging, and that heat is energy lost.
References
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