POWER CALCULATIONS

You can calculate the power P (in watts, symbolized W) in a dc circuit such as that shown in Fig. 1 using the following formula:
P = EI
where E is the voltage in volts and I is the current in amperes. You may not be given the voltage directly, but you can calculate it if you know the current and the resistance.

Remember the Ohm’s law formula for obtaining voltage: E = IR. If you know I and R but don’t know E, you can get the power P by means of this formula:
P = (IR) I = I²R
That is, take the current in amperes, multiply this figure by itself, and then multiply the result by the resistance in ohms. You also can get the power if you aren’t given the current directly.
Suppose that you’re given only the voltage and the resistance. Remember the Ohm’s law formula for obtaining current: I = E/R. Therefore, you can calculate power using this formula:
P = E (E/R) = E²/R
That is, take the voltage, multiply it by itself, and divide by the resistance. Stated all together, these power formulas are
P = EI = I²R = E²/R
Now we are all ready to do power calculations. Refer once again to Fig. 1.

Fig. 1. Circuit for working Ohm’s law problems.