How to Calculate Power in Watts

Understanding how to calculate power in watts could lower you cost of electricity.

If you want to know how to calculate power for an appliance and judge the cost of electricity, understanding watts and usage is essential. If you want to find out how to calculate power in watts, the equations are very useful because so much can be done with them. The level of math required to solve these equations is relatively low, which means they can be done with or without a calculator. This tutorial is important for anyone who wants calculate power usage properly.

Things You'll Need

• Calculator
• Number of Seconds
• Amount of Energy
• Amount of Current
• Amount of Volts
• Amount of Resistance

Instructions

How to Calculate Power

1. Using a calculator, determine the amount of energy (Joules) and time (Seconds) you wish to calculate power (Watts) by. Divide energy into time and you will get the rate of power (Watts) that is used.For example: .05(J) / 20(S)=.0025(mW)

   Use milliwatts because the amount of power used in most electronics is relatively low.

2. Determine the amount of current (Amps) and voltage (Volts) you wish to calculate power by. Multiply the number of amps and volts together. The product will give you the amount of power. For example: 001(mA) x 5(V)= .005(mW)

3. Look at Ohm's Law: V=IxR or voltage equals current times resistance. Using the previous formula, power equals amps times voltage (P=IxV), convert the formula to power equals current squared times resistance P=IxIxR. This will give you the calculation of power using resistance.

4. Squaring voltage and dividing it by resistance will also give you the amount of power. For Example:

   .005(mA) x .005(mA) x 50(ohms)=P

   .000025(mA)x 50(Ohms)=.00125(mA)

Tips & Warnings

• Do not use a calculator if you want to improve your mental math calculations.

• Practice these equations by plugging in power and calculate the time, energy, volts, amps and resistance.

Resources

• Read this Article in Spanish
• Power and Energy
• Power Calculator