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Watt Calculator

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How do you calculate watts? - Watt's equationOhm's law: volts, amps, and ohmsPower, Voltage, Resistance, CurrentExamples of conversion between volts, amps, watts, and ohmsFAQs

With our watt calculator, you'll have a better understanding of what is Watt's law and the electrical power unit. Would you like to learn how to find watts? What about what connects volts, amps, watts, and ohms? Well, to do that, we need to dive in and discover what the power equation is!

If you want to know how the type of current affects how to calculate watts in a circuit, check out our watts to amps calculator.

How do you calculate watts? - Watt's equation

Our calculator is based on the two laws that describe simple electrical circuits. One of them - Watt's law - states that:

Power = Voltage * Current - in symbols: P = V * I.

This power equation, as well as the power unit, were named after James Watt - a Scottish engineer. One watt is the power at which the work performed in one second is equal to one joule:

1W = 1J / 1s

In electrical circuits, one Watt is defined as the rate of work when a current of one ampere flows through a conductor which has an electrical potential difference (voltage) of one volt:

1W = 1V * 1A

So what is power? Power, in an electric circuit, is the rate of transferring electrical energy per unit of time. Learn more in the electrical power calculator.

Ohm's law: volts, amps, and ohms

Our watt calculator uses a second formula - Ohm's law. It states that:

Voltage = Current * Resistance, or V = I * R

What do those names mean?

Electric Current is a measure of the quantity of charge (electrons) passing through any point of a wire per unit of time. Its SI unit is an ampere [A].

Resistance describes the strength of a given wire to oppose the flow of electrons. The resistance unit is an ohm [Ω].

Voltage is the difference in electric potential between two points of a wire. The SI unit of voltage is a volt [V].

Power, Voltage, Resistance, Current

With Ohm's and Watt's equations you can calculate four variables - power, voltage, resistance and current. If you know the values of two of these variables, you can transform the above equations according to your needs. Underneath we list all of these transformations:

  1. Resistance:
  • R = V / I
  • R = V² / P
  • R = P / I²
  1. Current:
  • I = V / R
  • I = P / V
  • I = √(P / R)
  1. Voltage:
  • V = I * R
  • V = P / I
  • V = √(P * R)
  1. Power:
  • P = V * I
  • P = V² / R
  • P = I² * R

Keep reading to see a couple of examples where we learn how to find watts and calculate amps from watts and volts!

Examples of conversion between volts, amps, watts, and ohms

To use our watt calculator, all you need to do is input two numbers, and all the other fields will be filled on their own. But, if you want to learn how to calculate these things by yourself, here are some examples that you might find helpful:

Let's consider a 60 Watt light bulb with 120 Volts of electric potential. How to calculate amps from watts and volts? Find the correct formula, and input the numbers in the correct places:

I = P / V = 60 W / 120 V = 0.5 A

Your light bulb needs 0.5 amps of current.

Let's look at another example. A resistor has a voltage of 4 volts and a resistance of 8 ohms. How to find watts?
You need to combine both Ohm's and Watt's law. Then, you will get:

P = V² / R = (4V)² / 8Ω = 2 watts

Would you like to challenge yourself a bit? Check out the power factor calculator to find out more about the power equation and the components of power: real power, reactive power, and apparent power!


What does a Watt measure?

A Watt (W) is a unit of electric power (P) that measures the rate at which electric work is done when the potential difference (V) drives current (A) through a circuit.

P (in Watts) =  V (in volts) ⋅ I (in amps)

How many volts are in a Watt?

One Watt is the electric work done when a current of one amp passes through a circuit with a voltage of one volt.

1 W = 1 V ⋅ 1 A

How many volts are in one Watt depends on how much amps of current is flowing in the circuit. The higher the current in one Watt, the lower the voltage.

How many kilowatts are in a Watt?

One Watt is equal to 10-3 kilowatts.

1 W = 10-3 kW = 0.001 kW


1 kW = 1000 W

How many BTUs are in a Watt?

British thermal unit per hour (BTU/h) is a unit of power for heating and cooling systems sometimes used in North America and the United Kingdom instead of watts. It is commonly shortened from BTU/h to just BTU.

1 W ≈ 3.412142 BTU/h.

How many BTUs are in a 1500 Watt heater?

One watt is approximately equal to 3.412142 BTU/h. So a 1500 W heater is equivalent to a 5118 BTU/h rated heater.

How do I calculate the Watt hours of a battery?

To calculate the Watt-hours (Wh) of a battery, follow these steps:

  1. Find the battery's voltage (V) and amp-hours (Ah) from its specifications. For example, a 12V50 battery has 12 V voltage and 50 amp-hours capacity.

  2. Multiply the battery's voltage by its amp-hours to get the battery's capacity in Watt-hours:

    capacity (in Wh) = voltage × amp-hours

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