Watt’s Law Calculator: Why Should I Use It? - ShopSolar.com
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Watt’s Law Calculator

Our Watt’s law calculator guide will tell you the origins of the law, how you can use the calculation, and why you need to use it for your solar setup.

Ultimately, this law and its corresponding calculation allow you to calculate the power output of your solar panels and all the associated pieces of equipment in your setup. Understanding these concepts also allows you to do more complicated equations, like converting watt-hours to milliamp-hours.

This can help you choose other essential components of your setup, such as your inverter or solar generator.

What Is Watt’s Law?

what Is watt’s law calculator

Watt’s law is named after James Watt, a chemist and engineer from Scotland. He was the first to define the relationship between power, current, and voltage.

This is the relationship the law defines, and it’s represented as a mathematical equation: P = IV.

To understand this equation before using it, let’s go over what those symbols stand for.


“P” represents power in this equation. The unit used to measure power is watts. 

Not exactly sure what power is? Don’t worry, we’re here to make solar simple for you. Power is the measure of how much work a circuit can do.

This shows us how it’s useful in the solar power realm. When we know how much work our setup can perform, we know how much of our home we can power with it.

“I” can get a little confusing, because it represents current. Current is measured by amps, so the corresponding unit is followed by an A. 

It’s the amount of electric current flowing through a circuit at any given time.

“V” represents voltage, which is a measure of the volts in the circuit. This refers to the electrical potential difference between two points in the circuit. This difference is what causes electrons to flow, and power to be produced. 

Calculating Using Watt’s Law

Now that you know a bit more about the three variables, let’s dive into the equation derived from Watt’s law. As mentioned earlier, you can use this to measure the power output of your solar panel. 

If you have more than one solar panel, you can simply do the equation per panel, and add your results together.

This equation can also be used to calculate the output of your generator. If you’re in the market for one of those, have a look at our Zendure Superbase Pro 2000 - we promise it won’t disappoint.

calculator of watt’s law


Using Watt’s Law in practice requires using the calculation which describes the relationship between the three variables: power, current, and voltage. 

The law says that power is equal to the current multiplied by the voltage of a circuit. We’re going to think of your solar panel as a circuit to do this - because it is. It creates electrical power that you can use to power appliances in your home.

The equation looks like this: P = V x I.

Because these values are so closely related, we can use the same equation in a different form to figure out VA from Watts.

However, since we’re looking for the wattage, we don’t need to rearrange the calculation, as power is the variable we’re solving for. 

We simply need to plug in the values for the known variables, which are the voltage and current of the circuit.

Let’s assume your solar panel has a voltage of 12V, and a current of 7A. These are pretty common measures, but it’s possible that your panels won’t match these figures exactly.

To find the values for your solar panel, you can look in the user’s manual, or you can find the information on the back of your panel.

Back to the equation, we’re going to plug these numbers in to demonstrate how easy it is to use.

P = V x I

   = 12 x 7

   = 84W.

See? So easy! 


We’ve wrapped our heads around the basics of Watt’s Law and how to calculate it, but it doesn’t stop there. There’s a lot more we can do with these equations and we’ve put together some helpful guides to help you out.

Working out a Watt density calculator can help you figure out how many solar panels you need to meet your demands.

Watt’s Law isn’t everything though. You also need to know how to calculate watts-hours to amp-hours and a DC to AC watts conversion calculator.

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