Amps vs. Watts: How They’re Different

amps vs. watts

Amps vs. Watts: How They’re Different

Generally, people are aware of electricity, or at least its effects. The way it can be used to power our lights, AC, homes, favorite devices and even our new electric vehicles. As well as this, the potential dangers of electricity are well-established. However, when it comes to how electricity works and the terms and measurements used to describe it, things can get a little more complicated and less intuitive. Understanding exactly what things like amps and watts mean is crucial to safely take on any sort of electrical project, and can be hugely beneficial if you’re looking to save money on your energy bills. Without further ado, let’s get into a comparison and explanation of amps vs. watts.

Amps vs. Watts: Side-By-Side Comparison

DefinitionUnit of electric currentUnit of power, or the rate at which work is done
Equivalent toC / s: 1 coulomb, or 6.24 x 1018 electrons worth of charge moving past a given point in 1 second.J / s: 1 Joule per second. In SI units, 1 kg / m2 / s3. In CGS units, 107 erg / s
Unit SystemSISI
OriginNamed after André-Marie AmpèreNamed after James Watt

Amps vs. Watts: What’s the Difference?

Now we’ve defined amps and watts and their units, it’s time to look closer at the differences between them.


As far as the mathematical symbols used, things are kept fairly simple for amps and watts. To signify either, it’s simple — the first letter from both is taken to give A and W respectively. This is how amps and watts are denoted when used in calculations and equations, as well as how they’re measured and listed on electrical devices. 

The Definition of Amps

Amperes, or amps for short, are always defined within the International System of Units (SI), which was created by the Bureau International des Poids et Mesures (BIPM). This is a French organization responsible for establishing standardized scientific measurements. As such, 1 amp is equivalent to 1 coulomb’s worth of charge moving past a given point in 1 second. 1 coulomb is defined as the amount of electric charge carried in 1 second by a 1 amp current. Put simply, a coulomb quantifies an amount of electric charge and an amp measures how fast the charge is moving past the point in space. Therefore, an amp is the rate at which current is flowing. Amperage means the current that can flow through, and is measured in amps.

The Definition of Watts

In comparison, watts can be thought of as a unit of power, or the rate at which “work” is done. In this case, work refers to the energy transferred from one object to another. Therefore, power is the rate of energy transfer or the amount of energy converted per unit of time. Watts are used as the standard unit of power for quantifying the rate of this energy transfer. In practical terms, watts can be thought of as the power it takes to do something or the energy used by a device, such as turning on a light bulb. In this way, 1 watt is equivalent to 1 Joule of energy being transferred per second. A 40-watt light bulb, for example, requires 40 Joules of energy to be transferred per second to run optimally.

amps vs. watts
To light up properly, this 60-watt bulb requires 60 joules of energy to be transferred each second the light is on.


Other Ways to Define Watts

1 Watt is also equivalent to 1 kg.m2 / s3, or 1 kilogram meter squared per second cubed. The reason for this is that 1 Joule can be defined as the energy transferred when a force of 1 Newton displaces a mass through a 1-meter distance. A Newton is a unit of force defined as the force which gives 1 kilogram of mass an acceleration of 1 meter per second squared. Thus, 1 Newton is equal to 1 kg.m / s2, and 1 Joule is equal to 1 kg.m2 / s2 because it’s essentially defined as 1 Newton per meter. This all means that 1 watt is equal to 1 kg.m2 / s3

To make matters simpler, or potentially more complicated, watts can also be defined according to the centimeter-gram-second (CGS) metric system of units as 107 erg / s, or 10,000,000 ergs per second. An erg is equivalent to 1/ 107 Joules, so a quick conversion gives 1 watt as 107 x 1 / 107 Joules / s, which cancels out to 1 Joule / s.

There are many equations that can relate watts to amps, but the most common relates watts and amps to voltage:

  • W = A * V

where W is the power, measured in watts, A is the current, measured in amps, and V is the potential difference, measured in volts. This equation is often rearranged to give these variants:

  • V = W /A
  • A = W / V

Therefore, as long as you can measure 2 of these variables, you can calculate the other using these equations.

How Do We Measure Amps and Watts?

Both amps and watts can be measured using specific devices. To measure amps, we use an ammeter, which measures the current that an electrical device is using. If you want to calculate the power, or wattage, used by a system, then you can multiply the amps by the voltage using the equation previously mentioned. Most of the time there are set standards for voltage, usually according to geographic location. Because of this, you can usually simply check what the voltage in your area is and use this to calculate the wattage. As an example, if your device uses 10 amps with a voltage of 40 volts, then the wattage would be equal to 10 * 40, or 400 watts. You can also purchase wattmeters for calculating watts directly, but they work on the same principles. That is, they measure the voltage and amps and then calculate the wattage.

What are the Benefits of Measuring Amps and Watts?

We’ve covered exactly what amps and watts are, and how they’re measured. However, why would you want to measure these? There are many reasons, but the most common are to try and save money and ensure you’re carrying out electrical work under safe conditions. When you’re deciding on what type of wires to use in your wiring project, for example, it’s important to know how much current is flowing through. Generally, the more current there is, the bigger the wire needed.

When it comes to household bills, being able to calculate how much power your appliance is consuming can be incredibly helpful in cutting energy costs. Once you’ve calculated how many watts an appliance uses from the voltage and current, you can simply convert this to watt-hours by multiplying the watts by how many hours you use the appliance. By doing this, you can easily compare the power draw of several appliances to see which is the most efficient.

Calculating Energy Usage – An Example

For example, if you use your microwave for 1 hour a day, and you’ve calculated that the microwave uses 900 watts, then simply multiply the wattage by the hours to get a value of watt-hours per day. In this case, the watt-hours per day would be 1 * 900, or 900 watt-hours per day. Realistically, most microwaves these days display their wattage clearly as a power rating, but the calculation comes in handy when the wattage isn’t so clear.

Amps vs. Watts
It’s important to know the wattage of your microwave, as some foods have different cooking times for different wattage machines.


Reduce Your Wattage, Not Your Amperage

It’s not uncommon for people to believe that operating devices at a higher voltage will reduce their energy bills, because the current will be reduced. However, if we refer to the previous equations, we can see that this isn’t the case. For a simple illustration, let’s look at the case of a 240W bulb with voltages of 240V and 120V:

  • 240W / 240V = 1 A
  • 240W / 120V = 2 A

We can see that the amperage does indeed change when the voltage changes. However, the watts used by the bulb remain the same. Plus, since energy companies always bill in kilowatt-hours, the only realistic way to reduce your bills is to adjust the wattage used by your devices.

Amps vs. Watts: 7 Must-Know Facts

  • Amps are given the symbol A, and watts are given the symbol W
  • Amps measure the rate an electric current is flowing, whereas watts measure how much power is being used by a device, or the rate of energy transfer
  • Amps can also be defined as 1 Coulomb per second, and watts as 1 Joule per second
  • Amps can be measured with an ammeter, and watts with a wattmeter
  • Amps and watts are related to each other through the voltage of a system
  • Calculating amps and watts is important for safe electrical work and helping to reduce energy bills
  • A watt can also be defined as 1 kg.m2 / s3

Up Next…

Amps vs. Watts: How They’re Different FAQs (Frequently Asked Questions) 

What's the difference between amps and watts?

Amps measure the rate at which an electric current is flowing, whereas watts measure how much energy is being used by the device which the current is flowing through.





How are amps and watts related?

Amps and watts are related through the potential difference, or voltage, of a system. This relationship is illustrated with the equation W / V = A, where W is watts, V is voltage and A is amps.


Where did the terms amps and watts come from?

Amps, or amperes, are named after the French mathemetician and physicist André-Marie Ampère, and watts are named after the Scottish engineer and chemist James Watt. The units for amps and watts were defined by the BIPM, the International Bureau for Weights and Measures. They are defined according to the BIPM’s SI system of units.

Did the definition of an amp change?

Yes, in 2019, the definition of an ampere was changed. This was because the previous definition was hard to acknowledge in practice without a high degree of precision. An amp used to have a complicated definition, being the current maintained between two conductors of infinite length and 1 meter apart in a vacuum that would produce a force of 2 x 10^7 Newtons per meter of length. The new definition is a lot easier to illustrate, being 1 Coulomb’s worth of electric charge moving past a point in 1 second, where a Coulomb is equivalent to 6.24 x 10^18 electrons worth of charge.

Why do amps and watts matter?

It’s important to calculate amps when carrying out electrical projects, for instance to know what kind of wire to use to support the current. Understanding the watts, or power usage of an appliance, is useful when trying to make your home more energy efficient and reduce your bills.

How do you measure amps and watts?

The simplest way is to measure amps using an ammeter, check your area for the standard voltage and then calculate the watts using the equation W = A * V. However, you can also use a wattmeter, which will perform these calculations for you.

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