Is Watts A Power Or Energy?

Watts are a commonly used unit of measurement in electricity and electronics. But there is often confusion around what exactly watts measure – power or energy. Many people use the terms power and energy interchangeably, but they are actually distinct concepts.

This article will provide a clear explanation of what watts are, the key differences between power and energy, and why there is confusion between the two. We’ll look at how watts are used to measure power, how energy is measured in joules, and how watts and hours can be combined to quantify energy. Examples will help illustrate the practical differences between power in watts and energy in joules or watt hours. By the end, you’ll understand exactly why watts measure power and not energy.

Definition of Watts

A watt is a unit of power, defined as 1 joule per second. Power is the rate at which energy is transferred or converted per unit of time. A watt measures the rate of energy conversion or transfer.

Specifically, one watt is equivalent to one joule of energy being converted or transferred in one second. For example, a 100-watt lightbulb uses 100 joules of electrical energy every second to produce light and heat.

The watt is named after Scottish engineer James Watt, who helped develop the steam engine in the late 18th century. The watt was formally adopted as part of the international system of units (SI) in 1960.

Difference Between Power and Energy

the watt unit measures the rate of energy transfer or consumption per second

Power and energy are two related but distinct concepts in physics. While they are often used interchangeably in casual conversation, understanding the precise difference between power and energy is important.

Power is the rate at which energy is transferred, used, or transformed per unit time. Power refers to how quickly or slowly energy is being used or moved. Common units of power include Watts (W), horsepower (hp), and kilowatts (kW).

In contrast, energy is the capacity to do work. Energy exists in various forms such as electrical, mechanical, chemical, nuclear, or thermal energy. The common units used to measure energy are Joules (J) in the metric system and British Thermal Units (BTUs) in the imperial system.

The key distinction is that power describes the speed or rate at which energy is being used, while energy refers to the total amount available to do work. Power is about the flow of energy over time, whereas energy is the total capacity.

Power Measured in Watts

Watts are used to measure power, which is the rate at which energy is transferred or consumed. Some common examples of power measured in watts:

  • Lightbulbs are rated based on their wattage, like a 60W or 100W bulb.
  • Appliances like blenders and hairdryers have a wattage rating to indicate their power usage.
  • Electric motors and engines are often rated in horsepower, which can be converted to watts.
  • Audio speakers and amplifiers have a power rating in watts.

In all of these cases, the watt rating indicates how much power the device draws or uses when operating. The higher the watts, the more power is being used. Watts measure the rate of energy consumption, not the total energy capacity.

Energy Measured in Joules

While watts measure power, energy is measured in joules. A joule is a unit of energy defined as the work required to produce one watt of power for one second.

For example, if a 100-watt light bulb is on for 10 seconds, it uses 100 watts x 10 seconds = 1,000 joules of electrical energy. The joule is the standard international unit for energy, although there are some other units in use as well, like kilowatt-hours.

The key distinction is that a watt measures power (the rate at which energy is transferred or consumed), while the joule measures energy itself. So watts are used for power ratings, but joules are used to measure the total energy used, generated, or transmitted over time.

Watt Hours for Energy

While watts measure power, watt hours are commonly used to measure energy consumption over time. A watt hour is equal to the energy used by a device operating at 1 watt for 1 hour. For example, a 100 watt light bulb running for 10 hours would use 100 x 10 = 1000 watt hours of energy. The watt hour is useful for calculating electricity usage on utility bills, where the kilowatt hour (kWh) is frequently used. 1 kWh is equal to 1000 watt hours. By multiplying the power rating of a device by the time it runs, you can calculate energy usage in kilowatt hours. Watt hours and kilowatt hours allow us to quantify energy consumption, rather than just instantaneous power draw.

Common Confusion

There is a common misconception that watts measure energy rather than power. This stems from the fact that watts and watt-hours are both used as units in electricity, which can understandably cause confusion. People often use the terms power and energy interchangeably in everyday language as well.

The key distinction is that power is the rate at which energy is transferred or consumed. Watts measure power, specifically the rate of energy transfer per unit of time. For example, a 60W lightbulb uses 60 joules of energy per second. Joules are the unit of energy, while watts are the rate of energy consumption.

Watt-hours are used to measure energy consumption over time. For example, if a 60W bulb is left on for one hour, it will consume 60 watt-hours of energy. One watt-hour is equivalent to 3600 joules. So in this case, 60 watts * 1 hour = 60 watt-hours = 216,000 joules of energy.

The watt unit confuses people because it is present in both watts and watt-hours. But watts themselves only measure the rate of power, not the total accumulation of energy over time. The “hours” part of watt-hours is what makes it an energy measurement.

So in summary, watts measure power and watt-hours measure energy. Watts tell you how fast energy is being used, while watt-hours tell you the total energy used over time.

Practical Examples

To understand the relationship between power (watts) and energy (joules), let’s look at some practical examples with calculations:

A 60W lightbulb that is on for 1 hour will use 60W * 1hr = 60 watt-hours (Wh) of energy. We can also calculate this in joules: 60W * 1hr * 3600sec/hr = 216,000 joules.

An electric oven has a power rating of 220W. If it is used for 30 minutes, it will consume 220W * (30min / 60) = 110Wh = 396,000 joules of energy.

A vacuum cleaner with a 1200W motor used for 15 minutes will use 1200W * (15min / 60) = 300Wh = 1,080,000 joules of energy.

As you can see from these examples, watts (W) represent power while joules (J) or watt-hours (Wh) represent energy. Power x time = energy. But watts and joules are different units used for different purposes.


To recap, watts and joules are different units used to measure power and energy respectively. Watts measure the rate of energy consumption or production per second. For example, a 100 watt light bulb uses 100 joules of energy per second. Joules measure the total amount of work or energy. Leaving that 100 watt bulb on for 1 hour would consume 100 watts x 3600 seconds = 36,000 joules of energy. So watts describe instantaneous power while joules describe total energy. This difference can create confusion when comparing power ratings or energy usage. Just remember that watts are always power and joules are always energy. Keeping the units straight and doing unit conversions as needed clears up the distinction.


In conclusion, it’s important to understand the difference between power and energy when dealing with electricity. Watts are a unit of power, specifically measuring the rate at which energy is transferred or converted. Joules are a unit of energy, measuring the total amount of work done or heat produced. While related, power and energy are fundamentally different concepts.

The common confusion comes from the fact that watts and joules are both derived from the same base units (kg, m, s). However, watts measure instantaneous power delivery, while joules accumulate over time to measure total energy usage. Remember that power is the rate of energy flow, similar to your car’s speed versus the total distance traveled.

To summarize:

  • Watts measure power (the rate of energy transfer)
  • Joules measure energy (the total amount of work)

Understanding this key distinction will help when calculating electrical energy usage, converting between units, and discussing power delivery. The next time you see watts or joules used, consider if it refers to an instantaneous rate or a cumulative amount.

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