Unit of Power

Edited By Team Careers360 | Updated on Jul 02, 2025 04:59 PM IST

Introduction

Power is the measurement of doing work per unit time. Let us answer the question what is the unit of power? The SI unit of power is watt, which was given after the Scottish physicist James watt. To measure the capability of his modified steam engine, James watt discovered another unit of power which is horse power unit. But the unit commonly used to express the power of an engine is Watt, which is the practical unit of power. Let's move forward in order to fully grasp and understand what the unit of power is.

This Story also Contains

Introduction
What is power?
Unit of Power
How do we exert one watt power?
Other generally used units of power

Unit of Power - Definition, Unit, FAQs

What is power?

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Power is the defined as the rate at which electrical energy is consumed in an electrical circuit or amount of work done per unit time.

P=WT

Where,

P is the power
W is the work done
T is the time taken

Also, P=dWdT=d(F. r)dT

Where, w = F. r = force x displacement

P= F. drdT

P= F. V

Where, V=drdT=velocity

From P=WT

Unit of power in SI unit is =joulessecond = watt

From the above equation, we can say that the series of basics and physics that are making this work has prequels and sequels, concept that depend on each other and is going to help us understand it better. Joules depend on newton and watt which is the unit of power depends on joules.

Watt is exceedingly simple to understand. It is basically how much energy is used over an amount of time. Hence we can say that, if we spend one joule of energy in one second then that yield one watt.

And from

P= F. V

The unit of power is = ms-1 = kgm2s-3

Differential mathematical forms of electric power:

P=WT

Potential difference V = WQ

So, W = V.Q

P=VQT

From ampere’s law, I=QT

P = V.I

Now according to ohm’s law V=IR

So,

P = I2R

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The unit of power in SI unit is Watt. This name was given by a Scottish scientist James watt. He became very famous because of his contribution in improving the steam engine in 1781.

watt unit is nemed after the scottish inventor james watt

Let us now define unit of power: Watt

Watt can be defined as the power of appliance which does work or consumes energy at the rate of 1 joule per second. Watt is the measurement of how fast the work is done. The abbreviation for watt is W. Time doesn’t matter in the definition of joule, it is just getting the job of moving an object using one newton of force over one meter. If we use one newton force and we complete one meter distance, no matter how long the task took it is said to have one joule energy. Similarly the Power is simply where time matters and is taken into account and this can be seen in the expression joule per second which is a watt. Watt is used in electrical work. Any power of an electrical appliance tell the rate at which the energy is consumed. So, the power of an electrical appliance is defined in terms of watt. For example, a bulb of 30 watt tells us that it consumes the electrical energy at the rate of 30 joule per second.

Now to understand it better or relate to it let’s take a look at an example:

Let us suppose a man pushes a cart from point A to point B with an energy of one joule. If he covers the distance from point A to point B in one second then he spent one watt of power and if he take two second to cover the same distance then he only produces half(0.5) a watt because it took him twice as long to accomplish the task. Hence we can conclude that the less time it takes to accomplish the task, higher will be the power.

How do we exert one watt power?

The work done to lift a 102g (0.102 kg) through a distance of one meter in one second.

We know,

P=WT =f.dT =m.g.dT

Where, P = power, T= time taken, W= work done, f= force, m= mass,

g is equivalent to acceleration due to gravity, d= distance

After putting the values,

P=0.102 kg.9.81m/s^2 .1m1 s

P=1 Watt

Let’s take a look at another example,

How much do I do on my body when I do one pull up? Assume I raise 60 cm (0.60 m) and I have a mass 95 kg. It takes me 0.5 seconds.

We know,

P=WT =f.dT =m.g.dT

Where, P = power, T= time taken, W= work done, f= force, m= mass,

g is equivalent to acceleration due to gravity, d= distance

After putting the values,

P=95kg.9.81m/s^2 .0.60 m 0.5 s

P=1120 Watt

Now if we are clear with the unit watt let’s see another unit of power which is Horsepower unit.

1 horsepower is equivalent to 746 watt.

We measure a power of engine especially the car engine in horsepower. Before cars, trains and buses, horses were the primary vehicle for transportation and for any other commercial work. Horses were widely used for all kinds of power in 18th century and earlier. So, how much power is there in a horsepower? How horsepower came to be one of the most common ways that we measure the power of car engine? Horsepower is the unit of power. Well, the history of the horsepower goes back to the 18th century and the Scottish inventor, James Watt, is a reason behind this unit. The first commercial stem powered device is generally considered to be Thomas Savery’s steam pump which invented in 1698. By 1712, Thomas Newcomen had created a steam engine that could transmit continuous power. Both these early designs were primarily used to pump water for mines. The Newcomen engine was significantly more powerful and replaced the team of 100 horses that have been used to pump water out of a mine. In 1758, the Newcomen engine came to the attention of James Watt a Scottish inventor and mechanical engineer, who made a living by fixing and making scientific and mechanical instrument in Glasgow Scotland. He came up with the term horsepower and coined the horsepower unit. Hence we got, 1 horsepower is equivalent to 746 watts which is 0.746 kilowatts

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Other generally used units of power

Following are the all units of power:

Horsepower, abbreviated as HP and is equivalent to 746 watts.
Kilowatts, abbreviated as kW and is equivalent to 1000 watts or 103 watts.
Megawatts, abbreviated as MW and is equivalent to 1000000 watts or 106 watts.
Gigawatts, abbreviated as GW and is equivalent to 1000000000 watts or 109 watts.
Decibel-milliwatts, abbreviated as dBm and is equivalent to 1Watt = 30 dBm.
British thermal unit, abbreviated as BTU and is equivalent to 1Watt = 3.412 BTU/hr.
Calories per second, abbreviated as cal/sec and is equivalent to 1Watt = 0.2 calories per second

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NCERT Physics Notes:

Frequently Asked Questions (FAQs)

1. Is watt time dependent?

Yes we define watt as joules per second. Hence, it is time dependent.

2. Define unit of power.

Watt can be defined as the power of an appliance which does work or consumes energy at the rate of 1 joule per second. Watt is the measurement of how fast the work is done.

3. Who gave the term horsepower?

James watt

4. For small unit of power we use watt but for high power which unit we use?

Kilowatt

5. How is power related to force and velocity?

Power can be expressed as the product of force and velocity: Power = Force × Velocity. This relationship is particularly useful in situations involving constant motion.

6. What is the formula for electrical power?

The formula for electrical power is P = VI, where P is power in watts, V is voltage in volts, and I is current in amperes.

7. How does power relate to the concept of work in physics?

Power is the rate at which work is done. While work measures the total energy transferred, power tells us how quickly that transfer occurs. The relationship is: Power = Work / Time.

8. How does power factor affect electrical power in AC circuits?

Power factor is the ratio of real power to apparent power in AC circuits. A low power factor means that a significant portion of the apparent power is not doing useful work, reducing efficiency and potentially increasing costs.

9. What is instantaneous power?

Instantaneous power is the power at a specific moment in time. It's particularly useful when dealing with systems where power output varies over time, such as in alternating current circuits.

10. How is power relevant in thermodynamics?

In thermodynamics, power is often considered in terms of heat transfer rates or the rate of work done by or on a system. It's crucial in analyzing heat engines, refrigeration cycles, and other thermal processes.

11. What is meant by 'power hungry' in the context of electronic devices?

Power hungry' refers to devices or systems that consume a large amount of power relative to their function or compared to similar devices. This often implies lower efficiency or the need for more robust power supply systems.

12. How does altitude affect engine power?

Engine power typically decreases with increasing altitude due to lower air density. This results in less oxygen available for combustion, reducing the engine's ability to produce power.

13. How does gearing affect power in mechanical systems?

Gearing can change the balance between force and velocity in a mechanical system, but it doesn't change the overall power. Lower gears provide more force but less speed, while higher gears do the opposite, maintaining the same power output.

14. How does power relate to the concept of energy conservation?

The principle of energy conservation states that energy cannot be created or destroyed, only converted from one form to another. Power describes the rate at which this energy conversion occurs, always adhering to the conservation principle.

15. What is the relationship between horsepower and watts?

Horsepower and watts are both units of power. One horsepower is approximately equal to 745.7 watts. This relationship allows for conversion between the two units.

16. How does power consumption relate to energy bills?

Energy bills are typically based on the amount of electrical energy consumed, which is power multiplied by time. Higher power consumption over time leads to higher energy usage and, consequently, higher bills.

17. How is power measured in practice?

Power can be measured using various instruments depending on the context. For electrical power, wattmeters are used. In mechanical systems, dynamometers can measure power output. In some cases, power is calculated from measurements of other quantities like force, velocity, voltage, and current.

18. What's the difference between power and energy?

Energy is the capacity to do work, while power is the rate at which that energy is used or transferred. Energy is measured in joules, while power is measured in watts (joules per second).

19. What is the power equation in terms of torque and angular velocity?

For rotational motion, power can be expressed as the product of torque and angular velocity: Power = Torque × Angular Velocity. This is useful in analyzing rotating machinery.

20. What is the SI unit of power?

The SI unit of power is the watt (W), named after James Watt. One watt is equal to one joule of energy transferred per second.

21. What is the concept of power density?

Power density is the amount of power per unit volume or mass. It's an important consideration in designing compact or lightweight power systems, such as in mobile devices or vehicles.

22. What is meant by 'rated power' in electrical devices?

Rated power refers to the power consumption or output that a device is designed to handle under normal operating conditions. It's typically specified by the manufacturer and indicates the device's capacity.

23. How does temperature affect power output in solar panels?

Generally, the power output of solar panels decreases as temperature increases. This is due to increased resistance in the semiconductor materials, highlighting the importance of cooling in solar energy systems.

24. What is the difference between mechanical and electrical power?

Mechanical power involves the rate of doing physical work, often through motion or force application. Electrical power relates to the rate of electrical energy transfer. While the underlying concept is the same, they are measured and applied differently in practice.

25. What is power in physics?

Power in physics is the rate at which work is done or energy is transferred. It measures how quickly energy is used or converted from one form to another.

26. How is power related to work and time?

Power is directly related to work and inversely related to time. It can be calculated by dividing the amount of work done by the time taken to do that work: Power = Work / Time.

27. Can power be negative?

Yes, power can be negative. Negative power indicates that energy is being absorbed or received by a system, rather than being expelled or done by the system.

28. Why is power important in real-world applications?

Power is crucial in real-world applications because it helps us understand how quickly energy is being used or transferred. This is important for designing efficient machines, managing energy consumption, and optimizing performance in various systems.

29. How does power relate to efficiency?

Power relates to efficiency in that a more efficient system will require less power to perform the same amount of work. Efficiency is often expressed as the ratio of useful power output to total power input.

30. What is regenerative braking and how does it relate to power?

Regenerative braking is a process where kinetic energy is converted back into electrical energy during deceleration, instead of being lost as heat. This process effectively makes the power flow negative, charging batteries in electric vehicles.

31. What is the difference between average power and instantaneous power?

Average power is the total energy transferred divided by the total time taken, while instantaneous power is the power at a specific moment. Average power gives an overall picture, while instantaneous power provides detailed information about power variations.

32. What is the significance of peak power vs. continuous power?

Peak power is the maximum power a system can produce or handle for short durations, while continuous power is what can be sustained indefinitely. Understanding both is crucial for designing systems that can handle both steady-state and transient demands.

33. How does friction affect power in mechanical systems?

Friction typically reduces the useful power output of a mechanical system by converting some of the input energy into heat. This is why reducing friction is often a goal in improving mechanical efficiency.

34. What is the relationship between power and acceleration?

Power is related to acceleration through force and velocity. More power is required to achieve higher acceleration, as power equals force times velocity, and force is mass times acceleration.

35. What is the significance of power in the design of electrical grids?

Power is crucial in electrical grid design as it determines the capacity needed for generation, transmission, and distribution systems. Understanding power demands and fluctuations is essential for maintaining grid stability and meeting consumer needs.

36. How does air resistance affect power requirements in transportation?

Air resistance increases with velocity, requiring more power to maintain higher speeds. This is why vehicles require significantly more power to travel at high speeds, and why aerodynamics is crucial in vehicle design.

37. What is the concept of specific power in engineering?

Specific power is the power output per unit mass of a system. It's particularly important in aerospace and automotive engineering, where high power-to-weight ratios are desirable for performance.

38. How does power relate to the brightness of light bulbs?

The brightness of a light bulb is generally proportional to its power consumption. However, different technologies (like LED vs. incandescent) have varying efficiencies in converting electrical power to light output.

39. What is the relationship between power and torque in rotating machinery?

Power in rotating machinery is the product of torque and angular velocity. This means that the same power can be achieved with high torque and low speed, or low torque and high speed.

40. How does power scaling work in physics?

Power scaling refers to how power changes with size in physical systems. Often, power doesn't scale linearly with size. For example, the power needed to overcome air resistance increases with the cube of velocity.

41. What is the concept of power management in electronic devices?

Power management involves controlling and optimizing the power consumption of a device. This includes techniques like dynamic voltage and frequency scaling, sleep modes, and efficient resource allocation to extend battery life or reduce energy costs.

42. How does power relate to the concept of force fields in physics?

In the context of force fields, power can be thought of as the rate at which work is done on a particle moving through the field. The power depends on both the strength of the field and the velocity of the particle.

43. What is the significance of power in wave phenomena?

In wave phenomena, power is often expressed as the energy transported by the wave per unit time. This is crucial in understanding energy transfer in various wave types, from sound waves to electromagnetic radiation.

44. How does the concept of power apply in quantum mechanics?

In quantum mechanics, power can be related to the rate of change of expectation values of observables. It's relevant in understanding processes like stimulated emission in lasers or energy transfer rates in quantum systems.

45. What is the relationship between power and radiation in electromagnetic theory?

In electromagnetic theory, power is related to the Poynting vector, which describes the directional energy flux of an electromagnetic field. This concept is crucial in understanding antenna radiation patterns and energy transfer in electromagnetic waves.

46. How does power factor correction work and why is it important?

Power factor correction aims to bring the power factor of an AC electrical system closer to 1 by reducing reactive power. This is important because it improves efficiency, reduces energy costs, and helps maintain grid stability.

47. What is the concept of power spectral density in signal processing?

Power spectral density describes how the power of a signal is distributed across different frequencies. It's a crucial concept in analyzing random processes, communication systems, and various types of noise.

48. How does power relate to the intensity of sound waves?

The intensity of a sound wave is the power transmitted per unit area. Higher power results in higher intensity, which we perceive as louder sound. The relationship is logarithmic, which is why we use the decibel scale for sound intensity.

49. What is the significance of power in statistical mechanics?

In statistical mechanics, power can be related to the rate of energy transfer between microscopic states. This concept is important in understanding non-equilibrium thermodynamics and the behavior of complex systems.

50. How does power consumption change in superconducting materials?

Superconducting materials have zero electrical resistance, which means they can conduct electricity with theoretically zero power loss. This property has significant implications for power transmission and high-efficiency electrical systems.

51. What is the relationship between power and information in physics?

The relationship between power and information is explored in fields like information theory and quantum computing. For example, Landauer's principle relates the erasure of information to a minimum amount of energy dissipation, connecting information processing to power consumption.

52. How does the concept of power apply in fluid dynamics?

In fluid dynamics, power is often considered in terms of the rate of energy transfer in fluid flows. This is crucial in understanding phenomena like turbulence, drag, and the efficiency of hydraulic systems.

53. What is the significance of power in the study of black holes?

In the context of black holes, power is relevant in understanding phenomena like Hawking radiation and the energy released during matter accretion. The immense gravitational power of black holes makes them some of the most energetic objects in the universe.

54. How does the concept of power relate to the expansion of the universe in cosmology?

In cosmology, power is relevant in understanding the energy dynamics of the expanding universe. Concepts like the power spectrum of cosmic microwave background radiation provide crucial information about the early universe and its evolution.