Kilovolt-ampere (kVA) refers to special units of the SI system. It detects electrical power and is 1000 volt-amperes. With the help of this unit, the value is fixed, which is the absolute power of the alternating current.

Another unit - kilowatt equals (kW) the amount of energy that is consumed or generated by a device with a power of 1 kW for 60 minutes. It allows you to accurately assess the mechanical power of a device. Quite often, the question arises of how to translate kVA into kW, since this is required for specific technical calculations. However, you should first learn the specific terminology used in such operations.

Concepts and terms

The first step is to establish the difference between kVA and kW. It is known that in the first case the total power is reflected, and in the second - the active one. In the most ideal case, with an active load, these powers will be the same. Other types of load, such as electric motors or computers, create a component. In this regard, the activity of the full power increases, so it will be the square root of the sum of the squares of the active and reactive power.

The unit of apparent power is kilovolt-amperes, which is 1000 volt-amperes. The determination of this parameter for alternating current is carried out by the product of the effective value of the current in the circuit, measured in amperes and the voltage at the terminals, measured in volts.

The next unit to work with is watt (W) or kilowatt (kW). That is, 1 watt is such a power when work equal to 1 is performed for 1 second. As electrical or active power, 1 watt is equal to 1 ampere of constant current at 1 volt.

There is a special unit known as "cos phi" (cos f), which is the power factor. In essence, it will be the ratio of active power to total power, indicating linear and non-linear distortions in the electrical network that occur when the load is connected. The maximum value of the coefficient is one (1). A good and satisfactory indicator would be 0.95 and 0.90, respectively. A value of 0.8 is most suitable for modern electric motors and is considered an average. Coefficients 0.7 and 0.6 are the lowest and most unsatisfactory indicators.

Speaking more simple language, cos f means the losses occurring during the conversion of electrical energy into mechanical energy. These numbers will vary from device to device, but they add up to the overall current loss in the system.

Calculation examples

When calculating energy consumption, it is often necessary to convert one unit of measurement to another. This makes it possible to determine in advance the expected losses and to find out the full power characteristics.

The simplest conversion option would be to convert kVA to kW and vice versa. For example, 10 kVA is converted as follows: 10 kVA x 0.8 = 8 kW. The reverse conversion will look like this: 8 kW / 0.8 = 10 kVA.

From the point of view of the consumer, the kW value is the net power and the kVA value is the apparent power. For most calculations, a loss factor of 0.8 is used. Therefore, in order to transfer one unit to another, it is necessary to reduce the kVA by 20% and, as a result, you will get kW with a small error that does not affect the overall result of the calculations.

All manipulations with translations can be formalized in the form of the formula: P = S x cos f, in which P is the active power (kW), S is the total power (kVA), cos f is the power factor (losses).

After converting kVA to kW, a different formula can be used to reverse the process: S = P / cos f. This makes it possible to convert units that are used for all types of calculations.

When talking about the power of electrical appliances, we usually mean active energy. But many devices also consume reactive energy. This article explains what kVA is and what is the difference between kVA and kW.

Active and reactive energy

In an alternating current network, the magnitude of the current and voltage changes sinusoidally with the frequency of the network. This can be seen on the oscilloscope screen. All types of consumers can be divided into three categories:

• Resistors, or active resistances, consume only active current. These are incandescent lamps, electric stoves and similar devices. The main difference is the phase coincidence of the current and voltage;
• Chokes, inductors, transformers and induction motors - use reactive energy and convert it into magnetic fields and back EMF. In these devices, the current is 90 degrees out of phase with the voltage;
• Capacitors - convert voltage into electric fields. In AC networks, they are used in reactive power compensators or as current-limiting resistances. In such devices, the current is 90 degrees ahead of the voltage.

Important! Capacitors and inductors shift the current relative to the voltage in opposite directions and, when connected to the same network, compensate each other.

Active energy is the energy released on an active resistance such as an incandescent lamp, electric heater and other similar electrical appliances. In them, the phases of the current and voltage coincide, and all the energy is used by the electrical appliance. At the same time, the differences between kilowatts and kilovolt-amperes disappear.

In addition to active energy, there is reactive energy. It is used by devices in the design of which there are capacitors or inductive reactance coils, electric motors, transformers or chokes. They are also possessed by cables of long length, but the difference with a device with a purely active resistance is small and is taken into account only when designing long power lines or in high-frequency devices.

Full power

Under real conditions, purely active, capacitive or inductive loads are very rare. Typically, all electrical appliances use active power (P) together with reactive (Q). This is the full power, denoted "S".

To calculate these parameters, the following formulas are used, which you need to know in order, if necessary, to carry out conversion of kVA to kW and vice versa:

• Active energy is useful energy converted into work, expressed in watts or kW.

KVA can be converted to kW using the formula:

where "φ" is the angle between current and voltage.

These units measure the payload of electric motors and other devices;

• Capacitive or inductive:

Displays energy losses in electric and magnetic fields. Unit of measurement - kVar (kilovolt-ampere reactive);

• Full:

1. U - mains voltage,
2. I is the current through the device.

It is the total power consumption of a device and is expressed in VA or kVA (kilovolt-ampere). In these units, the parameters of the transformers are expressed, for example, 1 kVa or 1000 kVa.

For your information. Such devices of 6000 / 0.4 kV and a capacity of 1000 kVa are among the most common for powering electrical equipment of enterprises and residential areas.

Kvar, kVa and kW are related by a formula similar to the famous Pythagorean theorem (Pythagorean pants):

Important! It should be noted that a 10 kW electric motor cannot be connected to a 10 kVa transformer, since the electricity consumed by this device, taking into account the cosφ, will be about 14 kilovolt-amperes.

Reducing cosφ to 1

The reactive energy used by consumers creates unnecessary stress on the cable and starting equipment. In addition, you have to pay for it, as well as for an active one, and in portable generators, the lack of compensation increases fuel consumption. But it can be compensated by using special devices.

Consumers in need of cosφ compensation

One of the main consumers of reactive energy is asynchronous electric motors, which consume up to 40% of all electricity. Cosφ of these devices is about 0.7-0.8 at rated load and drops to 0.2-0.4 at idle. This is due to the presence of windings in the structure that create a magnetic field.

Another type of device is transformers, the cosφ of which decreases, and the consumption of reactive energy increases in unloaded devices.

Compensating devices

Different types of devices are used to compensate:

• Synchronous motors. When applied to the excitation winding voltage higher than the nominal, they compensate for the inductive energy. This allows for improved network performance at no additional cost. When replacing a part of asynchronous motors with synchronous ones, the compensation possibilities will increase, but this will require additional costs for installation and operation. The power of such electric motors reaches several thousand kilovolt-amperes;
• Synchronous expansion joints. These synchronous electric motors are characterized by a simplified design and a power of up to 100 kilovolt-amperes, they are not intended to drive any mechanisms and operate in the X.X. mode. Their purpose is to compensate for reactive energy. During operation, these devices use 2-4% of the active energy of the amount compensated. The process itself is automated in order to achieve the cosφ value as close as possible to 1;
• Capacitor batteries. In addition to electric motors, capacitor banks are used as compensators. These are groups of capacitors connected in a "triangle". The capacity of these devices can be changed by connecting and disconnecting individual elements. The advantage of such devices is simplicity and low consumption of active power - 0.3-0.4% of the compensated one. The disadvantage is the impossibility of smooth adjustment.

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1 kilowatt [kW] = 1 kilovolt-ampere [kVA]

Initial value

Converted value

watt exawatt petawatt terawatt gigawatt megawatt kilowatt hectowatt decawatt deciwatt sanewatt milliwatt microwatt nanowatt picowatt femtowatt attowatt horsepower horsepower metric horsepower boiler horsepower electrical horsepower pumping horsepower British horsepower British horsepower thermal unit (int.) per hour Brit. thermal unit (IT) per minute Brit. thermal unit (IT) per second Brit. thermal unit (thermochemical) per hour Brit. thermal unit (thermochemical) per minute Brit. thermal unit (thermochemical) per second MBTU (international) per hour Thousand BTU per hour MBTU (international) per hour Million BTU per hour ton of refrigeration kilocalorie (IT) per hour kilocalorie (IT) per minute kilocalorie (IT) in second kilocalorie (term) per hour kilocalorie (term) per minute kilocalorie (term) per second calorie (IT) per hour calorie (IT) per minute calorie (IT) per second calorie (term) per hour calorie (therm) per minute calorie (therm) per second foot pound-force per hour foot lbf / minute foot lbf / second pound-foot per hour pound-foot per minute pound-foot per second erg per second kilovolt-ampere volt-ampere newton-meter per second joule per second exajoule per second petajoule per second terajoule per second gigajoule per second megajoule per second kilojoule per second hectojoule per second decjoule per second decijoule per second centijoule per second microjoule per second per second nanojoule per second picojoule per second femtojoule per second attojoule per seconds joule per hour joule per minute kilojoule per hour kilojoule per minute Planck power

Thermal resistance

More about power

General information

In physics, power is the ratio of work to the time it takes to do it. Mechanical work is a quantitative characteristic of the action of force F on the body, as a result of which it moves a distance s... Power can also be defined as the rate at which power is transmitted. In other words, power is a measure of the health of a machine. By measuring the power, you can understand how much and at what speed the work is being done.

Power units

Power is measured in joules per second, or watts. Along with watts, horsepower is also used. Before the invention of the steam engine, the power of engines was not measured, and, accordingly, there were no generally accepted units of power. When the steam engine began to be used in mines, engineer and inventor James Watt began to improve it. In order to prove that his improvements made the steam engine more efficient, he compared its power to the performance of horses, since horses have been used by people for many years, and many could easily imagine how much work a horse could do in a given amount of time. In addition, steam engines were not used in all mines. In those where they were used, Watt compared the power of the old and new models of the steam engine with the power of one horse, that is, with one horsepower. Watt determined this value experimentally by observing the work of draft horses at a mill. According to his measurements, one horsepower is 746 watts. Now it is believed that this figure is exaggerated, and the horse cannot work in this mode for a long time, but they did not change the unit. Power can be used as an indicator of productivity, since as power increases, the amount of work performed per unit of time increases. Many have realized that it is convenient to have a standardized unit of power, so horsepower has become very popular. It began to be used to measure the power of other devices, especially transport. Although watts are used almost as long as horsepower, the automotive industry is more likely to use horsepower, and many buyers have a better understanding of when these units are used to indicate the power of an automobile engine.

Household electrical appliances power

Household appliances are usually marked with wattage. Some luminaires limit the power of the bulbs that can be used in them, for example, no more than 60 watts. This is because higher wattage bulbs generate a lot of heat and the luminaire with the socket may be damaged. And the lamp itself at a high temperature in the lamp will not last long. This is mainly a problem with incandescent bulbs. LED, fluorescent and other lamps usually operate at lower wattage at the same brightness and, if used in luminaires designed for incandescent lamps, there is no power problem.

The more the power of the appliance, the higher the energy consumption and the cost of using the appliance. Therefore, manufacturers are constantly improving electrical appliances and lamps. The luminous flux of lamps, measured in lumens, depends on the wattage, but also on the type of lamp. The higher the luminous flux of the lamp, the brighter its light looks. For people, it is the high brightness that is important, and not the power consumed by the lamp, so lately, alternatives to incandescent lamps are becoming more and more popular. Below are examples of lamp types, their wattage and the luminous flux they generate.

• 450 lumens:
• Incandescent lamp: 40 watts
• Compact fluorescent lamp: 9-13 watts
• LED lamp: 4-9 watts
• 800 lumens:



• Incandescent lamp: 60 watts
• Compact fluorescent lamp: 13-15 watts
• LED lamp: 10-15 watts
• 1600 lumens:
• Incandescent lamp: 100 watts
• Compact fluorescent lamp: 23-30 watts
• LED lamp: 16-20 watts

From these examples, it is obvious that with the same generated luminous flux, LED lamps consume the least energy and are more economical than incandescent lamps. At the time of this writing (2013), the price of LED bulbs is many times the price of incandescent bulbs. Despite this, some countries have banned or are about to ban the sale of incandescent lamps due to their high power.

The power of household electrical appliances may differ depending on the manufacturer, and is not always the same during the operation of the appliance. Below are the approximate capacities of some household appliances.



• Household air conditioners for cooling a residential building, split system: 20-40 kilowatts
• Monoblock window air conditioners: 1-2 kilowatts
• Ovens: 2.1-3.6 kilowatts
• Washers and dryers: 2-3.5 kilowatts
• Dishwashers: 1.8-2.3 kilowatts
• Electric kettles: 1-2 kilowatts
• Microwaves: 0.65-1.2 kilowatts
• Refrigerators: 0.25-1 kilowatts
• Toasters: 0.7-0.9 kilowatts

Power in sports

Performance can be judged by power not only for machines, but also for people and animals. For example, the power at which a basketball player throws the ball is calculated by measuring the force she applies to the ball, the distance the ball flew, and the time that force was applied. There are websites that allow you to calculate work and power during exercise. The user selects the type of exercise, enters height, weight, exercise duration, after which the program calculates the power. For example, according to one of these calculators, the power of a person who is 170 centimeters tall and weighs 70 kilograms, who did 50 push-ups in 10 minutes, is 39.5 watts. Athletes sometimes use devices to measure the power at which muscles are working during exercise. This information helps determine how effective their chosen exercise program is.

Dynamometers

To measure power, special devices are used - dynamometers. They can also measure torque and force. Dynamometers are used in various industries, from technology to medicine. For example, they can be used to determine the power of a car engine. Several basic types of dynamometers are used to measure the power of vehicles. In order to determine the engine power using dynamometers alone, it is necessary to remove the engine from the car and connect it to the dynamometer. In other dynamometers, the force to be measured is transmitted directly from the wheel of the vehicle. In this case, the car engine drives the wheels through the transmission, which, in turn, rotate the rollers of the dynamometer, which measures the engine power under various road conditions.

Dynamometers are also used in sports and medicine. The most common type of dynamometer for this purpose is isokinetic. Typically, this is a sensor gym equipment connected to a computer. These sensors measure the strength and power of the entire body or specific muscle groups. The dynamometer can be programmed to issue alarms and warnings if the power has exceeded a certain value. This is especially important for people with injuries during the rehabilitation period, when it is necessary not to overload the body.

According to some provisions of the theory of sports, the greatest sports development occurs at a certain load, individual for each athlete. If the load is not heavy enough, the athlete gets used to it and does not develop his abilities. If, on the contrary, it is too severe, then the results deteriorate due to the overload of the body. Physical activity during some exercise, such as cycling or swimming, is influenced by many environmental factors, such as road conditions or wind conditions. Such a load is difficult to measure, however, you can find out with what power the body resists this load, and then change the exercise pattern, depending on the desired load.

Do you find it difficult to translate a unit of measurement from one language to another? Colleagues are ready to help you. Post a question to TCTerms and you will receive an answer within a few minutes.

As you know, the power depends on the work and the time required to complete the work. Each electrical appliance, device or household appliance operates with a certain power, which is a physical quantity equal to the ratio of the work performed in a certain time by a certain force to a given time interval. The higher the power indicator, the more work the electrical appliance can perform in a certain time.

Electric power at home

A kettle, a hairdryer, a vacuum cleaner, a computer and an ordinary incandescent lamp - electrical appliances of various levels and capacities surround a person everywhere, making his life more comfortable and cozy.

To find out the power of electrical appliances that a person uses in everyday life, just look at the information posted on the body of this device. Electrical power is an indicator of how much energy the device consumes from the network during its operation.

The power of an electrical appliance not only affects the meter readings and the cost of paying for electricity, but also the quality of the wiring. Do not forget that excessive current can lead, at best, to automatic power off, and at worst to short circuits, damage to contacts and fire.

Therefore, it will never be superfluous to know the dependence of the power of the device on the electric current in the network. To do this, it is necessary to understand the difference between the total, reactive and active power of electrical appliances.

Capacities are different

As a rule, manufacturers in the technical characteristics of electrical appliances, equipment indicate the total power, measured in kilovolt-amperes (kVA). At the same time, the consumer, accustomed to the familiar kilowatts (kW), begins to get lost and does not understand how much power the device, power tool, etc. have. Both kVA and kW are units for measuring the power of an electrical appliance, equipment, technology.

At the same time, kilowatts show the actually used power of the device during active operation, and kilovolt-amperes show the power level of the device as a whole. The full power is consumed by the device. However, she does not fully participate in the operation of the equipment. One part goes to heating, action (active power), and the other is transmitted to electromagnetic fields along the circuit (reactive power).

Each electrical appliance has a certain power factor - a value that characterizes the device by the presence of reactive power at a certain proportion of the load. This indicator makes it clear how much the power level of the device shifts under load relative to the voltage. There are several main indicators of power factor:

• 0.80 is a bad indicator;
• 0.90 - satisfactory;
• 0.95 is a good indicator;
• 1.00 is perfect.

For example, in the technical characteristics of the rotary hammer, a power of 5 kW is indicated. Since it has a reactance during operation, it has a poor power factor (0.85). Accordingly, the total power required to operate the rock drill is 5.89 kVA.

But the power factor of a conventional electric kettle is unity. Thus, the level of power consumption and the power used by the kettle are the same.

Full and active power are different physical quantities that give a complete picture of the technical characteristics of an electrical appliance and the conditions necessary for its high-quality operation.

Volt-ampere (VA) is the unit of apparent AC power, referred to as VA or VA. The apparent power of alternating current is defined as the product of the effective values ​​of the current in the circuit (in amperes) and the voltage at its terminals (in volts).
Watt (W) is a unit of power. Named after the Scottish-Irish inventor-mechanic James Watt, it is denoted W or W. Watt is the power at which work equal to 1 joule is done in 1 second. Watt as a unit of electrical (active) power is equal to the power of a constant electric current of 1 ampere at a voltage of 1 volt.
When choosing a stabilizer or power plant, it should be remembered that kVA is the total power consumption, and kW is the active (spent on performing useful work) power. Apparent power is the sum of reactive and active power. Often, different consumers have a different ratio of total and active power. Therefore, to determine the total power of all consumers, it is necessary to add the total capacities of the equipment, and not active capacities.

Rated power

In the electrical industry, it is customary to determine the power of most consumers in watts. This is the so-called active power - the power released on a purely resistive load (Heaters, TVs, light bulbs, etc.). Active power is entirely used for useful work (heating, mechanical movement), and it is usually understood as power consumption. If the consumer is active (kettle, incandescent lamp, heating element), then other information about it is not required, such consumers are written (as a rule ) rated power in W, rated voltage and that's it. There are no questions about the cosine "phi" here. this "phi" (the angle between the current and the voltage of these consumers) is zero, the cosine of zero is 1, - hence, the active power ("P") is equal to the product of the current through the consumer and the voltage at the consumer multiplied by this notorious cosine "phi", those. P = I * U * Сos (fi) = P = I * U * 1 = P = I * U.
A simple example for ten with cos phi = 1:
Apparent power S = 10 kVA cos phi = 1
Then the active power P = 10 * 1 = 10 kW

For consumers who have in their composition not only active resistance, but also any reactive (inductance, capacitance), it is customary to write the value "P" in watts on the nameplate, and also indicate the value of the cosine "phi". The value of the cosine "phi" is determined by the parameters of these consumers themselves, or rather, by the ratio of their active and reactive resistances.
For example, a conventional electric motor has on the tag: P = 5kW, Сos (fi) = 0.8. This means the following: This motor, during operation (in nominal mode), consumes its full Total power (the sum of active and reactive power). Active power "S" equal to P / Cos (fi) = 5 / 0.8 = 6.25 kVa and Reactive power "Q" in the amount of U * I / Sin (fi).
To find the rated current of the motor, you need its Total power "S" and divided by the operating voltage (220), however, the current is usually indicated on the nameplate. The question may arise, why is the power in VA (volt-amperes) indicated on generators (transformers, voltage stabilizers)? How else to specify it? Let's say that a voltage of 10,000 VA is indicated on the voltage regulator. This should mean that if I hook up a bunch of heating elements to this transformer, then the power given by the transformer to the heating elements (in the nominal operating mode of the transformer) cannot exceed 10,000 W. Everything seems to fit. What if I want to load the voltage regulator with an inductor or an electric motor with Сos (fi) = 0.8? (a bunch of coils)? And this stabilizer will deliver power already 8000 W? And at Сos (fi) = 0.85 -8500 W. Then the inscription on the 10000 VA nameplate will no longer be legitimate. Therefore, the power of generators (transformers and voltage stabilizers) can be determined only in the Full power (in our case 1000 kVA), and how you will use it (Full power) is your business.
[i] Now you can go to the selection
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Power factor, cosine "phi"

It is the ratio of the average AC power to the product of the rms voltage and current. The largest value of the power factor is 1. In the case of sinusoidal alternating current, the power factor is equal to the cosine of the phase angle between the voltage and current sinusoids and is determined by the parameters of the circuit: Сos ф = r / Z, where ф ("phi") is the phase angle, r is the active resistance of the circuit, Z is the total resistance of the circuit. The power factor can also differ from 1 in circuits with purely active resistances, if they contain non-linear sections. In this case, the power factor decreases due to distortion of the voltage and current waveforms.
The power factor of an electrical circuit is the cosine of the phase angle between the bases of the voltage and current curves. According to another definition, power factor is the ratio of active and apparent energy. Power factor (Сos φ = Active power / Apparent power = P / S (W / VA) consumed by the load.
Power factor is a complex indicator that characterizes the linear and nonlinear distortions introduced by the load into the power grid.
Typical power factor values:
- 1.00 is the ideal value;
- 0.95 is a good indicator;
- 0.90 is a satisfactory indicator;
- 0.80 - the average of modern electric motors;
- 0.70 - low rate;
- 0.60 is a bad indicator.