To select the cross-section of power cables and wires when laying electrical networks for consumers, you need to know what power devices will be included in them. You can learn how to calculate the power consumption of an electrical appliance by understanding the very concept of power. For this, information from the school curriculum and basic concepts about current, voltage, and resistance will be enough. In addition, this knowledge is needed when purchasing household electrical appliances.
Electric current power
Total power and its components
Electrical power is a quantity responsible for the rate of change or transmission of electricity. The total power is denoted by the letter S and is found as the product of the effective values of current and voltage. Its unit of measurement is volt-ampere (VA; V A).
Total power can consist of two components: active (P) and reactive (Q).
Active power is measured in watts (W; W), reactive power is measured in vars (Var).
It depends on what type of load is included in the electricity consumption circuit.
Active load
This type of load is an element that resists electric current. As a result, the current does the work of heating the load, and the electricity is converted into heat. If a resistor of any resistance is connected in series to a battery, then the current passing through the closed circuit will heat it until the battery is discharged.
Attention! An example of a thermal electric heater (TEH) can be given as an active load in AC networks. The heat generated on it is the result of the work of electricity.
Similar consumers also include spirals of light bulbs, electric stoves, ovens, irons, and boilers.
Capacitive load
Such loads are devices that can accumulate energy in electric fields and create movement (oscillation) of power from source to load and back. Capacitors, cable lines (capacitance between conductors), capacitors and inductors connected in series and parallel in a circuit serve as capacitive loads. Sound power amplifiers and synchronous electric motors in overexcited mode also load the lines with a capacitive component.
Inductive load
When the consumer of electricity is certain equipment, which includes:
- transformers;
- three-phase asynchronous motors, pumps.
On the plates attached to the equipment you can see such a characteristic as cos ϕ. This is the phase shift coefficient between current and voltage in the alternating current network into which the equipment will be connected. It is also called the power factor; the closer cos ϕ to unity, the better.
Important! When a device contains inductive or capacitive components: transformers, chokes, windings, capacitors, the sinusoidal current lags in phase with the voltage by a certain angle. Ideally, the capacitance provides a phase shift of -900, and the inductance provides a phase shift of +900.
Cos ϕ values depending on the type of load
The capacitive and inductive components together form reactive power. Then the total power formula looks like:
S = √ (P2 + Q2),
Where:
- S – apparent power (VA);
- P – active part (W);
- Q – reactive part (Var).
If we display this graphically, then we can see that the vector addition of P and Q will be the total value of S - the hypotenuse of the power triangle.
Graphic explanation of the meaning of total power
How to determine?
To solve the problem of finding power, you can use various methods. All of them are available for use even with knowledge of physics and electrical engineering at the school curriculum level.
More often, power is found by determining the current; sometimes you can do without intermediate procedures and determine it immediately.
We look at the technical passport
Typically, the power consumption is indicated in the passport or description of the device and is duplicated on the nameplate. The latter is located on the rear wall of the case or its base.
If there is no description, this parameter can be found on the Internet, for which you just need to search by the name of the device.
The power indicated by the equipment manufacturer refers to peak power and is consumed from the network only at full load, which is quite rare. The resulting difference is considered as a reserve. At the regulatory level, this reserve is determined through the power factor.
Ohm's law to help
The power of most household electrical devices can be fairly accurately estimated experimentally and by calculation using Ohm's law, known since high school. This empirical law relates voltage, current and load resistance R as:
P = U 2 /R. U = 230 V, and the resistance is measured by a tester. The following is a simple calculation using the formula P = 48,400/R W.
For example, with R = 200 Ohm we obtain power P = 240 W.
The method does not take into account the so-called reactance of the device, which is created primarily by input transformers and chokes, and therefore the resulting estimate is somewhat overestimated.
We use an electric meter
When determining power from a meter, you can proceed in two different ways. In both cases, only the device under test should be powered from the household network. Without exception, all other consumers must be disconnected.
In the first approach, an optical meter indicator is used to measure power, the intensity of its flashes is proportional to the power consumption. The proportionality coefficient is indicated on the front panel in units imp/kWh or imp/kWh, Figure 1, where imp is the number of pulses (indicator flashes) per kilowatt hour.
Figure 1. Front panel of a household electricity meter with optical indicator
After turning on the device under test, you must begin counting the indicator flashes for 15 or 20 minutes. The resulting value is then multiplied by 3 or 4 (for a 20- or 15-minute measurement interval, respectively) and divided by the coefficient from the front panel. The result of the calculation gives the power of the device in kW, which in some cases is conveniently converted into Watts by multiplying by 1000.
Example. For the counter we have k = 1600 pulses per kilowatt hour. With a 20-minute measurement interval, the indicator worked (flashed) 160 times. Then the power of the device will be 160*3/1600 = 0.3 kW or 300 W.
The second approach also uses a 15- or 20-minute time interval, but the energy consumption is determined on a digital scale. For example, with a difference in readings over 20 minutes of 0.2 kW×hour, the power of the unit is 0.2 × 3 = 0.6 kW or 600 W.
Wattmeter
A modern household power meter or wattmeter is convenient to use because:
- it is connected directly to the open circuit, for which it is equipped with a plug and socket, see Figure 2;
- equipped with an easy-to-read digital indicator and equipped with internal automatic adjustment circuits, which eliminates errors in readings;
- has good weight and size characteristics.
The device is ready for use immediately after switching on.
Rice. 2. Digital household wattmeter
Its only drawback is its narrow specialization, so this device is rarely found in the household.
Negative effects of reactive load
Learn to easily calculate the power consumption of an electrical appliance
The reactive load does not perform any useful work. Fluctuations in the reactive component from source to consumer only cause parasitic losses. In addition, industrial enterprises are required to pay for the reactive energy supplied to them. This is due to the fact that most energy receivers are electric motors and transformers. The amount of electricity consumed (kWh) does not all go to useful work, but you also need to pay for its reactive component.
Capacitor compensation units will help solve this problem. After all, if you connect a capacitive load in parallel with an inductive load, you can reduce the effect of parasitic currents to a minimum. Such installations are installed at substations supplying consumers.
Formula for calculations
How to calculate electrical energy consumption
All data necessary to be substituted into the formula for calculations can either be measured or taken from the characteristics of the instruments used.
For your information. If the passport data indicates the value of cos ϕ, it means that the electricity received by the device will have a reactive component. This also needs to be taken into account when calculating.
The formula for calculation is:
P = I * U * cos ϕ,
Where:
- I – current in amperes;
- U – voltage in volts;
- cos ϕ – phase shift.
In the case of an active load, the phase shift is not substituted into the formula, and it has the form:
P = I * U.
Summarizing
So, where in real life you might need conversions from W to kilowatts and vice versa:
- The ability to apply such simple calculations in life will help you plan your electricity expenses;
- Knowing the units of measurement will help determine the wire cross-section for electrical wiring and select suitable protective devices;
- Different devices have different values, so you can adjust them to a single value and sum them up.
- Such knowledge is simple and necessary for every person, since such measurements relate to calculations in physics in high school classes.
Now you know how to correctly convert Watts to Kilowatts, how to do the opposite action and why this is needed. Don’t let physical quantities scare you, because making calculations is easy and interesting!
Features of calculation
To calculate power without having complete data on current and voltage consumption, you can use average characteristics. By consulting reference books, you can find out that lighting equipment can consume current up to 15 A. The maximum current of powerful devices reaches 50-60 A. The power factor, if it is not indicated or known, can be taken as 0.7 - this is the average value.
Single-phase voltage in a household network is 220 V. Its linear value in three-phase networks is 380 V.
Mathematical operations
The basic formula allows you to calculate an unknown quantity when the other two are known. For example, if the current consumed by the device is known I = 2 A and the network voltage U = 220 V, then the power consumption is equal to P = 2 * 220 = 440 W.
For example, it is known that the iron consumes 2 kW, and the voltage in the outlet is 220 V, then you can find the current strength for which the cross-section of the power cord is designed.
I = P/U = 2000/220 = 9.1 A.
In the case of fractional values, when using a calculator for calculations, the obtained values are rounded to tenths of the units of the desired value.
Basic formulas for calculation
Learn more about value ratios
Watts are derived units. This means that values are expressed using basic system units. One W is the power of one joule per second. So the power looks like this:
In addition, watts can be expressed in terms of joules and newtons.
People use prefixes to indicate that a value is a multiple of ten of its original value. Kilo is a common prefix, translated from Greek it means 1000.
If this prefix is used, it means that the indicator has been increased tenfold.
The formula for converting Watts to kilowatts will look like this:
Kilowatts refers to the power of many electrical appliances that we use every day.
These include vacuum cleaners, microwave ovens, and many other devices. Engines in cars are also calculated by this value, although in cars this value can also be represented in horsepower.
But horsepower is outside the system, and has been used since the moment when the power of a machine was calculated by the number of horses it could withstand. The kW value is quite easy to convert into them:
Thus, as can be understood from the above, 1 kW equals a thousand watts. Compared to the first formula (1), the second will look like this:
To convert W to kW, you need to increase the W value tenfold.
To obtain Watts from kW, you need to perform the reverse operation accordingly and divide the kW value by ten to the minus third power.
There are tables that can make the calculation much easier. For example:
Rules for calculating power consumption
In everyday life, when the need arises to independently determine the power consumption of electricity, do the following:
- determine the voltage required to power the device;
- find out the rated current from the passport data.
How to find out the power of an electrical appliance if not a single parameter is known? Household electrical appliances are designed for 220 V.
To determine power, it is permissible to measure current consumption. This can be done using an ammeter. It is connected to the circuit in series, having previously set the highest measurement limit - at least a hundred amperes. Current clamp meters will help you easily measure current, for which one of the conductors is grasped by the clamp sensor, and the readings are displayed on the display. Knowing the voltage, multiply it by the measured current to obtain the amount of power consumed.
Clamp meter
How much electricity does a kettle consume?
An electric kettle is a convenient household appliance that can provide its owners with boiling water in a matter of minutes.
It is necessary to calculate how many kilowatts a kettle consumes, taking into account the power of the device and the maximum volume of liquid that it can bring to a boil. The larger the displacement of the device, the more time it will take to heat the water, and accordingly the amount of electricity consumed increases. On the other hand, the high power of the kettle contributes to its fast operation. However, it requires a sufficient amount of electricity.
All electric kettles are different in their parameters and, accordingly, in their level of energy consumption.
To calculate how much a kettle consumes, you should perform the following calculations:
- The power of the device is taken from the passport;
- the time it takes to boil water in the kettle is calculated;
- electricity consumption per unit time is determined;
- the resulting value should be multiplied by the number of times the water is boiled;
- monthly electricity consumption is determined.
Based on the table, the power of the electrical appliance is in the range of 700-3000 W, which depends on the volume of the bowl, body material, displacement, type of heating element, and chemical composition of the water. The heating element can be open (spiral) or closed (plate) type. The first option provides a high rate of water heating and, accordingly, uses less energy.
The energy consumption of the device is also influenced by the housing material. In a metal bowl, water heats up faster. However, an additional amount of electricity is spent on heating the case. Glass also heats up quickly, but retains heat less well. Ceramic has a slow heating rate, but the water in the kettle will remain hot for a long time.
To reduce the kettle's energy consumption, unplug the appliance from the outlet when not in use. The required volume of water should be poured into it, without reserve. You should monitor the condition of the heating element by regularly descaling it.
Calculation of light bulb power
The selection of incandescent lamp power depends on the desired level of illumination of the living space. One 100 W light bulb, working in the dark for at least 12 hours, consumes 1.2 kW of power. For a month this will be 36 kW, for a year - at least 432 kW. If there are 10 light bulbs in an apartment, then the total annual consumption will be 4320 kW. If the price for 1 kW of electricity is 5 rubles, the amount turns out to be decent – 21,000 rubles. Therefore, replacing incandescent lamps with energy-saving light sources: LED lamps, LED strips and the like allows you to save money. In addition, reducing the power of such light bulbs does not reduce the luminous flux. The reduced supply voltage of LED strips also reduces the amount of power consumed.
Let's look at examples
In life, converting Kilowatts to Watts and vice versa occurs quite often. Let's look at a few examples where such calculations can be useful.
- The electric motor has a power of 1.5 kW. You need to understand what the engine power will be in W. Multiply kilowatts by a thousand. We get:
- Tables of values contain a nominal mechanical power of nine hundred watts. In Kilowatts we get the following:
Kilowatts are constantly encountered in our lives.
For example, when paying for electricity, it is often measured in kilowatts per hour of energy use. This shows how much energy an object consumes in one hour of operation. In this case, the power of devices is often calculated in Watts.
If we want to know how much 500 watts are spent per hour, we need to make the following calculation:
Power measurement devices
To measure P, you can use special instruments. A multimeter is suitable for this, to which you can connect a current clamp. How to measure power with a multimeter? The tester is switched on to the alternating voltage measurement mode; the clamps should grasp only one conductor connected to the load.
Measuring with a multimeter
Separating conductors in a cable is not always convenient. In addition, after measurements you need to calculate the power using the formula.
Power meter
To measure, you can use a special device - a wattmeter. The device is plugged into a power outlet, and the load whose power needs to be measured is connected to its output socket. The results of the measurement are displayed on the display in kilowatts.
Power meter
Measuring power with an electric meter
Using a residential electricity meter, you can also check the power consumption of an individual device. To do this you need:
- turn off all energy consumers, leaving only the device under test in consumption mode;
- note the readings at the current moment and record their values in an hour;
- subtract the latest values from previous readings;
- the result will be a measured value.
The main disadvantage of this block of actions is the shutdown of other necessary household appliances.
Information. When using this method, taking a moment, you can see if there is a hidden current leak and the serviceability of the meter. When all devices are turned off, the electric meter should stop.
How much does an electric boiler consume?
Electric boilers are installed in houses for heating and water heating. However, the simplicity of the design and ease of operation hides high energy consumption. Models of electric boilers differ in power, design, number of circuits and method of heating the coolant (heating elements, electrode or induction heating). Double-circuit boilers are used for heating and water heating. Boiler models are more economical than flow-through ones.
The choice of boiler is made on the basis of the required power that it must have to ensure heating of premises of a given area. When calculating, it should be taken into account that kW is the minimum power of the device required to heat 10 sq.m. of room area. Additionally, climatic conditions, the presence of additional insulation, the condition of doors, windows, floors and the presence of cracks in them, and the thermal conductivity of walls are taken into account.
Energy consumption
Calculating energy consumption for a house or apartment is not particularly difficult. To do this, you need to perform the following algorithm of actions:
- make a table of all electrical appliances used in the house, including lighting lamps;
- put in separate columns: device power, operating hours per day;
- for each energy consumer, calculate (by multiplying the power by the operating time) the average daily consumption;
- sum up all the obtained power values.
Such a calculation will give a real picture of electricity consumption. Using this data, you can control consumption and adjust the daily power consumption of each device.
It does not matter how power consumption is calculated or measured. The main task of the process is to correctly select the cross-section of conductors for wiring, supplying power cables and ensure that automatic protection is triggered. The cable supplying voltage to the room must withstand the simultaneous switching on of all consumers located in it for a long time. Its choice directly depends on the accuracy of determining the power of consumers.
Refrigerator: how many watts does it consume per hour?
When answering the question of which electrical appliances consume the most energy, the refrigerator will be first on the list. This device works around the clock. The actual energy consumption of the refrigerator is calculated taking into account the international classification of energy efficiency devices. This parameter is designated by a letter with a certain number of pluses; the more there are, the lower the level of electricity use.
The classification of a household appliance by energy efficiency is as follows:
- A++ - the highest class with maximum energy saving. Electricity consumption is 30% of the standard value;
- A+ - energy consumption - 30-42% of the standard;
- A - energy consumption - 42-55% of the standard;
- B - energy consumption - 55-75% of the standard;
- C – energy consumption – 75-90% of the standard;
- D – energy consumption – 90-100% of the standard;
- E - energy consumption - 100-110% of the standard;
- F - energy consumption - 110-125% of the standard.
However, the energy efficiency parameter is very average. Since the amount of electricity consumed by the refrigerator is affected by its operating mode, workload, and the number of door openings.
The refrigerator consumes the most energy of any electrical appliance.
Annual energy consumption corresponds to 220-460 kW. It is impossible to obtain an accurate result for a table of electricity consumption per day or month by simply dividing this value. Since energy consumption is influenced by a number of factors, such as freezing power, ambient temperature, and product filling level.
To reduce the energy consumption of the refrigerator, it is necessary to operate the device correctly, do not leave the internal space empty when it is turned on, do not open the door for a long time, do not put hot food in, check the condition of the seals, ensure that there is a gap between the refrigerator and the wall, regularly defrost, wash and dry the unit.
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