Calculation of UPS battery life. Uninterruptible power supplies: an attempt to develop a comprehensive testing methodology How to calculate the required UPS power
It is an integral guarantee of the reliability of the power supply system. The UPS parameters must be strictly comparable to the load that will be connected to the UPS. Otherwise, the uninterruptible power supply will not bring the desired benefit, and the money will be wasted.
How to calculate uninterruptible power? To do this, it is necessary to take into account a number of parameters, the key of which is power. If you buy a UPS that has less power compared to the load, it simply will not work. To accurately calculate power, you need to remember a little physics.
The load power factor, or otherwise Power Factor, is very important when calculating the power of an uninterruptible power supply. This figure shows what proportion of power the load actually consumes, that is, active power. If we consider the load as an ideal resistance, then in this case the coefficient value will be equal to unity, which is the maximum value. Capacitors and coils are not power consumers, so for them the coefficient value is zero. The equipment may have a predominance of both capacitive and inductive components.
Equipment with a capacitive component includes computers and servers. The inductive component is present in devices with electric motors, it can be a pump, an air conditioner, etc. This information is necessary in the case when the UPS will protect equipment of different types, since for the first the power factor tends to unity, and for the second it is in the range from 0.8 to 0.9. In this case, it is necessary to find the average power factor to get an accurate result.
How to calculate the power of a UPS, knowing the power factor of the load? To calculate the power, you need to multiply the rated power of the UPS by the power factor. The result of the operation is a number that shows the maximum active power that the uninterruptible power supply can serve. For example, the UPS power is 100 kVA and the load power factor is 0.9. In this case, the active load power will be 90 kW. The total load power should not exceed 90 kW, and it is better if it is somewhat less.
Such difficulties when calculating power can be avoided if you use an uninterruptible power supply as an indicator of the output power. In this case, the calculation of the uninterruptible power supply will be performed without errors. It is a big mistake to compare powers expressed in volt-amperes and watts, since the values differ significantly.
It should also be taken into account that the power consumed by the equipment may be slightly lower than the rated one. This can happen in a variety of cases. For example, if we consider computers, their power in most cases is determined by the power of the power supply. But not in all cases this calculation algorithm is correct. So, for example, a computer may have a power supply with a power of 450 W, but the total power of the computer components is only 120 W. There can be a lot of such features and they need to be taken into account when calculating an uninterruptible power supply.
Another situation that needs to be taken into account in order to calculate the operation of the UPS is related to the refrigerator. For example, it may have a power of 250 W, but it is worth considering that the refrigerator does not work all the time, but only at certain intervals. In this case, it is necessary to find out the annual electricity consumption. In calculations, you must use this value divided by 9. It should be noted that the load power must be calculated in watts.
On some sites you can find UPS power calculations online, but they cannot provide accurate data because they do not take into account such nuances. If you still decide to use such services, then in addition to the obtained result you need to add about 20%. It is important to think about the prospect of increasing load power. If the load increases in the future, it is better to immediately purchase a more powerful UPS. A similar situation is with services that allow you to calculate the UPS operating time online.
Battery calculation
If you need to calculate the UPS capacity for a given power and operating time, then a simple formula is used:
Capacity= 100*time*load power
Battery life is expressed in hours, and load power in kilowatts. Please note once again that power is not expressed in volt-amperes. For example, an uninterruptible power supply protects a computer with a power of 500 W (0.5 kW). The uninterruptible power supply must provide an operating time of 2 hours. Under such conditions, the formula that allows you to calculate the battery capacity for a UPS takes the following form:
100*0.5kW*8h=400 Ah
Thus, for a load with a power of 500 W, to ensure operation for 8 hours, a battery capacity of 400 Ah is required. This calculation of battery capacity for a UPS is applicable for batteries with a voltage of 12 V. In addition, you need to take into account that the formula is suitable for a long battery life, namely about 9-10 hours. This is due to the fact that the dependence of battery capacity on charging time is not linear throughout.
If the operating time is shorter, then corrections must be made. This is due to the fact that for a short time the discharge current is large and the battery transfers only a certain part of its capacity to the load. So, if you need a working time of 30 minutes, then the result must be divided by two, for 2 hours reduced by 40%, for 4 hours - 30%, for 6 hours - 40%. To determine the exact value, it is necessary to use the exact efficiency value of the inverter that is installed on the UPS and compare the data with the discharge curve of a certain type of battery.
After the total capacity has been found, it is necessary to calculate the number of batteries for the UPS. To do this, you need to divide the total capacity by the capacity of one battery. In our case, the total capacity was 400 Ah. Let's assume that the capacity of one battery is 50 Ah. In this case, we will need 8 of these batteries.
Working hours
Many users are interested in the operating time that a particular uninterruptible power supply can provide. How to calculate the operating time of an uninterruptible power supply? To do this, you need to know the power of the load connected to the UPS, the efficiency of the inverter and the total capacity of the battery.
The total calculation of batteries for a UPS is extremely simple. In most cases, uninterruptible power supplies contain standard batteries. To perform a total calculation of batteries for a UPS, you need to multiply their number by the capacity of one battery.
In order to calculate the battery life of a UPS, it is recommended to take the inverter efficiency equal to 0.85. The total load power must be expressed in watts. We talked about how to find it at the beginning of the article.
The UPS operating time is calculated using the following formula:
Time=total battery capacity*battery voltage*(inverter efficiency/load power)
The obtained value is approximate and may change during the service life of the uninterruptible power supply. The calculation of the UPS time is approximate, since the time depends on the wear of the battery and operating conditions, mainly on the air temperature. For example, an increase in temperature by one degree after 40°C reduces the battery capacity by 5%, which is very significant. For maximum service life, it is recommended to reduce the load on the uninterruptible power supply by 20% for every 10 degrees after 25°C. Or you can organize a good cooling system and not allow any temperature rise at all, for which the uninterruptible source will only be grateful.
If such calculations are incomprehensible to you, then you can contact specialists in this field or use a special calculator - a UPS calculation program. However, in this case, it is necessary to use proven software created by professionals to avoid mistakes and the wrong choice of UPS. The advantage of such programs is calculation. When calculating, you can select the type of core of the transformer. The calculations take into account losses that are possible in the core and copper wires.
There may be cases when absolutely accurate data is not necessary. In this case, you can use special tables that show the battery life for various types of uninterruptible power supplies. These tables include operating time depending on the capacity of the batteries and the total load power. This way you can compare your data with the table data and find out the approximate time.
Knowing how to calculate a UPS, you can make the most correct choice of UPS. Now you know that battery life does not depend on the power of the UPS or the total voltage of the battery, but on the capacity of the batteries. Therefore, when choosing a UPS, preference should be given to batteries with a larger capacity in accordance with the given power. This choice will ensure maximum autonomy.
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After about three to six months of operation, the cost of the data stored on a new work computer begins to exceed the cost of the computer itself. In the case of a network server, this situation can arise within a few weeks after its installation.
In 50 70% of cases, the cause of failures in the operation of electronic devices is poor-quality power supply. If there is a power failure, one incorrect data writing session can destroy the entire file system.
Even if failures do not lead to catastrophic consequences immediately, after some time the sensitive electronics of your PC may simply “revolt” due to constant on/off cycles.
In Russia, data from studies conducted in the USA by Bell Labs and IBM have become known. According to Bell Labs and IBM (USA), each personal computer is exposed to 120 power incidents per month.
Types of Power Failures
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Low voltage, voltage dips |
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Overvoltage |
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High voltage pulses |
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Electrical noise |
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Complete power cut |
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Harmonic voltage distortion |
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Unstable frequency |
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UPS characteristics:
output power, measured in volt-amperes (VA) or watts (W);
switching time, that is, the time it takes for the UPS (UPS) to switch to battery power (measured in milliseconds, ms);
battery life is determined by the capacity of the batteries and the power of the equipment connected to the UPS (measured in minutes, min.);
the width of the input (mains) voltage range at which the UPS (UPS) is able to stabilize the power without switching to batteries (measured in volts, V);
battery life (measured in years, usually 5 and 10 years).
Basic electrical parameters of UPS (UPS)
UPS output power
The output power of a UPS (UPS) is defined as the product of voltage (in volts, V) and current (in amperes, A).
The power consumed by a load is defined as the product of the UPS output power (in volt-amperes, VA) times the load's Power Factor (PF).
You should select a UPS that meets the following conditions:
P output power of the UPS (UPS) (VA), Wн power consumed by the load (VA),
PF power factor, which for personal computers is assumed to be 0.7.
Typically, the amount of power consumption is indicated on a sticker located on the back cover of the devices.
UPS output voltage waveform
An uninterruptible power supply is a temporary substitute for the electrical network for the equipment connected to it.
In an electrical network, the voltage has a sinusoidal shape or a shape close to a sinusoid. Of course, all computers and other equipment designed to be powered from an alternating current network are designed specifically for sinusoidal voltage. But almost all types of equipment, including computers, can operate more or less normally with a voltage that is very different from a sine wave.
Previously, some switching UPSs (UPS) had an output voltage in the form of a square wave (rectangular pulses of different polarities).
Rice. 1. Meander
To ensure that the RMS and peak-to-peak values of the rectangular voltage are equal to the corresponding values of the sinusoidal voltage, manufacturers of modern switching UPSs (UPS) have slightly changed the shape of the square wave by introducing a pause between rectangular pulses of different polarities.
Rice. 2. Meander with a pause.
UPS manufacturers call a voltage of this form “stepped approximation to a sine wave.” This shape of the curve allows, with correctly selected voltage amplitude and pause duration, to meet the requirements of different loads. For example, with a pause duration of about 3 ms (for a frequency of 50 Hz), the effective voltage value coincides with the effective value of a sinusoidal voltage of the same amplitude.
The actual shape of the output voltage of the UPS (UPS) with switching is shown in Fig. 3.
Rice. 3. Voltage and current oscillograms of a personal computer connected to a switched UPS.
The same oscillogram also shows the curve of the current consumed by the computer. Strong pulse currents consumed by the computer at the beginning and end of the rectangular pulse do not affect the operation of the computer. They are completely suppressed by the computer's power supply, the output of which is a constant voltage with a normal ripple level.
A computer protected by a switching UPS is powered by non-sinusoidal voltage only when the UPS is running on battery power (i.e. very briefly). When the UPS (UPS) operates from the network, the computer is powered by the mains voltage, smoothed using noise and pulse filters built into the UPS (UPS).
Noise reduction
Noise is small random deviations of voltage from the nominal value, mainly high-frequency ones. Noise is suppressed by UPS input filters. The degree of suppression depends on the frequency of the noise. On average, a UPS (UPS) noise suppression ranges from 10 dB at a frequency of 0.15 MHz to 50 dB at a frequency of 30 MHz.
Pulse suppression
There are several standards in the world that describe the requirements for UPS systems regarding surge protection.
The most common standard applies to typical office environments and involves testing a UPS by applying a 3000 V pulse to its input. Different types of UPS use different surge suppression technologies. Offline and line-interactive UPS (UPS) models typically use varistor surge protection. A simple and effective varistor shunt can suppress pulses with currents of enormous amplitude.
Efficiency
Efficiency is the ratio of the power consumed by the UPS load to the total power consumed by the UPS. The higher the efficiency, the more efficiently energy resources are used. The efficiency of a UPS (UPS) can range from 85 to 97% in different classes and under different operating modes of devices.
Battery life
For most conventional low-power office UPSs (UPS), battery life at maximum load is 4-15 minutes.
If the UPS (UPS) load is less than the maximum, the battery operating time increases. Due to the non-linearity of the battery discharge curve, this increase is not proportional to the decrease in load. If the load is halved, then the operating time can increase by 2.5–5 times, if it triples, then the time increases by 4–9 times, etc.
High-power UPSs and some low-power UPSs have the ability to increase battery life by replacing the battery with a larger battery or installing an additional battery. A larger battery can be installed in the same housing, or an additional battery housing can be installed.
Power factor. Watts and volt-amps
Knowing the power of the equipment connected to the UPS is necessary in order not to exceed the maximum permissible load of the UPS. But the load (or overload) of the UPS is determined not only by how much power is released in the load, but also by how much current flows through the UPS. Therefore, when specifying the maximum load for a UPS (UPS), the maximum apparent power in volt-amperes and the maximum active power in watts are usually indicated.
The UPS must be selected so that the maximum load power does not exceed the maximum power of the UPS.
The apparent power of the load must be less than the apparent power rating of the UPS (you need to compare volt-amperes VA). And the active power of the load should not exceed the rated active power of the UPS (UPS) (you need to compare watts W).
For different loads and different UPS systems, the limitation can be either total or active power. Most often (for computer loads) the limitation is total power.
How to choose the optimal UPS configuration for organizing uninterruptible power supply to equipment and household appliances in the house
It is quite difficult to answer the question about choosing the configuration of an uninterruptible power supply to ensure reliable power supply to heating and engineering systems, and household electrical appliances. Essentially, this is an equation with many unknowns. After all, it is not known in advance how bad the network power supply will be, and how long the power outages will be.
At the first stage, it is necessary to determine the total power of all energy consumers whose operation must be ensured in the absence of mains power supply. Based on this value, it is necessary to select a UPS with a power 20% higher than the maximum load value. After this, you need to determine the capacity of the external batteries, based on the required backup time.
The most optimal solution for uninterruptible power supply is to divide the load into several smaller groups of consumers. And solve the problem of providing reserves separately for different groups of consumers, depending on their importance. When choosing the configuration of an uninterruptible power supply and batteries, it should be taken into account that increasing the UPS power reserve does not lead to a linear increase in the reserve duration. To provide high load power, a more powerful UPS is required, and to ensure a long reserve time, it is necessary to increase the capacity of external batteries.
A simple way to calculate uninterruptible power supply backup time
The power reserve time is determined primarily by two parameters: the power of the payload and the total capacity of all batteries.
However, it should be noted that the dependence of the reserve time on these parameters is not linear. But for a quick rough estimate of slack time, you can use a simple formula.
T=E*U/P(hours),
WhereE - capacitybatteries,U - voltagebatteries,P - load powerall connected devices.
A refined method for calculating uninterruptible power supply backup time
To clarify the calculation of reserve time, special coefficients are additionally introduced: inverter efficiency, battery discharge coefficient, available capacity coefficient depending on the ambient temperature.
Taking these coefficients into account, the calculation formula takes the following form.
T=E*U/P*KPD * KRA * KDE(hours),
where KPD (inverter efficiency) is in the range of 0.7-0.8,
KRA (battery discharge ratio) is in the range of 0.7-0.9,
KDE (available capacity ratio) is in the range of 0.7-1.0.
The available capacity coefficient has a complex dependence on the temperature value and the speed of load application. The colder the air temperature, the lower the available capacity ratio. The slower the battery energy is consumed, the higher the available capacity coefficient.
Ready-made tables of reserve time values for uninterruptible power supply systems of the SKAT and TEPLOCOM series
One 12 Volt external battery required
| Capacity, in Ah | Load power, VA | ||||
| 100 | 150 | 200 | 250 | 270 | |
| 26 | 2h 18min | 1h 22min | 55min | 44min | 39min |
| 40 | 3h 37min | 2h 15min | 1h 36min | 1h 15min | 1h 09min |
| 65 | 7h 01min | 4h 00min | 2h 45min | 2h 12min | 1h 54min |
| 100 | 12h 00min | 7h 12min | 5h 00min | 3h 40min | 3h 26min |
Table of approximate reserve times
Requires two external 12 volt batteries
| Battery capacity, Ah | ||||||||||
| 100 | 200 | 300 | 400 | 500 | 600 | 700 | 800 | 900 | 1000 | |
| 2x40 | 9,37 | 4,06 | 2,31 | 1,51 | 1,36 | 1,22 | 1,07 | 0,53 | 0,39 | 0,34 |
| 2x65 | 16,15 | 7,12 | 4,40 | 3,02 | 2,29 | 1,56 | 1,44 | 1,36 | 1,28 | 1,11 |
| 2x100 | 27,11 | 11,55 | 7,33 | 5,23 | 4,12 | 3,05 | 2,44 | 2,22 | 2,01 | 1,49 |
| 2x120 | 32,37 | 14,52 | 9,44 | 6,10 | 5,11 | 4,12 | 3,14 | 2,51 | 2,33 | 2,15 |
| 2x150 | 40,47 | 17,40 | 11,24 | 8,19 | 5,57 | 5,07 | 4,17 | 3,28 | 2,57 | 2,42 |
| 2x200 | 54,23 | 24,48 | 15,47 | 11,27 | 9,09 | 6,50 | 5,45 | 5,08 | 4,31 | 3,54 |
Table of approximate reserve times
Requires 8 external batteries with a voltage of 12 Volts
| Battery capacity, Ah | ||||||
| 500 | 1000 | 1500 | 2000 | 2500 | 3000 | |
| 65 | 12h 20min | 5h 10min | 2h 55min | 2h 15min | 1h 40min | 1h 25min |
| 100 | 19h 25min | 8h 40min | 5h 20min | 3h 40min | 2h 45min | 2h 15min |
| 120 | 23h 05m | 11h 35min | 7h 00min | 4h 45min | 3h 30min | 2h 45min |
| 150 | 28h 55min | 14h 20min | 8h 45min | 6h 30min | 4h 50min | 3h 40min |
| 200 | 38h 30min | 19h 10min | 12h 45min | 8h 45min | 7h 00min | 5h 20min |
Line of UPS brands S.K.A.T. And TEPLOCOM provides the ability to organize reliable uninterrupted power supply to consumers of various capacities and purposes. Uninterruptible power supplies make it possible to organize uninterrupted power supply from a small heating boiler or circulation pump to powering the entire home or office. Specialized UPSs make it possible to organize uninterruptible power supply for particularly important objects, such as communication systems, communication equipment, security and control systems.
There are several ways to increase the payload power reserve time. All these methods follow from the formula for calculating the reserve time.
To increase the reserve time, you can increase the capacity of external batteries, reduce the payload, and create optimal operating conditions for the UPS and batteries.
First option- the simplest, but most expensive. To increase the battery capacity, you will have to buy more expensive batteries and a UPS that allows them to be charged efficiently. In addition to the cost of equipment, you will also need to allocate a special room designed for storing and operating batteries, equipped with a good ventilation system.
Second method- reduce the load. First of all, you need to divide the load into groups depending on the need to ensure uninterrupted power supply. If there is no electricity for a long time, then you will need to choose between the importance of ensuring the operation of engineering heating and water supply systems and the need to use a refrigerator or air conditioner. Thus, a modern refrigerator allows you to provide an acceptable temperature for about 20 hours, if you do not open it again. Another group of consumers is the lighting system; for lighting, you can use autonomous uninterruptible power supplies or emergency lamps with a built-in battery. Ultimately, you can sit by the light of a flashlight or a good old candle, anything is better than defrosting the heating system.
Third method is to improve the quality of UPS and battery maintenance. The most important points here are keeping the equipment clean and ensuring good temperature conditions. Separately, it is worth noting the need to properly charge the battery and conduct battery training. It often happens that there are no electrical problems and the batteries are not subject to discharge and charge cycles. As a result, after a few months the actual battery capacity drops sharply. To train the battery, it is necessary to use special equipment or simulate periodic power outages, allowing the batteries to work.
Buying a UPS that is more powerful than your needs is a waste of money. However, underestimating the required power of an uninterruptible power supply system is fraught with load loss, which is completely unacceptable. How to calculate this characteristic as accurately as possible?
To do this, you need to know the load's power factor (Power Factor, P), which determines how much of the power provided by the electrical source is actually consumed by the equipment (active power). If the load behaves as an ideal resistance, it absorbs all the power supplied to it, that is, P=1. An ideal capacitance (capacitor) or inductance (coil) does not consume active power at all (P = 0), since it does not convert electrical energy into its other types. During one quarter of the sinusoid period, energy is stored in the magnetic field of the coil or in the electric field of the capacitor, and during the other quarter it is returned to the network. Thus, in this case, only energy recirculation takes place, and the resistance of the coil and capacitor, in contrast to the active resistance of the resistor, is called reactive.
In real life, nothing is ideal, so the value of the load power factor is usually in the range from 0 to 1. In general, P is calculated as the ratio of the active power absorbed by the load (measured in watts, W) to the total incoming power (measured in volts). -ampere, VA):
power factor (P) = active power (W)/apparent power (VA).
In the presence of only harmonic distortion, the power factor is equal to the cosine of the phase angle between current and voltage, so it is often denoted cos φ. A load with a predominance of the capacitive component is characterized by a leading power factor (cos φ positive), and an inductive load - by a lagging one (cos φ negative).
The main load for a UPS is PCs and servers. A rectifier with a filter in the form of a capacitor is installed in the power supplies of these devices, so they have a certain capacitive component. The power factor of the simplest power supplies used in cheap PCs may not exceed 0.6 - this means that only 60% of the useful power supplied by the source goes into use. In reality, the situation is not so bad for typical PCs - their power factor is usually 0.8, so most low-power UPSs are designed to handle such a load.
As for modern servers, data storage systems and network equipment (switches, routers), the situation here is even better. They use power supplies with a power factor correction function, so its value approaches 1. But in calculations it is still better to consider such equipment as a load with a small capacitive component, and take the power factor equal to 0.95.
But air conditioners, which are often also protected using a UPS, are already a load with an inductive component, which is due to the presence of electric motors in their compressors. The power factor of this equipment is usually in the range from 0.6 to 0.8 (see).
How to estimate the average power factor of a load consisting of different types of equipment? Let's assume the following equipment is installed in the office:
PCs and servers, power 4500 VA, P=0.95 (leading);
Air conditioner, power 3000 VA, P=0.8 (delayed).
Then, to determine the average coefficient, the average deviation P from unity is first calculated:
(4500 VA×0.05– 3000 VA×0.2)/7500 VA = - 0.05.
Thus, the load will be inductive with P=0.95.
TWO POWER FACTORS
The specification of almost any UPS indicates its input power factor. This parameter has nothing to do with the output ratio and determines how the UPS itself (as a load) behaves in relation to the external network. In modern UPSs, where the rectifier is based on IGBT transistors, the input power factor is close to unity, which means that the source behaves almost like an ideal active resistance and introduces almost no distortion into the external network. The value of input P is entirely dependent on the UPS circuit design.
The output power factor for a UPS is determined by the load connected to it. Knowing this characteristic (along with the total power in VA), you can, by multiplying one by the other, obtain the maximum power in W that the source can handle. If the load power factor turns out to be greater than that specified for the UPS, the latter will still not be able to exceed the power in W calculated using the above method, and therefore will not provide the maximum VA value.
Let's look at the example again. Let there be a UPS with a rated power of 60 kVA, designed for a load with a power factor of 0.9. The maximum active power it can serve is 54 kW:
60 kVA×0.9 = 54 kW.
It will serve a load with the indicated full power, but less P, for example 0.8, without problems:
60 kVA×0.8 = 48 kW
But if P of the load exceeds 0.9, say equals 0.95, then it is no longer able to provide a power of 60 kVA:
60 kVA×0.95 = 57 kW > 54 kW.
As mentioned, the power factor of many types of modern IT and telecommunications equipment is close to 1, so you need to be very careful here. In order not to be mistaken, many experts today, when choosing a UPS, prefer to be guided by its output power in W.
If you find it difficult to determine the P coefficient, then for a complete guarantee you should choose a UPS whose power in W would be greater than the load characteristic in VA. But in this case, a significant overestimation of the UPS power is possible. For a more accurate calculation, you should first calculate the total load value (in VA), then its average P, and then, by multiplying both values, get the value in W. The UPS power in W must not be lower than the load characteristic expressed in the same units of measurement.
TWO MORE FACTORS
Two more coefficients serve as an important characteristic of the load: Crest Factor and Surge Factor. The first of them in Russian-language documentation is often called the peak coefficient (or peak factor). It is determined by the ratio of the maximum (peak) current value to its root mean square (RMS) value. For rectangular waves the crest factor is equal to unity, for an ideal sine wave it is 1.414 (√2).
Although we have called the crest factor a “load characteristic,” it is actually influenced by the characteristics of the power supply. Switching power supplies for computers consume current very unevenly, so for them the crest factor is usually from 2 to 3. But this is if the load is supplied with a pure sine wave. If the UPS produces a stepwise approximated sine wave (which is typical for sources with a power of less than 1 kW), then the crest factor is less than 2 (usually from 1.4 to 1.9). In general, the use of UPSs, network filters and surge suppression devices helps reduce the crest factor. This is certainly a positive point, since a high peak factor (high current) leads to strong heating of the elements of power supply systems.
Most UPSs are capable of maintaining a crest factor of 3 at full load (the value of this characteristic increases as the load decreases), so usually there are no problems. Even if the source does not provide the required peak current value, then, as a rule, the operation of the load power supply is not disrupted, only slight distortions in the electrical signal shape are possible. However, in large installations (for example, when the UPS serves a large number of PCs), such distortions can be so significant that they can lead to disruption of the load. Therefore, it is desirable that the peak factor supported by the UPS is not lower than the peak factor of the load.
To calculate the average crest factor of a load consisting of different types of equipment, we can recommend the same method as for calculating the average power factor. Let's look at our example:
PCs and servers, power 4500 VA, crest factor=3;
Air conditioner, power 3000 VA, crest factor=1.4.
The average crest factor can be calculated as follows:
(4500 VA×3 + 3000 VA×1.4)/7500 VA = 2.36.
If the peak factor stated in the UPS specifications is greater than the specified value, then there will be no problems.
The Surge Factor value (unfortunately, there is no established Russian term for this characteristic) determines how much the inrush current consumed by the load exceeds its rated value. For example, to start an electric motor, a large starting torque is required, so when turned on, refrigeration compressors consume a current several times higher than the rated current (see). The inrush current of a lighting system using conventional incandescent lamps can also significantly exceed its rated value. The fact is that the electrical resistivity of tungsten, from which incandescent filaments are made, largely depends on temperature: at 20°C its value is 55×10 -9 Ohm×m, at 1727°C - 557x10 -9 Ohm×m . Accordingly, the starting current will be approximately 10 times the rated current.
As for computers and servers, the Surge Factor value for them usually does not exceed 1.5, and most UPSs have sufficient overload capacity to guarantee reliable switching on and stable operation of these devices. If the load contains equipment with a high starting current, then the overload capacity of the selected UPS should be studied most carefully.
Having analyzed the factors discussed in the article, do not forget that in order to ensure stable operation of the equipment, the UPS power should be selected “with a reserve” - 15-25% more than required.
Alexander Barskov is the leading editor of the Journal of Network Solutions/LAN. He can be contacted at:
Power surges are the main cause of computer breakdowns. To protect devices from damage, install a UPS or uninterruptible power supply. It is used to eliminate various interference in the electrical network:
- A sharp increase and decrease in voltage;
- Sudden power outage;
- Electromagnetic interference;
- High frequency pulses.
The system unit, monitor, audio system, game joysticks, modems, printers and scanners are connected to the UPS. To ensure reliable protection for all devices, it is important to know how to choose the right UPS for your computer.
How to choose an uninterruptible power supply for your computer
Choosing a UPS for a computer begins with determining its type. There are three of them: backup, interactive and online devices.
- Backup uninterruptible power supply systems operate in two modes. If there is voltage in the network, they “filter” incoming currents and make them safe for equipment. In the absence of voltage, they act as a backup battery. In other words, if there is a power outage, you will be able to work with your PC for some time.
Advantage: low price
Flaws: relatively long response time (up to 15 ms), which may be critical for some types of equipment. - Interactive UPSs, unlike standby ones, are equipped with a built-in voltage stabilizer. If the load on the network has changed slightly, the device will correct it. Switching to battery operation occurs only when major changes occur in the network.
Advantage: fast response time, universal, suitable for both computers and all related equipment.
Flaw: not suitable for equipment with high starting currents. - Online UPS are classified as professional equipment. They convert the incoming alternating current into direct current, “pass” it through themselves and again output alternating current with an exact voltage of 220 V.
Advantage: Suitable for protecting highly sensitive and expensive equipment.
Flaws: very expensive and noisy, installed in rooms where there are no people.
Another important parameter is the battery life of the device. It is indicated by the manufacturer in the technical data sheet of the device and ranges from 10 to 50 minutes. May vary depending on the number of connected equipment.
How to calculate UPS power for a computer
First, determine the type of PC you have and decide what additional equipment you want to connect to it. Calculate their total power. Be careful: the power of the equipment is indicated in watts (W), and the UPS, as a rule, is indicated in volt-amperes (VA). You need to correctly calculate the power of the UPS for your computer yourself.
- A standard office computer includes a system unit, a monitor, speakers and a printer. Their total power is about 500 W. Convert to volt-amperes: 500*1.4=700 VA.
- A gaming computer consists of a system unit, one or two monitors, a powerful speaker system, as well as joysticks, steering wheels and other equipment. Gaming computers are much more powerful than office computers, so the approximate total power will be higher - about 800 W. We do the calculation according to the sample and get 1120 VA.
How to connect a UPS to a computer
Connecting a UPS to a PC is quite simple. It is necessary to have a surge protector - a tee.
- We connect the uninterruptible power supply to the switched on network filter. This is necessary to recharge the device's battery.
- We connect all equipment: system unit, monitor, speaker system to the UPS.
- Turn on the computer correctly. Press the UPS power button and wait until the green light comes on. It signals that the device is ready for use. Only after this do we turn on the computer. Only in this case will your equipment be reliably protected from power surges.
