How to choose the optimal UPS configuration for your organization uninterruptible power supply 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 decide on the capacity of the external batteries, based on the required reservation time.

Most optimal solution uninterruptible power supply will split 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

Need 8 external batteries voltage 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. So 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. Here are the most important points 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 it drops sharply real capacity Battery To train the battery, it is necessary to use special equipment or simulate periodic power outages, allowing the batteries to work.

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, this can be a pump, air conditioner, etc. This information is necessary in the case when the UPS will protect the equipment different types, since for the former the power factor tends to unity, and for the latter 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, which can be served by an uninterruptible power supply. 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

Time battery life is expressed in hours, and the 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 the battery capacity on the 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 term service it is recommended to reduce the load on the uninterruptible power supply for every 10 degrees after 25°C by 20%. Or you can organize good system cooling and not allowing 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 in order 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 the UPS you can do the most right choice 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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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.

Connect to uninterruptible power supply system unit, monitor, audio system, game joysticks, modems, printers and scanners. 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 precise voltage 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– device battery life. 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.

• Standard office computer includes a system unit, monitor, speakers and 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 much more powerful than office ones, therefore 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.

1. We connect the uninterruptible power supply to the switched on surge protector. This is necessary to recharge the device's battery.
2. All equipment: system unit, monitor, sound system- connect to the UPS.
3. 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.

Which UPS to choose? We raised this topic in the previous article and looked at the types of uninterruptible power supplies that manufacturers offer. Today we’ll talk about how to choose an uninterruptible power supply depending on your tasks and the type of your equipment, and also calculate the required UPS power.

What kind of uninterruptible power supply you need depends on several main points:

1. What kind of network problems do you want to protect your equipment from?
2. Design features of the equipment you want to connect to the UPS.
3. Planned load power on the UPS.
4. Required battery life.

So, in this article we will look at choosing an uninterruptible power supply, taking into account the following questions:

• We calculate the battery capacity for a known battery life.
• We calculate the battery life, knowing the capacity of the UPS.

Why do you need a UPS?

The answer to the question: which uninterruptible power supply to choose depends primarily on why you need it.

	For what?	What to buy
	Correctly turn off the computer and have time to save data during a power outage.	In this case, feel free to take inexpensive off-line UPS or linear-interactive with a battery life of 5-15 minutes.
	Provide power to equipment in the event of a long power outage.	If your equipment is suitable for a non-sinusoidal waveform, buy an off-line or line-interactive UPS, but with increased capacity, with the expectation of long battery life. You can read below how to calculate capacity. The largest reserve of battery life is with UPS with external batteries, due to the ability to increase the capacity with additional batteries (connected in parallel). Such uninterruptible power supplies are most often from the expensive category, with double conversion. If it is needed really long operating time, tens of hours, perhaps the best solution would be to purchase a generator.
	Protect equipment from overvoltage or undervoltage, dips, and equipment-dangerous shutdowns for a few seconds (our electricians like to pull the switch back and forth).	For these purposes you need UPS with AVR function (automatic adjustment voltage): line-interactive UPS or more expensive double conversion. Voltage stabilization in linear-interactive UPS is most often implemented in a stepwise, rough form; in online models the stabilizer operates smoothly.
	Protect sensitive equipment from as many electrical interruptions and disturbances as possible.	For these purposes only suitable uninterruptible power supply on-line type.

Note that if you only need power stabilization and do not need to ensure autonomous operation of the equipment during a power outage, it is more advisable to buy a separate stabilizer.

Also, quite often they use a combination of a stabilizer + an inexpensive UPS (the uninterruptible power supply is connected to the network AFTER the stabilizer). Such a tandem not only allows you to regulate the voltage if this is not provided in the UPS, but also extends the life of the UPS batteries.

What equipment are you buying a UPS to protect?

Which UPS to choose also depends on the design features of the connected equipment.

The general rule is this: you can connect almost any equipment to a UPS with the correct sine wave at the output; you just need to correctly calculate the power. Not all equipment can be connected to other UPS, especially the offline type.

Peculiarity	Optimal UPS type	Explanation
Elements sensitive to non-sinusoidal waveforms.		The most common case is devices with an electric motor, pump, compressor, including gas boiler pumps, as well as almost all household appliances: refrigerators, hair dryers, washing machines, electric drills, etc. A stepped sine wave, or even more so a meander, has a negative effect on an electric motor: eddy currents arise, inductive reactance drops, as a result the motor overheats until it burns out. In some devices, e.g. laser printers, copiers There may also be components that require a sine wave voltage to operate and will last much less when operated from a square wave or stepped waveform UPS.
Inductive elements (inductors, chokes).	UPS on-line type.	Quite often the question arises: is it possible to connect devices with an inductive load to a regular cheap uninterruptible power supply, for example, fluorescent lamps? In practice, they connect it, and everything seems to work. But it should be borne in mind that many manufacturers categorically do not recommend this and classify cases of uninterruptible power failure after connecting an inductive load as non-warranty. In addition, there have been cases where a reactive load damaged a UPS that was not designed for it.
Transformer (linear) power supply.	UPS on-line type.	When choosing a UPS for devices with transformer power supplies, you need to be wary of a UPS that does not produce a pure sine wave output. When powered with voltage in the form of a meander or stepped sinusoid, losses in the transformer increase, which, if it is heavily loaded, will lead to a decrease in the transformer’s resources by tens of times. Also in practice, there have been cases when the UPS itself, to which such a load was connected, burned out. On the other hand, quite often equipment with low-power transformer power supplies, for example, radiotelephones, works quietly in tandem with an off-line UPS. However, many manufacturers, as in the case of inductive loads, most often do not recommend connecting transformer power supplies to conventional UPSs. How to distinguish a transformer power supply from a regular switching power supply? If we are talking about an external power supply, then a pulse power supply is usually light and small, while a transformer power supply is heavier and larger, due to the fact that the transformer itself is located inside it. The type of built-in power supply is more difficult to determine; here you need to rely on the manufacturer’s documentation. The good news is that in most cases, switching power supplies are now used in electronic equipment such as modems, switches, routers, and computers.
Structural elements sensitive to power quality.	Only on-line UPS type.	Almost everyone knows that equipment is sensitive to voltage drops in the network, or constantly under (over) voltage. However, the quality of power supply is determined not only by voltage. Sensitive telecommunications, audio-video, measuring, and medical equipment also react negatively to: unstable power frequency, radio frequency interference in the network, harmonic voltage distortion, nanosecond and microsecond voltage pulses. All this can not only distort the operation of equipment, but also shorten its service life.
	On-line UPS with power corresponding to the load.	Equipment with electric motors, pumps, compressors and other structural elements that consume a large amount of electricity at the time of start-up cannot be connected to low-power UPSs. Inrush currents can exceed standard consumption by 3-7 or more times.

How to calculate the power of a UPS?

In order to choose the right uninterruptible power supply, you need to calculate the total power of the equipment that you are going to connect to it. Power values ​​can be clarified in technical specifications(passport or instructions for equipment).

Let's look at a hypothetical example.

We want to connect to the UPS:

• 250 W computer,
• 60 W LCD monitor,
• 2000 W air conditioner (cos φ = 0.8).

There is one point here: even if the power of all devices is expressed in one unit, in this case in W, you need to calculate two powers: in volt-amperes and watts.

Power in volt-amperes and watts - what's the difference?

Power, which is expressed in volt-amperes (VA, VA) is called full power. It shows the actual load of the equipment, taking into account active and reactive ones.

Power, which is expressed in watts (W, W), is called active power.

These are two different quantities, and both need to be taken into account when choosing a UPS with the power you need. This is especially important if you are going to connect a reactive load to the UPS, since in such equipment the apparent and active power can differ significantly.

Calculation of power in volt-amperes.

To convert active power (in watts) to total power in volt-amperes, we use the formula:

Where:

• VA - apparent power,
• W - active power,
• P - equipment power factor.

If the equipment belongs to the active load, and this is almost all network, telecommunications equipment, lighting and heating devices, that is, equipment without inductance, without reactive power, and computer technology With power supply units with power factor adjustment (APFC), the current factor can be taken equal to 1, or better with a small margin - 0.95.

If you are going to connect a UPS laser printer, air conditioning, fluorescent lamps - equipment that has electric motors and the like, everything where there is inductance and reactive power, as well as computers with power supplies without APFC, the current power factor must be looked at in the device passport or on the sticker on the back wall. For this technique it is most often indicated. The power factor is designated as Power Factor (PF) or cos φ.

In the case where the manufacturer has not indicated the value of the power factor, but the load is clearly not fully active, you can take the most common value: 0.7.

Let's return to our example.

The power supply in the computer does not have power factor adjustment, so we take the P value equal to 0.7. It's the same on the monitor. In total we get the full power:

• for a computer with a monitor: (250+60)/0.7 =442 VA,
• for air conditioner: 2000/0.8 =2500 VA,
• Together: 2942 VA.

So, should we buy a 3000VA uninterruptible power supply? Take your time, it's not that simple.

Calculation of power in watts.

Most often the simplest case occurs - when the power is in watts, it is also called active power, is already indicated in the documentation for the equipment. If not, you can convert the power from volt-amps to watts using the same methodology as for total power.

Let's calculate the power of our equipment in watts:

• computer with monitor - 310 W,
• air conditioner - 2000 W,
• Together: 2310 W.

In our online store, among UPS for 3000VA, for example, there are:

How to calculate the required capacity of an uninterruptible power supply?

Usually, when choosing an uninterruptible power supply, we have some specific requirements for the time during which it will support the operation of the equipment connected to it in the event of a power outage. Many manufacturers indicate an approximate range, for example, they write that depending on the load, the battery life will be 4-20 minutes. Or they indicate that when working at maximum load this time will be 5 minutes.

But this is approximate, and we need to be absolutely sure that the UPS we purchased will provide battery operation for a certain list of equipment. Or calculate how long our chosen UPS model will hold our load.

We calculate the battery capacity for a known battery life

For calculations we need:

• The total active power (in watts) of the equipment that we are going to connect to the UPS (W).
• Battery life (T).
• Rated battery voltage.

We use the formula:

Where:

• T - time of planned autonomous operation (h),
• P - power of connected equipment (W),
• KPD - efficiency of the uninterruptible power supply (you can take about 0.85).

And the formula for converting capacity in Wh to capacity in AH:

Let's say we need the computer and monitor from the example above to work for 2 hours after a power outage.

Capacity (Wh) = 2 * 310 / 0.85 = 730 Wh.

However, battery capacity is usually indicated in ampere-hours. To convert watt-hour capacity to amp-hours, you will need to specify the rated voltage of the batteries.

For 12V batteries:

Capacity (A*h) = 730/12 == 60.83 ≈ 61Ah.

For 24V batteries:

730/24 = 30.42 ≈ 30Ah.

Since most often a UPS uses 1-2 batteries, less often 4, with a capacity of 7-9AH, it will be difficult for us to select a standard UPS for such total capacity values. It is best to buy an uninterruptible power supply with the ability to connect external batteries and select the capacity according to your needs.

UPS catalog with the ability to connect external batteries.

UPS efficiency (approximately 0.85).

We use the formulas:

• V - rated battery voltage (V),
• AH - capacity of one battery (AH),
• N is the number of batteries.

• E - total capacity (Wh),
• KPD - efficiency of the uninterruptible power supply (by default you can take 0.85,
• P is the power consumption of the connected equipment.

Let's take for example UPS PowerCom BNT-800AP USB. The manufacturer claims a battery life of 5 minutes at maximum load. How long can our computer and monitor work with a power consumption of 310 W?

Total capacity (Wh) UPS = 12V * 7.2AH * 1 = 86.4 Wh.

Time = 86.4*0.85 / 310 = 0.237 hours ≈ 14 minutes.

Conclusion

Now let's briefly summarize.

In order to select a UPS, you must:

• Define, what type of UPS do you need.
• Calculate the required total and active power of the UPS, taking into account starting currents and a small margin.
• If you need to maintain power for a certain time, calculate how much UPS capacity is needed for this. And depending on the calculated capacity, buy a regular uninterruptible power supply or a UPS and a set of additional batteries for it.

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