Step 1

Establish whether you really need a UPS.

Sudden loss of power and power surges are two of the main causes of damage to computers and other sensitive electronics. In order to protect your electronic devices against power supply interruptions, you need a battery backup. Uninterruptible Power Supply (UPS) units contain a big batteries inside, providing a buffer against power supply interruptions. This buffer can range from a few minutes to an hour or more depending on the size of the unit.

A UPS offers protection against drops in line voltage, brownouts, blackouts, and other power supply issues.

How do you select a UPS?

Some UPS units are tiny desktop units designed to keep a lightweight desktop computer running for 10 minutes or less, and others are walk-in-freezer sized units deployed in data centers to keep an entire bank of servers running through a black out.

Major Categories of UPS

A.Passive (standby or off-line) and Line Interactive UPS

These technologies are most common for personal workstations and point of sale applications They are typically single phase equipment with size ranges of 350 VA – 2000 VA for passive and 500 VA to 5000
VA for line-interactive.

Passive UPS’s are the simplest type Under normal conditions AC power passes straight through to the UPS load When the input power supply goes outside of specifications, the UPS transfers
the load from input power to the internal DC to AC power inverter Passive UPS’s do not correct for voltage or frequency deviations under “normal” operation.

Line-interactive is similar to the passive technology except it has circuitry that attempts to correct for standard voltage deviations Frequency deviations under “normal” power operation are not corrected.

Equipment Notes:
These devices tend to be electrically / harmonically very noisy. A single small UPS is not a significant concern, but applications with multiple UPS’s can be problematic Passive UPS technology typically has normal tolerances of 10-25% on voltage and 3 hertz on frequency. If the input source goes outside of
these tolerances, the UPS will switch onto the UPS battery source .Some line-interactive units may have frequency tolerances factory set to 0 5 hertz .These units will need to have their frequency tolerance
increased to a minimum of 2 hertz

B.Double-Conversion

This technology is most common for critical load applications. Double-conversion UPS’s constantly rectify AC to DC and then invert the DC back into AC. This configuration results in an output that corrects for voltage and frequency deviations. There are single and three phase models covering small through large applications. Most UPS applications larger than 5000 VA use double conversion technology .This approach is also the preferred technology for generator applications.

Equipment Notes:
Double-conversion UPS’s that are single phase or unfiltered three phase models tend to create a significant level of electrical/ harmonic noise. This is illustrated by harmonic current distortions that are greater than 35%. When three phase models are supplied with harmonic filters (current distortion less than 10%), this concern is no longer an issue.

Step 2

How to size a battery back-up systems

AC power measurements are related as follows:

VA = Volts x Amps only if PF = 1. Where PF=Power Factor and VA=Volt-Amperes. If PF is smaller than 1 then Volts x Amps = Watts and Watts ÷ PF = VA

Volts = 230 typical, Amps = Load Current and Power Factor = between 0 and 1

Power factor is a number between 0 and 1 which represents the fraction of the load current which provides useful energy (Watts) to the load. Only in an electrical heater or incandescent light bulb is the power factor equal to 1; for all other equipment, some of the load current flows into and then back out of the load without delivering watts. This current, composed of distortion or reactive currents, is the result of the nature of the electronic load. The important point to understand is this distortion or reactive current, which is forced to exist due to the load, causes the load V-A rating to be larger than the load-watt rating.

How to calculate battery runtimes.

Factors to consider include,

1. Load

The bigger the load that one wants to support, the greater the capacity of the batteries should be. In our example we will use a load of 1000W.

2. Required runtime

The longer the required runtime, the greater the battery capacity will have to be. In our example we will use a required runtime of 1 hour.

3. Charger Type

The battery capacity is limited to the size of the charger. Generally, the battery capacity should be no more than 12x the maximum charge current i.e. a 5A charger can only accommodate 60AH of batteries (5 x 12 =60). However, if discharges are expected to be less frequent than once in every 10 days, one may in extreme cases go to 20x the maximum charge current. We will assume that our UPS has a 10A charger.

4. DC Bus

The DC Bus is the voltage required by the inverter to operate and dictates the number of batteries in series required to drive the inverter. This information is available from the UPS supplier and should be clearly indicated in their spec sheets. DC busses range from 12V (1 x battery) to 480V (40 x batteries). We will assume a DC bus of 36V (3 x batteries).

5. UPS Efficiency (Inverter)

Inverters use some of the energy supplied by the batteries to run the internal electronics and so not all of the battery capacity is available to run the load. Also, some of the energy is lost due to cabling and connections particularly if there are long runs of DC cables. For our example we’ll assume a 70% inverter efficiency.

6. Battery Type

There are many types of batteries available on the market today .As a rule we use fully sealed, maintenance-free, deep cycle lead-acid batteries which are common to the industry. It is important to remember that batteries discharge exponentially faster at higher loads than at lower loads, so if a battery provides 1 hour runtime at 5A it will provide significantly less than 30min at 10A – usually in the order of 10 to 15% less. For total accuracy it is important to refer to the discharge curves of the particular battery manufacturer however, PHD uses a few rules of thumb to increase runtime accuracies.

1. For runtimes below 2 hours, a factor of 1.5 is applied to the final required battery current.
2. For runtimes above 2 hours, a factor of 1.3 is applied to the final required battery current.

Calculation

From the above information we have the following:
Load = 1000W
DC Bus = 36V (3 x 12V batteries)
Required runtime = 60mins
Inverter efficiency = 70%
Charger = 10A

First we need to calculate the current required to run the load and the inverter:
I = Load / (70%) / (DC Bus)
I = 1000 / 0.7 / 36
I = 39.68A
From the above we can see that to supply our load and inverter for 1 hour, the battery will have to supply 39.68A for 1 hour = 39.68AH. However, using our rule of thumb for the exponential nature of battery discharge curves, we need to increase the required current by a factor of 1.5
I = 39.68 x 1.5
I = 59.52

A 59.52AH battery will therefore be required to provide 1 hour’s runtime to the load of 1000W. However, manufacturers do not manufacture batteries of 59.52AH so the next biggest standard size should be selected. In this case a 65AH battery. Finally, since the DC Bus requires a 36V input 3 x 12V, 65AH batteries connected in series will be required to complete the system. The above calculations may seem a little daunting
and many consumers may feel it unnecessary to learn something that they may need once in a lifetime. So here’s a basic rule to ensure that when comparing quotes one compares apples with apples. Remember “the devil is in the details” so always ask for a breakdown when getting a quote for a backup system. Ensure that the UPS, batteries, cabinets and installation are each quoted on a separate line with a full description including the number and capacity of batteries. That way, one is easily able to understand the discrepancies between competing quotes.