What Size UPS?

UPS manufacturers spend a lot of time working out methods of sizing. Much of this comes from the consumer or light commercial market, where the general population is not necessarily electrically knowledgeable. The UPS is very much like a battery. It has a load rating for a period of time. In a battery, this would be so many amp/hours (AH). The UPS typically has a Volt-ampere (VA)/min rating.

The most difficult part, obviously, is to calculate the correct VA. A good rule of thumb to remember is that most computers and other such devices have a poor power factor; therefore, using the device wattage is not recommended.

The Power Factor is the real Power (measured in Watts) divided by the Apparent Power. So:

Power Factor (pf) = Watts of Average Real Power / (Volts * Amperes)

Another way of expressing this:

Power = Voltage x Current x Power Factor

Power Factor - Most desktop PCs present a nonlinear load to the AC supply. The voltage and current are somewhat out of phase. This is because they have a power supply design known as a “capacitor input switch mode power supply”. Typical PC systems have a power factor of .65, which means that the apparent power (VA) is 50% larger than the actual power (Watts).

Information Technology equipment like servers, routers, hubs, and storage systems, now use a different power supply design known as “Power Factor Corrected”. These devices do not generate harmonics. Ten years ago, these devices were nonlinear loads like PCs, but today all of these loads are subject to international regulation IEC 1000-3-2, which require them to be made with the “Power Factor Corrected” power supplies.

To determine the UPS capacity you will need, follow these simple steps:

1. List all equipment to be protected by the UPS on your worksheet.

2. Read the nameplate data on each of the devices listed in Step 1. Write down the voltage and amperage for each device listed in Step 1.

3. Multiply the voltage by the amperage of each device to calculate the Volt/Amps (VA). Some equipment, such as PC power supplies, may be marked with a power consumption measured in Watts. To convert Watts to VA, simply divide Watts by 0.65 (for a power factor of 0.65), or multiply by 1.54.

4. Total the VA for all devices you want to protect with the UPS and enter it in the "Subtotal".

5. Multiply the subtotals found in Step 4 by 0.25 and enter it as the "Growth Factor". This number takes into account room for future growth. This growth factor allows for a 5% rate of growth for each year over a five-year period.

6. Add the "growth Factor" to the Subtotal" to get the "Required VA".

Now you can select the appropriate UPS model by choosing a model that has a VA rating at least as large as the "Required VA" you calculated in Step 6.

 UPS SizingMost UPS manufacturers will provide sizing assistance or easy to use programs, to aid in specifying their products.

Laser printers generally do not require support by a UPS. When they do, however, care should be taken when calculating the load of such devices. The data plate rating of a laser printer might state, for example, 120 volts at 2 amps or 240 VA. This only relates to the running load. When the fuser in the laser printer comes on, the load will increase to around 5 amps or 600 VA. It is recommended only that a quality surge protector for protection from lightning strikes and electrical noise be in place to protect the printer, and to leave it off the UPS so as not to drain the battery life and short-change the other devices the UPS protects.

 Lasers and UPSes Don’t MixThe official CompTIA line is that you do not put laser printers on a UPS.

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