The majority of UPS systems sold today utilize a VA (Volt-Amperes) rating. The difference between watts (W) and volt-amperes (VA) is that Watts accounts for an inefficiency called the Power Factor (PF) and VA does not. The Watts rating is going to be more accurate, unfortunately, UPS companies like to advertise the VA rating because it makes their products look better, the VA number is always higher.
- The most basic formula for determining the necessary UPS protection may be expressed by the following:
- Amps * (120 Volts) = Watts / Power Factor "PF" = VA
- Amps or "A" is the current pull of the system Voltage is a constant of 120 in North America Watts or Wattage for AC current = Volts x Amps x Power Factor "PF" Power Factor or "PF" is a number between 0 and 1, the watts after reactance loss Volt-Amperes or "VA" is Watts / PF or Watts without PF loss taken into account
- Older PC systems had less efficient power supplies with PF rating around 0.8, while newer PC power supplies are much more efficient with PF ratings often at or above .95. Incidentally, an ordinary incandescent light bulb has a perfect PF of 1.
Backup Time: How Long UPS Power Lasts
To calculate the size of the required UPS add up the power requirements in watts. A fluorescent lamp of 200 equivalent "watts" literally gives the same light output in lumens as a standard 200 Watt incandescent lamp. Please note it probably uses 45 Watts of power. Read the package or the label on the ballast unit itself. Do not confuse the misuse of "watts" to describe light output.
To further confuse things the power requirement of most devices is listed according to W=VA or Direct Current Watts = Volts * Amps. This is the formula for DC (direct current) power. It provides a built in safety factor for electricians and homeowners because AC (alternating current) is more efficient. FYI: 60 Cycle Alternating Current Watts = Volts * Amps * Sin 45 Degrees. The Sin of 45 Degrees is .707. It is recommended you use the DC formula to calculate the power requirements of each device.
- The backup time is the time your equipment will run off of UPS power during an electrical outage. This backup time is proportional to the total load connected to the UPS. Although not a direct linear proportion, the total backup time will decrease with the more devices you connect to your UPS.
- As an example a quality UPS rated 1500VA will offer 5 minutes of battery runtime at full load, while offering 14 minutes of runtime at a half load of 750VA. The Tripp Lite OMNIVS1500XL, a line-interactive UPS, was referenced for these figures. Most UPS manufactures consider a full load VA rating with a safe PF of .8 even though modern power supplies are much more efficient. This is beneficial to the consumer since no one is going to have a power supply less efficient than a PF of .8.
- When the VA rating or Watts power rating is exceeded then a UPS is effectively overloaded. Keep in mind it is technically necessary to be aware of both the Watts and VA load when selecting an UPS. Once the power factor of the UPS is known, a quick calculation using the Watts rating of the UPS output can be done for comparison with other UPS products and to assure that sufficient "real power" is available from the UPS to power a connected load.
