How to choose a UPS?
Step 1: Calculate the power required for the installation
a) Read the consumption in Amps, Watts or VA of each device to be protected.
Here are formulas you need to know:
- Apparent power (VA) = Voltage (V) x Current (A)
- Effective power (W) = Apparent power (VA)/Power factor* (cos)
* In most cases, power factor = 0.7
b) Add up the values found (in the same values).
c) Choose an inverter with a higher power than this sum of the connected loads (ideally about 1.25 higher than total load).
Step 2: Choose the right technology for the installation
Off Line technology
The equipment is normally powered by the mains and the inverter only takes over in the event of a power cut or a significant drop in mains voltage. This switchover to battery power takes a little time (a few milliseconds), which is not too much of a problem for computers with switch-mode power supplies, but it can cause problems for more sensitive equipment. Off-line UPS systems are the most economical but should be used only at workstations or for individual use.
Line Interactive technology
Line-interactive UPS systems are usually implemented with Automatic Voltage Regulation (AVR) buck/boost transformers. They provide voltage regulation in the mains line supplying the load during normal operation. High-end line-interactive UPS also offers surge and noise suppression.
This voltage regulation works by reducing the number of mains battery transfers and putting less stress on the battery. This topology offers slightly better running cost efficiency. Application areas are networking equipment (router, modem), games consoles, kiosks.
On Line Double Conversion technology
An On Line Double Conversion UPS system converts power twice:
- first, the AC power from the mains (with all its voltage peaks, disturbances and other anomalies) is converted into DC power.
- secondly, the DC voltage is in turn converted to a strictly regulated AC voltage by the inverter. This AC output can even have a different
frequency from the AC input, which is impossible with a Line Interactive inverter.
All power supplied to the load equipment goes through this double conversion process when the AC input is present.
The current is continuously delivered by the inverter, ensuring a constant voltage and no interference. The device to be protected is therefore completely disconnected from the mains. Tests show that On Line inverters have the most stable output voltage and the best immunity to interference. They should therefore be used on all critical servers.
Other technologies exist, but they are mostly derived from these three basic models !
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