Which of the three UPS types is the right one for me?

Most people are unaware that there are three different types of UPS system designs, all of which can be used in different ways to support different backup loads.
They are:

  • 1. Offline UPS
  • 2. Line-interactive UPS
  • 3. Online Double-conversion UPS

First of all, anyone buying a UPS must remember that, as with everything in life, you get what you pay for.

That said, many large UPS corporate companies with worldwide coverage such as Riello AROS, Schneider APC, Eaton etc., make use of high productivity countries to produce some of their product ranges.

Generally speaking, the majority of single-phase UPS systems sold into South Africa are produced and labelled in China. Often companies will even use the same Chinese manufacturer with some “tweaks” to make it their own product.

It cannot be said that all UPS units are the same. There are three distinct different technologies available with the best, of course, costing you the most, but giving you the best backup power solution for your critical load.

1. Offline UPS

An offline UPS is a UPS system where the battery charger is only big enough to charge the battery, and the inverter cannot run at its rated VA capacity for extended periods. It is designed to the absolute technical limits of the inverter’s components, which generally start overheating after 20 minutes or so.

This is the simplest and cheapest form of UPS system and therefore the most commonly bought by unsuspecting and uninformed UPS users who are often disappointed after their purchase.

Besides charging the battery, an offline UPS system is always off and the inverter will switch on in a few milliseconds and feed the emergency connected load with its inverter until the backup battery is flat. General IT loads are usually not affected by the switch-on time of the inverter.

Most offline UPSs will not work or charge the internal backup battery if the generator supply feeding it is of poor power quality found in small diesel- and petrol generator sets that are built as cheaply as possible for entry-level backup power generation solutions. They usually do not have a pure Sinewave inverter but rather a modified sinewave or quasi-sinewave which is not good for use with motor-type loads and printers as they require Sinewave supply to work and last properly. Remember printers are fitted with motors like many other small devices.

An offline UPS is a cheap and simple way to backup most loads, however, more sensitive loads will falter during the switch-on time for the inverter from when the mains fails. If you think your load will survive bad power supply and slow switch-on time then an offline UPS is your cheapest choice, but maybe not the wisest.

Dirty power damages sensitive loads

An offline UPS does not filter or clean any power from the mains under normal mains supply conditions and therefore does not offer the user any power filtering. Many people think that it is the power failures alone that damage sensitive loads such as IT; that is a myth. In fact, the greatest culprit in causing hardware and software failures, in all types of electronic equipment, is the bad power that exists in the mains supply grid, especially in Africa.

Offline UPS systems are generally only available up to a maximum of 3 kVA with the most popular size being a 650 VA for home use on a PC or network switch.

2. Line-interactive UPS

A line-interactive UPS is a step up towards the best from the offline UPS types; it plays the middle man between the worst and best technologies.

Line-interactive UPSs usually have a battery charger that only charges the UPS backup battery. The inverter is always on but it doesn’t supply the load. It follows the mains AC supply so that if the mains AC supply goes out of limits it takes over. Because the inverter is on the switch-on to inverter time is generally much shorter, and thus far better, for use with sensitive loads.

Line-interactive UPS systems usually have an AVR (automatic voltage regulator) fitted inside. This is a transformer type device that will help regulate the mains supply feeding the load under normal power supply conditions, making the load supply under normal power conditions more stable. They are also sometimes fitted with filters to limit the amount of electrical “noise” on the supply grid from affecting the critical loads.

Generally, this middle-man UPS is only available up to 3 kVA with some manufacturers producing it up to 5 or 10 kVA. It is cheaper to build with lower-rated power components of the same VA rating when compared to the much superior and next level UPS which is the online double-conversion UPS system.

3. Online Double-conversion UPS

An online double-conversion UPS system is by far the best UPS backup system that money can buy; affording the user ultimate power stability for the critical loads connected to this type of UPS inverter. An online double-conversion UPS system works 24/7! And offers the best UPS solution for any user.

This type of UPS system is fitted with a much bigger rectifier that charges the backup battery while supplying the inverter all the time. The UPS inverter is continuously rated to supply 100% full load and it recreates the Sinewave inverter power to ensure that the critical connected loads are receiving the best and cleanest Sinewave power that is available.

Because the inverter is always feeding the connected UPS loads, the mains power supply variations and spikes, dips, brownouts, power failures, etc., are not felt by them as they always have steady inverter power supply.

When the power supply to the UPS rectifier fails, the inline battery backup simply stops charging and supplies power to the inverter input. The inverter cannot distinguish between the battery charger power and the battery backup power and thus there is no switch-on time. It just merely continues to operate.

Because an online double-conversion UPS system protects the UPS user’s loads from mains power supply failures as well as all the bad things that affect sensitive loads on the mains power supply grid, this type of UPS system design is the best by far, but it is not the cheapest.

At the end of the day, you get what you pay for when it comes to a UPS system, the life quality, and reliability of the inverter’s critical loads.