What is the difference between a UPS and a home inverter?

In South Africa, the public discussions in the workplace and at home is the dreaded rolling blackouts and power cuts. Everyone is talking about home inverter systems, 200AH batteries, and deep cycle batteries are all buzz words.

With all the load shedding issues that disrupt our electricity supply for hours at a time, sometimes weeks on end, “Emergency Power Experts” began setting up shop on every corner. This industry has incorrectly sold “inverter solutions” to the public, causing a certain amount of confusion about backup power solutions.

I hope to answer a few niggling questions about standby power and emergency power systems.

An inverter is used to convert battery power, also known as DC, into 220 VAC + Neutral. A three-phase inverter is, essentially, three inverters in a unitary box with each inverter supplying its own phase. The Neutral point is common to all three phases.

AC and DC Power

An inverter performs the complete opposite function of a rectifier. A rectifier, or battery charger as it is commonly known, is used to convert mains AC power to battery DC power which charges the battery.

DC power is also often used for industrial applications such as powering PLC’s and emergency DC pumps in power stations and heavy industry. It can also be used for backup batteries for PABX and telecoms systems such as cell phone tower equipment backup. This is why you don’t often see a generator backup at a cell phone tower; it uses big batteries to supply its telecoms equipment directly, for as long as 24 hours.

Emergency backup to DC loads with a rectifier and battery is the most effective and efficient way of preventing critical loads from going down when the power fails.

What does a UPS consist of?

A UPS consists of a rectifier (converts AC power to battery DC power) with its connected backup battery and an inverter (converts DC battery power back into AC power) in one complete unit. Because a UPS has these two conversions happening it is less efficient than a load been supplied by DC through a rectifier. Thus a UPS uses more power than a rectifier with its connected DC load.

An inverter is thus one of the key critical components used in making a UPS. However, an inverter cannot work without a DC supply, so it must have a rectifier supply.

A UPS and home inverter are both used to power emergency loads

With the rolling blackouts, there has been a wave of emergency power experts popping up all over the place. This industry has incorrectly sold “inverter solutions” to the public.

In actual fact, a so-called “home inverter” is normally just an offline UPS system. This means that the load is usually fed by mains 220 VAC power, and when the power fails, the inverter will switch on and run off the connected battery to supply AC power to your emergency loads. When the AC mains power returns, the inverter switches off, and the AC power will feed the load.

But it is still necessary to have a battery charger to recharge the battery. Thus the so-called “home inverter” is actually a UPS as it has both a rectifier to recharge the battery and an inverter.

A home inverter is typically fitted with a big beefy rectifier so that it can charge big backup batteries for long discharge run times. This is the same thing as a UPS with extended backup battery charger often with an L at the end of the UPS products model number.

So a home inverter is a UPS, there is no mistake about that. It’s just the incorrect marketing of the home inverter that has taught the general South African public this phrase that has become a common household word when discussions are held around power failures and backup emergency systems.

As the home inverter is aimed for sale to the man on the street, it is very much a price-driven item. This means that the build quality of home inverters is generally much worse than UPS systems which are more widely used in everyday life.

What battery is best?

Unfortunately, the home inverter market has brought along with it a mass of battery “experts”. There are thousands of home inverters being sold all over South Africa with incorrect batteries, which are, amongst other things, a fire risk.

A typical battery used only in South Africa for emergency power backup, and NOWHERE else in the world, is the semi-sealed maintenance-free flooded lead-acid automotive technology battery, also incorrectly referred to as a “deep cycle” battery.

These batteries are designed and intended for the automotive industry and are NOT intended for emergency backup. When they get old, they are prone to explode, sending chunks of battery flying and lots of free-running acid, which can cause significant damage to their surroundings and people if they are nearby.

The correct battery product to use on a UPS system is the high rate VRLA maintenance-free battery, which is specially designed for UPS use.

Another battery product specific to the battery backup industry is the properly designed deep-cycle VRLA which has technical features built into it that makes it very resistant to long deep discharges during extended power failures lasting several hours.

Choosing what is right for you

The home inverter user has a massive range to choose from without the technical knowhow or savvy needed to mix and match the correct UPS system with the right battery for the required emergency load.

  • Price is NOT the main consideration when buying a reliable standby power system.
  • Be sure you get advice from a reputable company with a good track record and technical expertise.
  • Make sure that there are spares available locally for the system you choose.
  • Choose a battery that is safe and fully recyclable in South Africa.

To get the right combination for your power supply needs takes expertise, but in the end, it will ensure many years of safe emergency backup power availability.