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Generator or Uninterruptible Power Supply – A Short Guide to the Difference

It can be difficult to know the difference between a generator and an uninterruptible power supply (UPS) and which one is best for you – especially if you don’t have an electrical or mechanical background.

A generator offers a back-up power source, while a UPS protects critical appliances and electronics against power surges. But there’s slightly more to it than that, as this article explains.


A generator may be diesel or liquid petroleum gas (LPG) powered and can be installed to operate in ‘prime power’ or ‘standby power’ mode.

A generator may require a few minutes before it restores electricity to appliances and electronics following a power interruption. For that reason, a generator on its own may be insufficient to keep all the critical systems running during a power outage. A generator typically has a transfer switch that lets you power computer systems, interior lights, and other critical electrical circuits in the building.

A generator’s transfer switch is linked to a power inlet box, which looks like an electrical outlet connected to the side of a building. So, if the power goes out, you can connect the generator to the power inlet box. However, you must ensure all the box’s transfer switches are set to “off” before doing so.

You won’t have to worry about a transfer switch if you’re using a UPS. Instead, a UPS is generally plugged into an electrical outlet and any electrical devices that need to be protected during a power outage. A UPS protects one or more electrical devices, depending on the type of system uses.


When the mains power supply is present, the UPS will provide filtering of spikes and electrical noise and smooth out sags, surges and brownout conditions. When the mains power supply fails, the uninterruptible power supply will provide power to the connected load from a stored energy source; normally a valve regulated lead acid (VRLA) maintenance free or lithium-ion battery set.

The UPS may be a standby, line interactive or on-line system. On-line uninterruptible power supplies are recommended for critical IT loads due to the quality of their output waveform and continuity; whether powered via the mains power supply, battery set or a standby power generator. This type of UPS is often referred to as a ‘double-conversion’ (AC-DC-AC) or ‘triple-conversion’ (AC-DC-DC-AC) system.

A UPS usually includes a battery that keeps the system running for a limited time. The system also works near-instantaneously, as it can detect the moment a power outage occurs.

If you are considering a generator or UPS it may be worth considering both to ensure your business is fully protected against power interruptions.

Generator and UPS Combination Systems

As a standby generator can take several seconds to come up to full speed (power), it is common in critical infrastructure environments to use a generator and UPS system combination.

The UPS system is installed in-line between the mains power supply and the load(s). A generating set is connected to the mains power circuit and monitored via an automatic mains failure (AMF) panel. When the mains power supply fails, the AMF panels signals the generator to start. The UPS uses its battery set to power the load until the generator can provide a sufficiently stable supply in terms of voltage and frequency. At this point the UPS synchronises to the generator output and the remaining battery charge conserved or recharged.

When the mains power supply returns and proves stable, the UPS synchronises to the waveform of the mains power supply. Once connected the generator is instructed to power off and the UPS now supports the critical load. The battery set is recharged, and the generator is in standby mode once more awaiting then next power interruption or outage.


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