More about Inverters and UPS

  • Posted on April 15, 2019

Last month we spoke about alternative power sources to consider due to grid disturbances and solar energy was discussed. This week we take a look at other alternate power sources namely UPS’s and Inverter systems

Do You Need a UPS or an Inverter?

Uninterruptible Power Supplies (UPS) and inverters are both used to deliver electric power, and as a result they are often confused with each other. However, a UPS is a more sophisticated device with a wider range of functions – actually, a UPS uses an inverter as one of its internal components.

In simple terms, an inverter is a device that receives power from a direct current (DC) source, such as a battery or a solar module, converting it to an alternating current (AC) supply that is suitable for appliances. A UPS unit also performs power conversion, but it adds functions such as instant response and energy storage.

The best way to understand the difference between UPS units and inverters is by comparing them with air conditioners and compressors. Just like a compressor cannot deliver space cooling by itself, a stand-alone inverter cannot perform all the functions of a UPS.

How Does an Inverter Work?

As discussed above, the main function of an inverter is converting electric power from DC to AC. Note that inverters only convert power, and they cannot generate or store electricity by themselves. As a result, if you disconnect an inverter from its DC source, the electric supply is interrupted.

Like any piece of electrical equipment, inverters have a rated power they can handle. In solar photovoltaic systems, for example, most residential installations use inverters below 10 kW, while medium-sized commercial installations are likely to exceed 100 kW. In addition to having a power rating, inverters are also designed to operate within certain ranges of DC and AC voltage.

Variable frequency drives (VFD) are often called “inverters” because many models can produce three-phase power from a DC input. However, this naming practice can cause confusion, since the main purpose of a VFD is controlling motor speed by adjusting voltage input. Just like UPS units, VFDs use an inverter as one of their main components, but there are additional components and functions not found in a stand-alone inverter.

How Does an Uninterruptible Power Supply (UPS) Work?

A UPS is one of those devices that has a self-explanatory name – providing electric power without interruption, especially during blackouts or power grid disturbances. However, to be uninterruptible, a UPS must perform two vital functions:

  • Energy storage, which allows the UPS to continue delivering electricity if the main electric service is interrupted. The energy storage function is normally accomplished with batteries and a charge controller.
  • Instant response, so that all equipment connected to the UPS can continue operating when the blackout occurs. UPS units are normally used with computers and key data centres to prevent information loss during electric service interruptions or disturbances.

Note that inverters can also be used as backup power supplies, if combined with an energy storage system. However, a conventional inverter is not capable of the seamless transition delivered by a UPS. Although inverters can respond in less than one second, they are not fast enough to prevent data loss in IT equipment; UPS units are much faster, responding in a matter of milliseconds.

UPS Units and Inverters: Summary of the Main Differences

Inverter

UPS

Main function DC to AC power conversion Backup power with no interruption
Energy storage No, but many inverter models can use external storage Yes, includes built-in storage and modular addons to extend time
Response speed Around 500ms Around 10ms
Power input Only DC, needs a charge controller to fill external batteries with AC power AC and DC options
Output connections Only AC terminals Normally includes receptacles to plug in appliances.

Assuming the same rated power, a UPS unit is normally more expensive than an inverter, given its additional components and functions. UPS units are necessary when the application demands continuous power during a blackout, but inverters with external energy storage a more cost-effective option when this function is not needed. For example, you would not want to leave a data centre without power (UPS), but a brief disconnection of your lighting system can be tolerated (Inverter with energy storage).

When you charge batteries using the main electric service, note there are two power conversions involved. First the AC supply is converted to DC for battery charging, and when you use the batteries as a power source their output is converted back to AC. Keep in mind that battery charging requires DC power, and a device called a rectifier is required when using an AC input – UPS units include it, but an external charge controller is required if you have batteries connected to an inverter.

Combining UPS Units and Inverters

Since UPS units are more expensive, it does not make sense to size them for hours of operation with no power supply. A smarter approach is to have a short-term UPS capacity, providing time for a larger inverter system to take over the load.

  • An inverter with energy storage can be used as a direct power source for less critical loads such as lighting.
  • UPS loads can remain connected during an extended blackout, and you can simply recharge the UPS batteries with the inverter output.

Note that energy efficiency measures let you operate longer with backup power. For example, if you replace fluorescent lights with equivalent LED products that consume 50% less energy, they can last twice as long with backup power.

The best configuration changes depending on the loads present in your building. For example, an office with plenty of computers and communication equipment generally needs a higher UPS capacity. On the other hand, a storage area that only uses ventilation and lighting can use a conventional inverter with no problem. With a professional assessment of your property, you can determine the best configuration possible.