Inverters and uninterruptible power supply (UPS) units can both produce AC power from DC sources, and they are often confused for this reason. However, a UPS is a more sophisticated device with more functions, and it actually uses an inverter as one of its internal components.
In simple terms, an inverter receives electric power from direct current (DC) sources like batteries or solar panels, and it provides the alternating current (AC) used by most appliances. A UPS also has this function, but it has additional features like instant response and energy storage.
UPS units and inverters can be compared 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.
The main function of an inverter is converting electric power from DC to AC, as discussed above. Note that inverters only convert power, and cannot generate or store electricity by themselves. If you disconnect an inverter from its DC source, the AC voltage supply is interrupted.
Variable frequency drives (VFD) are often referred to as “inverters” because many models produce three-phase AC power from a DC input. However, this naming practice causes confusion, since the purpose of a VFD is controlling motor speed by adjusting voltage and frequency. Just like a UPS, a VFD uses an inverter as one of its internal components, but it has additional functions.
An uninterruptible power supply or UPS has a self-explanatory name - it provides electric power without interruption, especially during blackouts and power grid disturbances. However, uninterruptible power is only possible when two conditions are met:
Note that inverters can also be used as backup power supplies, when combined with energy storage systems. However, a conventional inverter cannot achieve the seamless transition offered by a UPS. Inverters can respond in less than one second, but they aren't fast enough to prevent data loss in IT applications. UPS units are much faster, on the other hand, responding in a matter of milliseconds.
| DEVICE | 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 addons to extend battery time. |
| Response speed | Around 500 milliseconds. | Less than 10 milliseconds. |
| Power input | Only DC, requiring a charge controller to fill batteries with AC power. | AC and DC options. |
| Output connections | Only AC terminals. | Normally includes receptacles to connect appliances directly. |
Assuming the same rated power, a UPS is normally more expensive than an inverter, given its additional components and functions. UPS units are necessary in applications that demand continuous power during a blackout, but inverters with external batteries are cost-effective when this function is not needed. For example, you would not want to leave a data center without power (UPS), but a brief disconnection of your lighting system can be tolerated (inverter + energy storage).
When you charge batteries using the main electric service, there are two power conversions involved. The AC supply is first converted to DC for battery charging, and the battery output is converted back to AC. Battery charging requires DC power, and you need a rectifier when using an AC input. UPS units include this component, but an external charge controller is required if you have batteries connected to an inverter.
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 having a short-term UPS capacity, providing time for a larger inverter + battery system to take over the load.
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 large UPS. 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 installations and equipment, you can determine the optimal configuration.