Inverters and uninterruptible power supply (UPS) units can produce AC power from DC sources and are often confused for this reason. However, a UPS is a more sophisticated device, with more functions and, in fact, uses an inverter as one of its internal components.
In simple terms, an inverter takes electrical power from direct current (DC) sources, such as batteries or solar panels, and supplies the alternating current (AC) used by most appliances. A UPS also has this function, but has additional features such as instantaneous response and energy storage.
Make sure your appliances have a reliable backup power system.
UPS units and inverters can be compared to air conditioners and compressors. Just as a compressor cannot provide space cooling on its own, a stand-alone inverter cannot perform all of the functions of a UPS.
How does an inverter work?
The main function of an inverter is to convert electrical energy from DC to AC as discussed above. Please note that only inverters convert energy and cannot generate or store electricity by themselves. If you disconnect an inverter from its DC source, the AC voltage supply will be interrupted.
- Like any electrical equipment, inverters have a rated power. For example, most residential solar power systems use inverters below 10 kW, while medium-sized commercial installations will likely exceed 100 kW.
- In addition to having a rated power, inverters are designed to operate within a certain current voltage range, both on the DC and AC sides.
Variable frequency drives (VFD) are often called “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 to control motor speed by adjusting voltage and frequency. Just like a UPS, a VFD uses an inverter as one of its internal components, but has additional functions.
How does a UPS work?
An uninterruptible power supply or UPS has a self-explanatory name – it provides electrical power without interruption, especially during blackouts and power grid disturbances. However, uninterrupted power is only possible when two conditions are met:
- Energy storage , which is used by the UPS when electrical service is interrupted. The energy storage function is typically performed with batteries and a charge controller.
- Instant response , so that all equipment connected to the UPS can continue operating when the blackout occurs. For example, UPS units are used in data centers to protect information and hardware when there are problems with the power grid.
Note that inverters can also be used as backup power sources when combined with energy storage systems. However, a conventional inverter cannot achieve the seamless transition offered by a UPS. Drives can respond in less than a second, but they are not fast enough to prevent data loss in IT applications. On the other hand, UPS units are much faster, responding in a matter of milliseconds.
Inverter and UPS Unit: Summary of Main Differences
| DEVICE | INVERTER | UPS |
| Main function | DC to AC power conversion. | Uninterrupted backup power. |
| Energy storage | No, but many inverter models can use external storage. | Yes, it includes built-in storage and add-ons to extend battery life. |
| Response speed | About 500 milliseconds. | Less than 10 milliseconds. |
| Power input | DC only, requiring a charge controller to supply the batteries with AC power. | AC and DC options. |
| Output Connections | AC terminals only. | Typically includes receptacles for connecting 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 require continuous power during a blackout, but inverters with external batteries are economical when this function is not required. 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 electrical service, there are two energy conversions involved. AC power is first converted to DC to charge the battery, 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.
Combining UPS Units and Inverters
Since UPS units are more expensive, it doesn't make sense to size them for hours of operation without a power supply. A smarter approach is to have a short-term UPS capability, providing time for a larger inverter + battery 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 and ventilation.
- UPS loads can remain connected during an extended blackout and you can simply recharge the UPS batteries with the inverter output.
Please note that energy efficiency measures allow you to operate longer with backup power. For example, if you replace fluorescent lamps 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 lots of computers and communications equipment often needs a large UPS. On the other hand, a storage area that only uses ventilation and lighting can use a conventional inverter without any problems. With a professional assessment of your facilities and equipment, you can determine the ideal configuration.
