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:
- Energy storage, which is used by the UPS when the 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. For example, UPS units are used in data centers to protect information and hardware when there are power grid issues.
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.
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.