DC UPS
Uninterruptible power supplies that use direct current (DC UPSs) are engineered to supply secondary power to ensure the uninterrupted operation of critical systems and equipment. Direct current power systems are common in applications where you would expect to find an AC UPS. Examples of systems that rely on direct-current power include most removable storage devices, like tape drives and hard disk arrays, as well as solar power panels and the sealed batteries commonly used to store their energy. These might seem like systems that can easily switch over to AC power, but once you do, they commonly require an AC-to-DC or DC-to-AC conversion, which is not as energy-efficient as just staying with direct current in the first place.
In places where the main power source is direct current (DC) or where efficient use of space is a must, direct current uninterruptible power supply (DC UPS) systems come into their own. They do not need the AC-DC conversion stage, which the other two types—online and offline—not only have to make but also often have to use in conjunction with a backup power inverter. And because DC UPSs escape both energy-wasting and heat-generating conversion and packing efficiencies that their AC counterparts cannot, a lot of reason can be adduced to say that DC UPSs are simply way cooler kinds of systems.
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AC UPS vs DC UPS
An Uninterrupted Power Supply (UPS) is an electrical device that provides backup power via a battery to a load when regular utility power has been lost. Depending on the UPS, some can provide protection against voltage spikes or power surges that help protect any equipment that is connected to the UPS. UPSs are not intended to be used for long periods of time. Typically, they are only used for short periods of time to provide critical backup power until an alternative power source can be provided.
When using an AC power supply, everything that is downstream of the UPS will be on battery backup in the event of a power failure. In many cases, the AC UPS will be wired somewhere at the beginning of the power distribution circuit to ensure that almost everything in the panel has battery backup in the event of power loss, including the DC power supply and all DC related hardware. Anything upstream of this UPS, will not have battery backup and will not function during a power failure.
When using a DC UPS, the same principle will apply. Everything downstream of the DC UPS will remain energized in the event of a power failure, however, if the control panel has any AC powered devices, they will not receive any kind of battery backup in the even of a power failure. So in general, an AC power supply will protect more components within the panel. With that said, in most cases, AC UPSs are much larger and can take up a great deal of space in a control panel. Also, it is common to see control panels that have almost all of the critical hardware using DC power. So, if space allows and there are various AC powered devices that need to remain running in the event of a power failure, an AC UPS would make a lot of sense. Conversely, if space is limited and there are no essential pieces of hardware requiring AC power, a DC UPS might be the better option.