A UPS is an electrical device that provides battery backup power to a load (e.g. a computer) when the primary power source (mains or utility power) fails or fluctuates excessively. When the electricity supply flows adequately again, the UPS transfers the load back to the primary power source again. Different UPS models vary in design, battery capacity and in size.
(a) Online UPSs:
In a nutshell, with this technology the load doesn’t receive electricity directly from the AC outlet. On entering the UPS, The AC power is directed to a rectifier, which turns the AC into DC power. This DC power is then directed to the UPS batteries. From the batteries, this DC power is then directed to the UPS inverter, which turns the DC into AC power again. The end result is that the batteries are always connected to the inverter and the load is continuously supplied with uninterrupted, clean, sinusoidal power. In the event of a power failure there is no break in power.
(b) Offline UPSs:
Many other UPS designs have been developed e.g. offline, standby, line interactive, parallel online, etc. These are generally less complicated and therefore cheaper products. They supply the normal primary source AC power to the load and only in the event of a power failure (or unacceptable fluctuation etc.) the UPS switches over to the batteries for backup power. During this switching over to battery backup, there is a very short break in power (typically 3ms). The cheapest designs also don’t produce a perfect sine wave. Many of the smaller loads (PCs, monitors, TVs, routers, switches, lights, cameras etc.) have been designed to accommodate brief breaks in power (e.g. with capacitors.) and they often don’t need a perfect sine wave. So on the smaller end, say up to around 3 kVA, the cheaper offline UPSs are more popular, and the more expensive online UPSs are only required if that particular load can’t handle a 3ms break in power and/or needs pure sine wave power.
Different UPS battery capacities:
(a) Standard internal battery:
The UPS was initially designed to just give enough uninterrupted power for that short space of time before the standby power kicks in and takes over. So you’ll find that your standard UPS has internal batteries for 10 to 15 minutes battery backup on say 70% average load. This means that you can’t add on more or bigger batteries externally at a later stage. The UPS charger and all other components have been sized to only handle the internal batteries that this UPS was supplied with. Increasing the batteries will over-work some of the components and consequently damage the UPS.
(B) Long run UPS with external batteries:
A long run UPS will be more expensive than the standard type UPS, because it’s been built with a bigger charger and all of the components are upsized in order to be able to handle the bigger batteries and the prolonged DC running time. Light loads such as routers, switches, lights, cameras etc. can run for 24 hours and more on a long run UPS.
Once your UPS Supplier has established the maximum VA/W/A that your load would pull, he can then size the required UPS for you, and thereafter, he’ll be able to give you pricing on different run time options (battery size dependent). A long run UPS is only economically viable up to a point, (e.g. 8 hours on a 3 kVA load) and thereafter, it makes more sense to start looking at other solutions (e.g. generators or solar).