Back-up power – four options

By J Palmer, PowerMan

 

Published in:

Electricity+Control, January 2014 (pages 36 - 37)

Enquiries: jonathan@powerman.co.za

 

Download the full article on Back-up power - four options in PDF format.

 

Eskom has made no secret of the fact that it is struggling to meet the country’s power demands as power failures have become more frequent and longer lasting. Previously when purchasing an uninterruptible power supply (UPS), clients would look for units that would give them enough time to shut down their computers, but as businesses have become more dependent on computers, this approach is no longer viable.

Businesses need solutions that can keep them running and fully operational without interruption. The real decision facing consumers is not if they need power back-up, but rather how much of their business do they need to keep running, and for how long. As a rule of thumb, in the absence of other requirements, the run time should be two to four hours.

There are four options for back-up systems with extended runtimes: Inverter with large battery pack; Generator; UPS with extended battery packs; UPS and generator combination. Before the user decides on which option to choose, certain things need to be taken into consideration.

The UPS or inverter must be sized to match the load that it is going to run. The user must never run the UPS at 100% load. Always calculate the load by looking at the specifications on the equipment. These will normally give a voltage and current rating or voltage and Watt. For the sake of sizing the UPS, take each piece of equipment and multiply the voltage by the current. If there are two voltages, use the lowest figure. This will give the VA rating. If there is no current rating then assume that Watts = VA for the purpose of sizing the UPS.

Having established the load requirement, choose the battery pack. Unfortunately, these calculations require the discharge graph available for the batteries. Divide the total load by the nominal battery voltage and that will give the current on the battery, armed with that information look at the battery graph and confirm the run time that the battery will give. The best solutions are a UPS with extended battery packs or the UPS and generator combination.

 

Take note:

  • There are four options for back-up systems with extended runtimes.
  • The UPS or inverter must be sized to match the load that it is going to run
  • The best solutions are a UPS with extended battery packs or the UPS and generator combination.