| UPS Systems Sizing and Efficient Uninterruptible | | | | Real and Apparent Power, and Load Power Factor |
| Power Supplies | | | | Most loads are actually sized in Watts (or Kilowatts |
| Sizing uninterruptible power supplies and UPS systems | | | | – kW). This is known as the ‘real power’ |
| is more important than ever today as an oversized | | | | of a load. Rear panel rating plates on equipment |
| UPS system will lead to increased running costs and | | | | typically lists their maximum Watt rating and these |
| and under-sizing can leave little enough load capacity | | | | can be summed to provide an overall power |
| for future expansion. Correctly sizing a UPS system is | | | | calculation. |
| also important for efficiency. Normally the lower the | | | | The next approach in UPS sizing is to look at the |
| load on a UPS, the less efficient it will run and the | | | | Amps being drawn by a system and the mains |
| higher the running costs. | | | | power supply voltage (Vac) it is being powered from. |
| Sizing UPS systems is not a complex topic but it is | | | | Combining the two using the formula VxA=VA |
| one of increasing importance given the higher power | | | | provides the ‘apparent power’ measure or |
| Blade servers now found within datacenters and the | | | | VA (kVA = 1000VA). |
| rapid growth that IT systems can experience within | | | | As well as rear panel rating plates, manuals, |
| a short time frame. | | | | datasheets and online portals can also provide useful |
| UPS Systems Sizing | | | | specification data sources for sizing information. Some |
| The optimum loading for any electronic system is | | | | UPS manufacturers also maintain equipment |
| 80% of overall capacity. Loading a UPS system to 90 | | | | databases that can be accessed by companies such |
| or 100% capacity is more than acceptable as well but | | | | as Critical Power Supplies to help assess a client’s |
| can introduce two weaknesses. The first is that it | | | | demand. |
| leaves little room for expansion but more importantly, | | | | Real and apparent power are linked by a term known |
| an on-line UPS may be forced into bypass by small | | | | as Power Factor (pf). If only the Watt rating is |
| surges in load demand. | | | | provided it is common practice to total this and then |
| This is a normal UPS safety feature (and one that | | | | divide the final sum by a power factor of 0.8 to give |
| protects the load) but it can expose the load | | | | an overall kVA value. However, load power factors |
| momentarily, until the demand decreases, to the | | | | can vary from anywhere between 0.5 and 1.0 (unity) |
| vagaries of the mains power supply itself. Most | | | | and care must be taken with this approach. In |
| on-line UPS are capable of working for predetermined | | | | addition power factors can be ‘leading’ or |
| periods on overload and these are usually specified in | | | | ‘lagging’ and sizing for three-phase loads and |
| terms of 110%, 125%, 150% and 200% of capacity. | | | | generators adds further levels of complication to the |
| However, the higher the overload, the less time the | | | | calculation. |
| UPS will continue to power the load from its inverter | | | | Two final considerations include runtime and |
| supply. Line interactive or standby UPS do not | | | | redundancy. The runtime or battery backup required |
| provide the same level of inherent safety as an | | | | will influence overall system design, configuration, |
| on-line UPS and without a built-in automatic bypass | | | | logistis and floor space requirements. For runtimes |
| facility, will generally shutdown after a short period on | | | | lasting several hours, it is a commonly adopted |
| overload. | | | | approach to use additional battery packs. For large |
| UPS Sizing Considerations | | | | three phase UPS this approach can be limited by |
| When sizing a UPS system it is important to start | | | | space availability and budget. In such instances a |
| with a classification of the potential loads to be | | | | standby generator can be more favorable. |
| protected: | | | | Business continuity is normally the primary driver for |
| - Critical Loads: are the IT, telecoms and and electrical | | | | installing a UPS solution. Resilience is a key factor that |
| systems that are critical to business continuity. They | | | | is driven by the levels of redundancy required. |
| can include Blade file servers, telecoms systems, PCs, | | | | Normally this is referred to as parallel redundancy or |
| storage devices, security and building management | | | | N+X where X equates to the level required. For |
| systems. These systems may be so vital that it is | | | | example, a 100kVA in an N+1 redundant configuration |
| important to power them through even long duration | | | | would require two 100kVA UPS systems to be |
| mains power failures, which will affect the size of the | | | | installed, where one module alone could power the |
| UPS battery chosen – or even overall power | | | | load if the other is removed out of service for |
| solution in terms of combining a UPS and generator | | | | maintenance or develops a fault. |
| solution. | | | | Future expansion is off course a final factor to |
| - Essential Loads: are vital to the organisation but not | | | | consider. It is normal practice to add a 20% |
| critical. These could include some lighting, heating and | | | | expansion factor (multiply by 1.2). In addition, if the |
| ventilation systems. Some of these systems may | | | | future electrical installation is allowed for, additional |
| well have their own emergency standby power | | | | UPS modules could be installed to meet expansion at |
| facilities built-into them. | | | | a later date. |
| - Non-essential Loads: are the systems that are | | | | UPS sizing is a relatively straight forward process. |
| non-critical. A measure of their appropriateness for | | | | The larger the load and system, the more complex |
| UPS protection is to consider their impact on | | | | the calculations and considerations, especially in terms |
| day-to-day operations when removed (for | | | | of resilience and capacity. For assistance in sizing your |
| maintenance). | | | | loads, designing a resilient system and selecting a UPS |
| From the list of critical and essential loads calculations | | | | system, consider a Critical Power Supplies Site |
| can then made of the overall load size. | | | | Survey by one of our Power Protection consultants. |