UPS Learning Center
This page is to help users and buyers of UPS make more knowledgeable decisions in selecting and using UPS.
Topics
- I. Power Factor Explained
- II. Major Differences in Types of Single Phase UPS
- Power Factor is often misunderstood as being related to UPS efficiency. It is not. A UPS is rated in both VA and Watts (or kVA and kilowatts). Power Factor is the relation between watts and kva. Divide watts by VA and result is power factor. It never can go above 1.00. Ideally the UPS output power factor will closely match the IT equipment input power factor.
- Modern IT equipment has relatively high power factor, typically .90 to near 1.00. If a 450 watt server has a 0.9 power factor it will draw 500 VA (450/500 = 0.9). For an IT room with 100 of these servers the load would be 50,000 VA / 45,000 Watts. In kVA and kW this is 50 kVA/ 40 kW. The UPS powering this needs to meet both numbers or the UPS will be overloaded. A 50 kVA UPS with 0.90 output power factor will be perfect, 50 kVA times 0.9 = 45 kW. A 50 kVA UPS with a 0.85 output power factor will be too small as it can only power 50 kva time 0.85 = 42.5 kW of load. Similarly a UPS rated 45 kW with a 1.0 power factor will be too small as it can only power 45 kW / 1.0 = 45 kVA of load. (division used to go from kW to kVA).
- What to do? Generally a 0.9 to 0.95 range power factor is best for powering IT equipment. A user really won't know the exact power factor in advance, nor how it changes over time. In the 1980's and 1990's IT equipment had low power factors, often under 0.60 but modern equipment has higher numbers.
- For those interested to know why
UPS manufacturers don't all have higher output power factors, two things apply.
- First, to meet kVA, the wire and breakers have to be properly sized by UL and code so they are always met. For higher power factor output ratings batteries and heat sinks need to be heftier, i.e. more costly. Designing a low power factor UPS is cheaper.
- Second, small UPS for work stations and home use tend to have low output power factors. However, the smallest units tend to be near 500 VA or higher. Workstations and home computers typically draw only about half load on these UPS and this are fine. The application demands low cost UPS, the low cost low power factor satisfies the need so all parties are satisfied for the environment.
II. The major differences in types of single phase UPS include:
- Line Interactive vs On-Line Technology - Line interactive passes on utility power to the output of the UPS either directly or through a voltage regulating component. On-line uses utility power to create DC power inside the UPS and this DC power is utilized in an inverter section to create a new clean output voltage waveform. Line Interactive is generally lower cost. On-line is higher cost. When utility power is lost, a line interactive UPS has a short break in power to the load before creating power from the battery through an inverter. On-line UPS do not have a power break when utility is lost as it is already using a DC power source and its inverter is always on. Loads are better protected from power problems with an on-line UPS. Line interactive UPS must transfer to battery when incoming frequency varies from 60 HZ. On-line UPS do not need to transfer when incoming frequency varies. Frequency may be an issue to consider if generators are to power the UPS, particularly smaller generators.
- Isolated vs. Non-isolated UPS. An isolated UPS includes a transformer which isolates outgoing power from incoming power. It also re-establishes the neutral and ground wires to a common point. This is a good thing. It is especially helpful in large environments where the load is far from the last transformer upstream. For instance, in a commercial building or hospital the last transformer may be hundreds of feet away and that causes a difference between neutral and ground. An isolated UPS corrects this problem.