Working with our APC SmartUPS SC 620, this PSU was stable at loads up to 375 watts when powered by the mains but couldn’t switch to the batteries even at 280 watts.
Cross-Load Voltage Stability
The +12V and +5V voltages are within 3% of the required levels. These power rails are especially stable at high loads.
The +3V voltage is also closer to the required level at high loads, but its maximum deflection is 4%.
As opposed to most PSUs in this review, the OCZ-ZS550W isn’t very stable at very low loads but its overall performance in this test is good.
Output Voltage Ripple
The high-frequency voltage ripple isn’t strong, yet there are voltage spikes above the permissible limits.
The same goes for the low-frequency ripple.
Temperature and Noise
The PSU is cooled by a 135mm Globe Fan RL4Z B1352512M fan. It won’t be easy to find a replacement if the default fan fails. On the other hand, fans with ball bearings usually have a very long service life. The fan has 11 blades and a rated speed of 1200 RPM. It is partially covered with a piece of plastic to optimize air flows.
Together with the Chieftec SPS-650C, the OCZ-ZS550W is the best in this review in terms of acoustic comfort. It is even quieter at low loads, its fan rotating at 650 RPM or less. This speed is maintained until a load of 230 watts.
Then the fan accelerates smoothly, reaching 1000 RPM at 430 watts and 1125 RPM at full load.
The PSU is silent at low and medium loads and quite comfortable at high loads.
Efficiency and Power Factor
At the reference loads of 20%, 50% and 100% this PSU was 83.9%, 86.1% and 83% efficient. The peak efficiency of 86.3% was spotted at 240 watts. Thus, the OCZ-ZS550W confirms its Bronze status.
The power factor couldn’t reach 98% at high loads, unlike in most other PSUs with active power factor correction.
We can see no problems about the standby source.
The OCZ-ZS550W has good electrical parameters and comes at an attractive price. It is virtually silent at medium loads. Its long cables make it suitable for large system cases.