This is true when a UPS is required--but what happens if a UPS isn't needed?
Say what? Who would ever want to deploy servers without UPS
There are certain circumstances where a UPS is not needed:
- Services with sufficient geo-redundancy that a power failure at any one site doesn't have appreciable impact on the overall service availability
- Lower-priority services for which an infrequent service outage would be acceptable
In situations like this, how does a -48Vdc system stack up? Let's look at the data in the report from The Green Grid mentioned above:
- The best AC power supplies to go from 240Vac down to 12Vdc peak out at around 93% efficiency [Figure 31].
- The best DC rectifiers (with batteries) to go from 240Vdc down to -48Vdc peak out around 96.5% efficiency [Figure 29].
- The best DC power supplies to go from -48Vdc down to 12Vdc peak out at almost 95% efficiency [Figure 31].
Taken together, the 96.5% rectifier efficiency x the 95% power supply efficiency equate to ~91.7% efficiency, slightly less than the 93% efficiency of a pure AC to 12Vdc power supply solution.
However, this is using rectifiers with tightly regulated -48Vdc outputs designed to work with batteries along with wide-ranging inputs. This is a mis-match! It's understandable why this has traditionally been done (for applications needing battery backup), but it's overkill for applications not needing battery backup.
Since most -48Vdc power supplies can handle input voltages from -42Vdc to -56Vdc (or a wider range), think what could happen with a DC rectifier with a loosely regulated output well within this range. If a DC rectifier was allowed to vary its output voltage between -44Vdc and -54Vdc, the net efficiency of the -48Vdc system could meet or beat the approach with a straight AC power supply.
Without battery backup, a -48Vdc system could match an AC system; even with full-time battery backup, the -48Vdc system is within ~1.5% of the AC system without battery backup.
Next: the story gets even better when redundancy is considered...