Let's start by looking at the power supply unit (PSU) component by itself. Based on the information in the quantitative analysis by The Green Grid, high-efficiency AC and DC power supplies look like this when compared to each other:
The graph shifts to the right when redundant power supplies are considered. Since there are numerous different voltage converters in a server (modern servers often have in excess of 25 voltage rails used internally), it's really impractical to try to duplicate every voltage converter in a server--at least if you want it for a reasonable price. However, servers with redundant power supplies provide three principal benefits:
- Connectivity to separate primary power sources (i.e., different utility feeds)
- Protection against failure in upstream power equipment (i.e., failure in a PDU)
- Cabling problem or service failure (i.e., accidentally unplugging the wrong server)
In contrast, a DC system has no phasing issues to deal with. Therefore, DC-based equipment has two main options: full duplicate power supplies (like AC) or using a technique called diode OR'ing (or FET OR'ing) to safely combine power from two separate DC sources as inputs to a single power supply. [Since there are numerous downstream power converters that are not redundant, there's no need for the power supply itself to be redundant--it just needs to be fed from multiple inputs.] Many DC power supplies do this today, as this approach is commonly used in the highly-reliable telecommunications system with -48Vdc systems. The result is a wider gap between the net AC power supply efficiency and the DC power supply efficiency:
Taking this a step further, look at the typical operating point for servers vs. their power supply ratings. For example, look at the various published reports for SPECpower_ssj2008: you'll notice there are numerous cases where the power supply shipped with the system is 2-4 times the maximum power draw in the sytem. If the power supply in a system is 2x the necessary power, then the system would normally operate in the left half of the graph immediately above. If the average power is considerably less than the maximum power draw, then the system could spend the bulk of its time operating at the 25% load level or less in the graph above.
At these lower loads, the efficiency benefits of -48Vdc systems become more apparent, even when there's no UPS in the picture. If an installation uses UPSes, the efficiency gap widens further in favor of -48Vdc.