Introduction
The traditional approach to the specification and selection of UPS systems has focused almost solely on system reliability, as represented by the mean time between failure (MTBF) provided by manufacturers and consulting engineers. Two issues are now conspiring to move efficiency, as much as reliability, to the forefront in UPS evaluation: (1) a focus on total cost of ownership (TCO) over the lifetime of the system, and (2) public and private environmental initiatives, as exemplified by “green building” certification programs and demand-side management programs offered by utility companies.
There are two major contributors to UPS inefficiency: the inherent losses of the UPS modules themselves, and how the system is implemented (i.e. right-sizing, redundancy). Oftentimes, when specifying UPS systems, the only efficiency value considered is the best case value published by manufacturers. This is misleading and will be explained further.
A hypothetical example is perhaps the best way to demonstrate how this practice can have a material effect on a company’s electrical expense. Consider two 1 MW UPS systems from two different manufacturers. UPS system 1 and UPS system 2 have identical published efficiencies (93% at full load), are operated in a 2N architecture, use an electrical cost of $0.10 / kW hr, and support a 300 kW load. Many would argue that there would be no difference in the annual electrical cost of operating these two systems. This is a flawed statement except for emergency or maintenance scenarios, UPSs are never operated at a 100% load level in a 2N configuration since each side of the “N” has to be capable of supporting the full load if one side fails. Therefore, the maximum design load on each UPS in normal operation cannot exceed 50%. In reality 2N systems rarely achieve even 50% load on each system. Some field surveys indicate that 2N data centers operate at 20-40% of their 2N capacity.1 For this example, a typical 30% load is assumed, where each UPS supports 150 kW. Each UPS in system 1 incurs an annual electrical cost of $10,470 in power losses vs. $28,322 for each UPS in system 2. Since there are two UPSs in each system, the electrical losses are doubled to $20,940 and $56,644 per year, respectively. These UPS losses manifest themselves as heat which must be removed by the cooling system. Assuming each kW of heat requires 400 watts for the cooling system to remove it, an additional $8,376 vs. $22,651 per year is required2. In this example, a typical data center lifespan of 10 years, results in a total cost of UPS system losses of $293,165 vs. $793,021 as shown in Table 1. So, how is it that the electrical losses between two seemingly identical UPS systems can differ by almost a factor of three?
The answer lies in the efficiency curves of both UPS systems and how they are sized against the load. An improvement of 5 percentage points in the efficiency of a single UPS can result in an electrical cost reduction between 18% and 84% depending on how much load is on the UPS. This is illustrated later using two UPS designs currently on the market.
In order to meet today’s efficiency and environmental demands, UPS manufacturers can utilize three factors to improve the efficiency of large UPS: technology, topology, and modularity. Together these factors can reduce the electrical UPS losses in the form of heat energy (kW). This paper explains the efficiency curve and will discuss common errors made in evaluating UPS efficiency. It will show how technology, topology, and modularity allow manufacturers to improve UPS efficiency. For a discussion on full data center efficiency see APC White Paper #113, “Electrical Efficiency Modeling for Data Centers”.
UPS Efficiency Curve
If there is only one UPS efficiency number listed on a UPS data sheet, it is almost certainly quoted at 100% load (rated load) and at various other favorable system states such as fully charged batteries, nominal UPS input voltage, and optional input transformers and filters disconnected or not installed. The fact is that most UPS manufacturers quote UPS efficiency at 100% load because it represents the very best efficiency the UPS will attain. Unfortunately, very few customers will ever reap the benefits of this efficiency because they will never reach 100% load.
