The traditional way to scale up a UPS system is to buy another system to handle whatever power capacity you need, tie it into the existing system and test it all out. The problem is, the system must be powered down before a new UPS can be brought online, which of course creates downtime – potentially significant downtime.
An alternative is to buy a UPS system that can scale “on the fly” in response to load growth without incurring downtime. While such a system may cost more up front, an analysis of the true cost of downtime incurred while expanding a legacy system shows the added cost may be well worth the money.
Consider a company that wants an N+1 UPS design for a 4,800 square foot (446 square meter) data center with an ultimate build-out capacity of 50 watts per square foot (538 watts per square meter). That translates to a total of 240 kW of UPS power in order to have sufficient redundancy to take one of the modules off-line without losing the ability to stay on conditioned power.
To keep up-front capital costs down, the IT manager plans to build the system in 80 kW increments as power requirements increase over time. To start, he employs two 80 kW UPS modules in parallel so that the initial 80 kW load can be supported by either module. As the data center grows, he will add another 80 kW UPS module, and then another still later when the data center reaches its full capacity of 240 kW. At that point, he’ll have four 80 kW UPS systems running in parallel, enough to power the entire data center even when one is out of commission.
Now the IT manager develops a similar plan but using a scalable, rack-based UPS system. On day one, to meet the 80 kW initial load, he’ll have to buy a 80 kW unit. But the scalable rack-based system has redundant 10 kW power modules within the unit, providing an N+1 configuration without the need to buy a second 80 kW unit. The same goes for when he has to add the additional two 80 kW increments, so a total of only three 80 kW units are required.
Still, the cost of the four legacy 80 kW units may well be less than three of the rack-based systems. But that’s not the end of the story.
Legacy UPS systems have a “critical bus” where the power supplied by any one UPS module is coupled with power from other modules. As modules are added to the system, the critical bus must be powered down to tie in the new module and the function of the system as a whole has to be tested through a commissioning exercise. To implement the expansion of the data center described above requires conducting this exercise twice in a production data center, which means two shutdowns. A typical shutdown to tie in and test a legacy UPS module is 24 hours, if there are no problems.
The cost of down time will vary by company but even at a modest $10,000 per hour, the cost for 48 hours of downtime for the two shutdowns is $480,000. Add in the cost of the various application specialists and technicians required for the job and the tab can easily grow to well over $500,000.
Deciding which system is really more cost-effective requires this kind of long-term cost analysis. Learn more about the factors that go into that analysis by reading the APC by Schneider Electric white paper, “Reducing the Hidden Costs Associated with Upgrades of Data Center Power Capacity.”
