For electrical contractors and other IT service and equipment providers, helping customers decide which uninterruptible power supply (UPS) is best for their needs requires asking lots of questions to determine exactly what those needs are, with respect to the loads the customer needs to protect and how much run time is required.
The first issue is to determine whether the customer needs a single- or 3-phase UPS. That requires taking a look at the loads the UPS will protect, to assess their voltage range, or kVa value. Generally, loads of 20kVA or less can safely use a single-phase UPS. Larger loads will likely need a 3-phase UPS.
If it’s determined that a 3-phase UPS is required, the next question is whether to use a 3/1 configuration or 3/3. A 3/1 UPS takes in 3-phase power but puts out single phase while a 3/3 takes in 3-phase power and likewise delivers 3-phase to the downstream loads.
Here again, the answer will largely be dictated by the downstream loads the UPS is protecting. IT equipment such as servers typically use single-phase power while medical equipment such as MRI machines or large factory floor machines may use 3-phase power. Although, some customers prefer the 3/1 configuration because it keeps things simpler by obviating the need for load balancing, which is required with the 3/3 configuration.
While how much energy your downstream loads draw is one step in determining which UPS to use, the next is to determine how much run time you need. A UPS can be sized to provide just one minute of backup power, or five or far more.
The answer depends on whether there’s a generator on site that will take over should utility power go out. If so, you really only need a minute or two of UPS power to keep the loads going until the generator kicks in.
If the UPS will be shouldering the load by itself, then you have to determine how much time the customer needs to gracefully shut down the equipment the UPS is protecting. Depending on how much equipment there is, that likewise can vary greatly. Although, as outlined in this previous post, some UPSs can be programmed to shut down attached loads on their own, greatly speeding the process.
Going through this Q&A exercise will help you determine how much run time the customer really needs and, given the rated capacity of the loads it is protecting, how large the UPS needs to be.
Although, in practice, you wouldn’t want to get anywhere close to maximum UPS capacity. A UPS should be sized to run at no more than 60% to 70% of its rated capacity, and 40% to 50% is more common – both for safety and to leave room for future expansion.
The amount of run time required may play into which type of UPS battery is best for the customer. The options in that regard are expanding as Lithium-ion batteries are now becoming available for UPSs, offering the same energy storage capacity in a smaller size and weight as compared to lead-acid batteries. Learn more about what Li-ion batteries mean for UPSs.