UPS Operating Modes: Simple Question, Complex Answer

We recently had a short-term neighborhood power failure due to a storm and a downed tree. As it lasted only about eight hours, it was a minor inconvenience, but major reminder of how much we depend on electrical power at home as convenience and necessity. Not surprisingly, the informal neighborhood discussion turned to the desirability of a home uninterruptible power system, which ranges from about $5,000 to $15,000 installed, depending on size and installation issues. These home systems are usually powered by a generator fueled by piped natural gas or an on-site propane tank, (Figure 1); some have a battery bank as well or even exclusively. (The generator-powered system is often referred to generically as a Generac system, as that vendor is the market leader in the U.S. with about 60% of the market, while Kohler is second.)

Figure 1: In a typical generator-driven home UPS, the house’s power source transition from grid to generator occurs about ten seconds after grid power and the generator is started. (Image source: This Old House)

One of the neighbors remarked that he doesn’t understand why these consumer generator-powered UPSs have a “break time” of about 10 seconds until power is restored. Commercial systems with battery backup have either no switchover time or one on the order of 10 milliseconds, depending on topology. After all, the uninterruptible power system at the hospital, for example, usually is one which assures no power interruption. His view was “what’s the big deal with making it so there is no interruption?”

While it’s easy to ask that question, the answer is not obvious. The technical reality is that a system designed to ensure no power interruption at all is a much more complicated, costly system with many more tradeoffs than one which is designed to allow for a brief interruption. There are two broad classes of uninterruptible power systems, with several variations within each:

• Offline: also called a standby architecture, this is the most-common home/consumer approach. The load (house) normally runs directly from the utility AC line and grid, Figure 2. When the system controller senses a loss of power — as different from a brief sag or brownout — it starts the generator (not needed for battery-supported system).

In the off-line UPS approach, the load normally operates from the grid-provided AC line while the backup source (generator or batteries) is on standby if needed. (Image source: EtechnoG)

That generator’s AC output is rectified and converted to line-equivalent AC via an inverter. The battery, if present, obviously does not need this rectification stage. Key to this operation are the “static switches” that provide a power-transfer function. When power fails, one static switch disconnects the AC line and the other connects the backup generator (or battery ) to the load. For battery-back up systems, there’s also provision for charging the batteries from the AC line when power is on. Most systems use an automatic sense/control for generator start-up and switchover, although manual system are available for about $500-$1000 less. But what to do you if you are not home when the power fails?

• Online: To offer continuous operation totally without any power gaps, the online approach — sometimes referred to as “double conversion online” is used, Figure 3. AC grid power continuously charges the batteries, and even under normal conditions, the load operates from the batteries via the inverter. This sounds very simple and logical, but it brings some major technical challenges including inverter design, switchover, and battery sizing and management issues to maximize run time and battery life.

For the online UPS, the load operates from the UPS as source, which is continuously recharged via the grid; the generator is activated while the system is still running on batteries. (Image source: EtechnoG)

Some commercial systems use batteries only and do not have the generator, which trades one set of limits (battery runtime) for another (generator issues. Some home systems such as the Tesla Powerwall or the Generac PWRCell also use batteries exclusively, and they are recharged via the grid or solar panels, but there is significant added cost and complexity. Some locales have zoning regulations regarding having installed charged batteries above a certain capacity in residential areas (electric vehicles are a carve-out exception, of course).

In some ways, the best system combines both battery and generator: the battery carries the load through short disruptions with little or no interruption while the generator is used for longer outages. As always, it’s a complex set of tradeoffs among cost, complexity, and capability.

Of course, this is a highly simplified overview of the reality of uninterruptible power system architectures, sub- architectures, and variations (some widely used, some proprietary). Nonetheless, it does indicate that the answer to the seemingly simple question of why most home UPSs have a switchover interruption is neither simple not obvious.

What’s your view on generator versus battery/generator versus battery-only uninterruptible power systems? What about online versus offline UPSs? Going beyond the UPS-only questions, have you encountered other simple-sounding questions from well-intentioned friends but that have complicated, deep-dive answers?

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