BESS, UPS, generator, and STS: What is the difference?

"We already have a battery, so we have emergency power, right?" That question often comes up when manufacturing companies talk about electricity supply security, and it rests on an understandable but imprecise assumption. An industrial BESS, a UPS, a standby generator, and an STS solve four different problems, and the four terms are often mixed together because they all concern keeping something running when the power supply fails.

This article clears up the terminology. The aim is not to recommend one technology over another, but to explain what each category actually does, how quickly it responds, how long it can deliver power, and what it requires in terms of maintenance and design. Finally, we look at how the four categories typically work together in an overall architecture, and where ZynexGroup's standard model fits in.

BESS: large energy capacity, many possible purposes

An industrial BESS (battery energy storage system) is a battery container with significant energy capacity that can charge and discharge power to or from the electrical installation. The fundamental property that sets a BESS apart from the other three categories is that it can be optimised commercially in the market and, if it is deliberately designed for it, also contribute to backup.

Historically, battery containers have been used in two fundamentally different ways. One is as a dedicated, uninterrupted power source for critical loads, where the state of charge is typically held close to full capacity, and where the battery is normally not used for other purposes at the same time, a model that resembles a large UPS. The other, and today more common, approach, described in NREL's review of the role of battery containers in energy resilience, is to use the capacity flexibly for several purposes at once, such as price optimisation, peak reduction, and participation in system services markets. The two approaches place different demands on how the system is controlled and designed, and a system optimised for one purpose is not automatically optimised for the other.

This is where the most important point in the entire article sits: a battery can technically deliver power instantly, without the start-up delay of a combustion engine. But using that ability to actually supply selected critical loads when the collective electricity grid is unavailable requires a deliberately designed system, not only sufficient battery capacity. The system must be able to measure grid voltage correctly, carry out automatic switching, and safely separate the installation from the failed grid, a process often called islanding. Two consulting engineers who specify this type of installation describe precisely the increased system complexity this requires: measurement, automatic control of breakers, and in some configurations an additional source of voltage reference. A BESS is, in other words, not automatically a UPS. It can become a UPS-like resource for selected loads if it is designed for that.

UPS: fast, but typically short-duration protection

A UPS (uninterruptible power supply) is the category that responds fastest. It delivers near-instant backup power from internal batteries or, in some larger configurations, flywheels, and is typically used to protect equipment where even a fraction of a second without power can cause damage, data loss, or serious operational disruption: computers, servers, telecommunications equipment, control systems, and other sensitive electronics.

There are broadly three types of UPS. A standby UPS supplies the equipment directly from the grid and switches in the battery only when a fault is detected. A line-interactive UPS actively regulates voltage within a certain range and uses the battery only for larger deviations. An online UPS, also called double conversion, keeps the equipment permanently isolated from the raw grid supply by converting the power twice, which gives the highest protection against voltage problems, but costs more and uses more energy in operation. The latter type is typically used where power quality is unstable, or where the equipment is particularly sensitive, as in data centres and industrial control systems.

Common to all three types is that backup time is typically short, often from a few minutes to a few hours depending on battery size and load. A UPS is therefore rarely designed to supply entire production lines for longer periods. Its role is to cover the short, critical period until either power returns, the equipment can be shut down safely, or a slower but more enduring source such as a generator can take over.

Generator: long-duration backup, but slower start-up

A standby generator is a combustion engine coupled to an electrical generator, typically fuelled by diesel, natural gas, or bottled gas. During a power outage, an automatic transfer switch (ATS) detects the loss of grid voltage, commands the generator to start, and transfers the load once the generator has reached stable voltage and frequency. That start-up process uses the generator's own time to reach full power, which by nature is a slower path to full supply than a source that is already ready, such as a UPS. That is precisely why generators in critical installations are often combined with a UPS or a battery that covers the period while the generator builds up to full voltage.

The generator's strength is the opposite of the UPS's: it can deliver long-duration backup, in principle as long as there is fuel, whereas a UPS is limited by its battery size. Engine-generator units are therefore typically used to supply critical facilities such as hospitals, data centres, and pumping stations in situations where an outage can last hours or more.

That long-duration capacity comes at a cost in the form of requirements for maintenance, testing, and fuel management. A standby generator requires a systematic maintenance programme, including oil and filter changes, battery checks, cooling system checks, and regular load tests, to be ready for operation when it is actually needed. The importance of that maintenance is documented: a US study referenced in NREL's material on battery containers for resilience indicates that poorly maintained diesel generators have only about a 50 percent probability of lasting 48 hours continuously. Fuel also degrades over time if it is not continuously checked and replaced, which can weaken reliability further. On top of that come emissions considerations: generators are typically subject to environmental and emissions regulation, which in practice can limit how much a given generator may run for anything other than emergency power and testing, and which places requirements on permits and setup.

STS: the electronic switch that makes the transition imperceptible

A Static Transfer Switch (STS) is not an energy source. It is an electronic switch with two inputs and one output that monitors two independent power sources and transfers a critical load from one to the other if the active source falls outside acceptable limits for voltage or frequency. The international standard for STS performance and testing, IEC 62310-3, defines the requirements for how such a controlled transition between two AC sources must be specified and tested.

STS is relevant when the transition between two power sources must happen without a noticeable interruption for critical equipment, such as PLC controls, servers, or automated production lines that can stop or misinterpret even a short voltage change as a real outage. A research review of STS technology in industrial and critical installations describes how this type of equipment has been developed specifically to protect sensitive loads against voltage dips, voltage swells, and short interruptions that an ordinary mechanical switch cannot handle quickly enough.

ATS vs. STS: mechanical versus electronic switching

The difference between an Automatic Transfer Switch (ATS) and an STS is fundamentally about how the switching itself is performed, and that is where the technical answer to "ATS or STS" sits. An ATS uses electromechanical components, relays, contactors, or motor-operated breakers, to physically move the load between two sources, typically between grid and generator. An STS instead uses semiconductor components with no moving parts for the same task.

That difference in the mechanism itself has a direct consequence. Because an ATS switch requires physical movement of components, there is typically a short, noticeable break in the power supply during the transfer itself. An STS electronic switch has no corresponding physical movement and can therefore complete the transition markedly faster, often so fast that sensitive equipment does not register the event at all. That makes STS relevant where even a short interruption can trigger faults or stops, while an ATS is typically sufficient and more robust where a short break is acceptable, for example general lighting, ventilation, or ordinary building services tied to a generator. The two technologies therefore do not compete directly for the same task. They are sized for different requirements on transition time, and the choice between them depends on what interruption the connected load can actually tolerate.

Typical architecture: layers that complement each other

In professional contexts, robust electrical architectures are often designed as several layers of technologies that complement each other rather than replace each other, a layered logic that recurs in professional reviews of how that type of architecture is typically built for critical installations. The grid is the primary source. A battery system can provide longer-duration support and commercial flexibility. An optional STS switches selected critical circuits between sources without a noticeable interruption. An optional generator provides long-duration, fuel-based backup during longer outages. And UPS units are placed closest to the most sensitive equipment to cover the shortest and most critical fraction of a second.

Each layer has its own role, and none replaces another: immediate, short-duration protection from the UPS; longer-duration support and market flexibility from the battery system; long-duration, fuel-based backup from the generator; and the speed of the transition itself determined by the switch, whether it is an ATS or an STS. Which of these layers make sense for a given company depends on which loads need protection, how long an interruption they can tolerate, and how long backup must be able to be maintained, not on which technology sounds most comprehensive.

What ZynexGroup offers in the standard model and as options

In ZynexGroup's standard model, an industrial BESS is primarily a tool for commercial optimisation: market flexibility and, where it makes sense, peak reduction, as part of a fixed contractual availability payment. The standard model does not automatically include emergency power, backup, or UPS function. That follows directly from the mechanical and design difference this article has walked through: delivering power to selected loads during an outage requires deliberate design, not only sufficient battery capacity.

STS, emergency power, backup, and reserved capacity are therefore options in our model, never something that comes automatically. An option always requires a concrete analysis of the customer's electrical installation, critical loads, switchboard structure, and risk level, and a separate technical design that decides which loads should be prioritised, how quickly the transition must happen, and how long supply must be able to be maintained. We do not recommend a generator or UPS as part of the architecture we help specify. They are general categories in the landscape of backup technology that this article describes to provide a shared language, not elements we supply. When an option such as STS or reserved capacity makes sense after a concrete analysis, we coordinate the process and work with partners on the installation itself on site.

Overall perspective

BESS, UPS, generator, and STS solve four different problems, and the difference is fundamentally about two things: how quickly the technology responds, and how long it can keep the load running. A UPS is fastest, but typically short-duration. A generator is long-duration, but slower to start and requires ongoing maintenance. An STS is not a source, but an electronic switch that makes the transition between two sources imperceptible for sensitive equipment. And a BESS has large energy capacity that can be used commercially, for peak reduction, or, if it is deliberately designed for it, as part of a backup solution, but it is not automatically a UPS. Mixing the four categories together, as in the assumption that "battery means emergency power," typically leads to either disappointment or oversizing. Keeping them separate is the first step if a company genuinely wants to assess what makes sense for its particular critical loads.

Have the potential assessed for your facility

If you are considering whether an industrial BESS, alone or combined with an option such as STS or reserved capacity, makes sense for your production, the starting point is always your own data: grid connection, consumption profile, and a description of which loads are most critical for operations. We use that type of data for an initial assessment of whether there is a match between your situation and our standard model, and whether there is a concrete reason to look more closely at options. You are welcome to contact us here.