The uninterruptible power supply spent fifty years as one of the least discussed assets on site: power electronics backed by a room of lead-acid batteries whose most visible job was covering the seconds between a grid failure and the generators taking load. Three changes have made it one of the most interesting parts of the building. Lithium-ion has become the default for new large installations, the grid now pays storage to help hold frequency and AI loads have broken the assumptions the whole architecture was built on. Assessment practice has not caught up with any of the three.
Why this matters
The battery room communities never see is becoming the place where a data centre can either strengthen the grid or quietly add risk. Chemistry, fire safety, recycling obligations and grid support are all decided years before switch-on. Assessment practice barely looks at any of it.
Every conventional data centre carries two layers of backup: generators for the long haul and a UPS for the gap, the seconds of ride-through between a grid disturbance and stable standby power, plus continuous conditioning of whatever the grid delivers. Generators got the public attention because they are visible and audible at the fence line. The UPS hid indoors, on valve-regulated lead-acid batteries sized for minutes, replaced every five or so years, chosen because the technology was proven, cheap and understood. That era is ending faster than most consent documents acknowledge. The UPS is becoming one of the defining elements of a facility’s backup philosophy rather than simply its bridging power.
The quiet takeover
Lithium-ion has moved rapidly from alternative chemistry toward the default choice for new large data centre UPS installations, particularly in hyperscale and colocation facilities. The drivers are prosaic: around 60 per cent less floor space for the same capacity, service life stretched from five years toward twelve, which removes one or two full battery replacements from the facility’s life. The switch is not automatic virtue. Lead-acid is one of the most recycled products on earth, with recovery rates near total; lithium-ion’s end-of-life pathways are still maturing and its supply chain carries its own footprint. A facility swapping chemistries trades a solved recycling story for a better operating one, which is exactly the kind of trade an assessment should see stated rather than assumed.
From cost centre to grid asset
The structural change is what the battery does while nothing is wrong. A lead-acid string waits; a lithium-ion UPS can work. Microsoft’s Dublin campus demonstrated the model, with UPS batteries certified to provide frequency response to the Irish grid; the direction has since hardened into policy: Ireland’s regulator decided in December 2025 that new data centre connections must bring generation or storage matching the maximum import capacity they request, alongside additional renewable supply for most of their annual demand. Google has argued the same logic for its battery-backed sites, that storage which strengthens the grid beats a generator that only ever waits. For Australia the implication is significant. Every megawatt of suitably designed storage inside a facility represents potential frequency response, demand flexibility or other grid services, subject to chemistry, reserve requirements, the connection agreement and market registration. That last qualification matters: a gigawatt of installed UPS is not a gigawatt available to the market. If large new loads are increasingly expected to demonstrate how they interact with a renewables-heavy grid, storage embedded inside the facility should not automatically be treated as a private resilience asset. Its potential contribution to system stability belongs in the assessment conversation.
AI broke the duty specification
The third force is the load itself. AI accelerator clusters can produce rapid power swings in milliseconds, at magnitudes conventional AC UPS architectures were never designed to accommodate; training clusters can move tens of megawatts between synchronised compute phases faster than any generator or conventional plant can follow. The industry’s answers are arriving in layers. Power-dense chemistries such as nickel-zinc are being packaged specifically for the swing duty, supercapacitors are appearing at rack level to absorb sub-second transients before they propagate upstream and the deeper fix is architectural: the shift toward 800 volt DC distribution, which strips out layers of AC conversion. That may not eliminate the UPS function so much as break it apart. Power conversion, ride-through and transient buffering that once lived together in a central AC machine can migrate to different layers of the facility, from rack-level supercapacitors handling millisecond excursions to DC-connected storage nearer the utility interface handling seconds to minutes. Storage remains fundamental; where it sits and how it meets the power train is what changes. We flagged that signal in our analysis of 800 VDC earlier this year; the practical point for assessors is that the AI facility arriving in an Australian application may carry a fundamentally different backup architecture from the one the template conditions assume.
The technologies compared
| Technology | Energy density and duty | Emissions and end of life | Service life | Standing |
|---|---|---|---|---|
| VRLA lead-acid | Low density; minutes of autonomy in dedicated rooms | Mature chemistry; near-total recycling, the best end-of-life story in industry | ~5 years, temperature-sensitive | The fading incumbent |
| Lithium-ion (LFP) | ~60% less footprint for equal capacity; minutes to hours | Thermal-runaway risk requires engineered detection, separation and suppression; recycling infrastructure developing rapidly | 10-12 years | Increasingly the default for new large installations |
| Nickel-zinc | Power-dense rather than energy-dense; built for high-rate swings | Aqueous, non-flammable electrolyte; avoids lithium-ion thermal-runaway characteristics | Long cycle life in bridging duty | Positioned for the high-rate swings of AI training loads |
| Flywheels | Seconds of ride-through, no chemistry at all | No chemicals, no thermal runaway; bearings and vacuum systems to maintain | 20+ years | Niche but persistent, often paired with DRUPS |
| DRUPS (diesel rotary) | An alternative UPS architecture rather than a storage chemistry: ride-through and generation in one rotating machine, no batteries | Diesel emissions profile when running; no battery room at all | 25+ years | Established in Europe and on large sites; couples the facility to diesel |
| Supercapacitors | Sub-second buffering; cycle life in the hundreds of thousands, so ageing is driven by time and temperature rather than use | No thermal-runaway pathway | 15+ years | Complements storage rather than replacing it; appearing in rack-level designs for the fastest transients |
The table’s honest reading mirrors the generator one: nothing wins on every axis. Lead-acid still owns end-of-life, lithium-ion owns the economics, nickel-zinc and supercapacitors own the new AI duty and DRUPS quietly asks whether the battery room should exist at all.
Four architectural tendencies
These are tendencies rather than rules. Topologies differ between operators, between generations of design and between sites.
Colocation tends to retain conventional centralised architectures, because tenant service level agreements specify autonomy and redundancy, which makes chemistry substitution far easier than topology change. Lead-acid rooms give way to lithium-ion on refresh cycles. Hyperscale cloud has greater freedom to distribute backup and redundancy through the IT and electrical architecture together, including rack-level battery backup in some designs, with fleet software carrying part of the redundancy. That freedom is what makes grid-interactive duty practical to contemplate. Edge can make battery-only resilience viable where workloads fail over elsewhere or shut down gracefully, although generator-backed designs remain common where local continuity matters. AI campuses are increasingly forced to separate very fast transient buffering from longer ride-through, which pushes storage to several different layers of the electrical architecture at once.
At the far end of the spectrum sits the same counterexample as the generator question. Cryptocurrency mining runs its mining load without a UPS at all. The flexibility runs both ways. It can shed in seconds when the grid is short, which is why these loads participate in the frequency control markets. It can also soak up generation nobody else wants when prices go negative, acting as a buyer of last resort for a renewable generator facing curtailment. A conventional facility buys storage so the compute is protected from the grid; mining leaves the compute unprotected so it can do both of those jobs for the grid instead.
What an assessor should ask
An assessor should be able to get clear answers to five questions.
- Which chemistry and architecture? Stated explicitly, because fire design, ventilation and emergency response differ fundamentally between a lead-acid room, a lithium-ion installation certified under UL 9540A test regimes and a flywheel hall. Local fire services need to know which they are walking into.
- What is the end-of-life commitment? Battery fleets at data centre scale are a recycling obligation that belongs in the application, not discovered at decommissioning.
- Can the installation support the grid? Storage that can deliver frequency response is a public benefit an assessment should recognise and, where appropriate, encourage.
- Does the autonomy claim match the workloads it protects? An edge site claiming battery-only backup and an AI campus claiming conventional UPS coverage are making very different engineering assertions. At least one of them deserves scrutiny.
- What does the storage do when the grid is healthy? The traditional question was what happens when the grid fails. The more useful one is what this asset does for the other 99.99 per cent of its life.
The bottom line
The UPS is where the industry’s three transitions meet, in what used to be one room: the chemistry transition, the grid-services transition and the AI architecture transition. It is also the layer communities never see and assessments rarely question, which made sense when the batteries simply waited and makes none now. The generator question, as we argued in our companion piece, is what actually needs backup, for how long and why. The UPS question is sharper: whether the storage between the grid and the compute is only ever a private resilience asset, or a working public asset as well. Australian assessment practice should start asking these questions now, because the answer is designed into a facility years before anyone flips the first switch.
A topical commentary from the Digital Infrastructure Institute. Companion to our analysis of diesel backup and its successors. For assessment practice, see our State & Territory Frameworks.
Sources: Microsoft — Dublin datacenter batteries supporting the Irish grid; The Register — Microsoft’s Dublin datacenter to feed the grid; Arthur Cox — Ireland’s new connection policy for data centres; DCD — Microsoft to roll out battery-sharing worldwide; Global Market Insights — data centre battery market; UL 9540A test standard