The UPS is the converter. The energy store is what actually carries the load through an outage, and it decides floor loading, room temperature limits, fire code obligations, refresh intervals and end-of-life cost. It is also, routinely, the last thing specified. Every model in the UPS systems catalog pairs with one of three storage approaches, and the distance between those three is wider than the distance between the converters they attach to.
This guide is one of four spokes beneath the data center UPS and energy storage hub, which sets out how topology, storage, sizing and frame architecture fit together. Read it alongside UPS topologies explained and how to size a UPS.
Key Takeaways
- Runtime depends on usable energy at end of life, at the allowed depth of discharge, at the lowest room temperature the design tolerates — four qualifiers vendors rarely state together.
- VRLA is the cheapest line on the purchase order and the most expensive over a holding period, because the string is a consumable that gets replaced inside the life of the UPS.
- Lithium-ion trades purchase premium for density, ambient tolerance and cycle life, and brings NFPA 855 into the permitting conversation by name.
- Flywheels remove the battery room entirely: the VYCON VDC delivers 300 kW for about 16 seconds, which covers a generator transfer tested to NFPA 110.
- Match the store to the machine before assuming a retrofit: the Mitsubishi Electric 9900AEGIS runs a 480 V class DC bus, and the replacement store has to sit inside that window.
What you are really buying#
Runtime is a function of usable energy rather than nameplate power. A UPS rated 500 kW with five minutes of autonomy needs a store sized for the discharge rate at the depth of discharge the manufacturer allows, at the end of the battery's warranted life, at the lowest room temperature the design tolerates. All four of those qualifiers move the answer, and vendors do not always state them.
Ask for the runtime curve, the load point it assumes, and whether the figure is quoted at beginning of life or end of life. A five-minute figure at beginning of life and full load is a different machine from a five-minute figure at end of life and part load.
Table 1 — How the three storage approaches differ on the dimensions that drive cost#
| Dimension | VRLA | Lithium-ion | Flywheel / kinetic |
|---|---|---|---|
| Energy density | Lowest | Highest of the three | Minimal energy, very high power |
| Footprint | Largest | Smallest for equivalent autonomy | Compact, no battery room |
| Ambient temperature | Most sensitive; heat accelerates capacity loss | Tolerates higher ambient | Mechanically limited, no chemistry effect |
| Typical autonomy | Minutes | Minutes | Seconds |
| Mid-life replacement | Expected inside UPS life | Rarely | None; mechanical maintenance interval |
| Primary code reference | OSHA 1926.441, NFPA 70 | NFPA 855, NFPA 70 | NFPA 70 |
| Disposal path | Established lead recycling | EPA-regulated handling | No chemical disposal |
| State-of-health reporting | Manual testing | Integrated battery management | Mechanical monitoring |
Valve-regulated lead-acid#
VRLA remains the default in a large share of installed rooms. It is inexpensive per kilowatt-hour, universally supported, and understood by every service organization in the market. Its weaknesses are density, temperature sensitivity and life: capacity falls as the string ages, and elevated room temperature accelerates that decline sharply.
The practical consequence is a refresh cycle. A VRLA string is generally treated as a consumable to be replaced during the life of the UPS, which means the replacement belongs in the total cost model from day one. Battery room work sits under OSHA 1926.441 for ventilation, eye-wash provision and handling, and the electrical installation follows NFPA 70.
Lithium-ion#
Lithium-ion cabinets are denser, tolerate higher ambient temperatures, cycle far more times, and carry integrated battery management that reports state of health continuously. For a retrofit into a constrained room, the footprint saving alone often decides the case. The cost premium at purchase is recovered over a longer service life, provided the site keeps the UPS long enough to collect it.
The obligations are different rather than lighter. Energy storage system installations are addressed by NFPA 855, which covers spacing, separation, ventilation and fire protection, and the authority having jurisdiction will ask for it by name. Disposal is also a regulated path: the EPA publishes handling guidance for used lithium-ion batteries and treats improper disposal as a fire risk in the waste stream.
Plan the thermal design against the ASHRAE Datacom series. Higher tolerance and no requirement are different things.
Flywheel and kinetic storage#
A flywheel stores energy mechanically and delivers high power for a short time.
300 kW for about 16 seconds — the VYCON VDC spins to 18,500 rpm and attaches to an existing DC bus, which makes it the most retrofit-friendly of the three approaches.
675 kW at 480 V — the Active Power CleanSource HD is a battery-free flywheel UPS, removing the battery room rather than shrinking it.
At megawatt scale the Piller POWERBRIDGE is a kinetic energy store paired to a host UPS, offered as a direct replacement for a battery string, and never as a standalone unit. It is commonly specified with the Piller UNIBLOCK UBT+ rotary machine.
Table 2 — Kinetic storage and rotary machines in the SecondWatt catalog#
| Model | Type | Published rating | Attachment |
|---|---|---|---|
| VYCON VDC | DC-bus flywheel | 300 kW for ~16 s, to 18,500 rpm | Existing static UPS DC bus |
| Active Power CleanSource HD | Battery-free flywheel UPS | 675 kW at 480 V | Standalone UPS |
| Piller POWERBRIDGE | Kinetic energy store | Megawatt class | Host UPS, replaces battery string |
| Piller UNIBLOCK UBT+ | Rotary UPS | Megawatt class | Standalone |
Seconds of autonomy sound thin until you set them against the generator start sequence. Where standby generation is installed, exercised and tested to NFPA 110, the store only has to cover transfer. In exchange the site loses the battery room, its ventilation, its temperature limits and its replacement cycle. Generator availability is the dependency to check first — current generator lead times run long enough that a flywheel-only design commits you to a delivery date.
Comparing on total cost#
Build the comparison over the intended holding period across five lines: purchase, installation and room works, cooling and ventilation, mid-life replacement, and disposal. VRLA usually wins line one and loses lines four and five. Lithium usually loses line one and wins line four. Flywheels remove lines four and five almost entirely and add a mechanical maintenance interval.
Efficiency belongs in the model too. Standby losses differ between chemistries and between charger designs, and ENERGY STAR publishes tested efficiency data on a common method, which compares better than vendor headline figures. For facility-level energy modeling, the Lawrence Berkeley National Laboratory data center energy usage report remains the standard reference.
Current purchase-side benchmarks by rating and topology are in the data center UPS pricing guide, which is where the line-one number should come from before the other four lines are modeled.
Matching the store to the machine#
Some UPS platforms are chemistry-agnostic and some are not. Confirm that the charger profile, the DC bus voltage window and the battery management interface are all supported before assuming a lithium retrofit is possible on an installed unit. The Mitsubishi Electric 9900AEGIS, for example, runs a 480 V class DC battery bus, and any replacement store has to sit inside that window.
Where the UPS is bought on the secondary market, the store is almost always a separate purchase with a separate lifetime. Treat the converter and the energy store as two procurement items. The same split applies across the wider backup power stack, where lead times on diesel and hybrid plant move independently of the UPS itself.
FAQ: UPS energy storage selection#
How long does a data center UPS run on battery?#
Long enough to cover the generator transfer, which is the design intent for most data center installations. Battery-backed systems are commonly specified for five to fifteen minutes of autonomy; flywheel systems are specified in seconds, because the transfer itself takes seconds when the generator plant is exercised and tested to NFPA 110. Any autonomy figure needs its four qualifiers attached: load point, depth of discharge, beginning or end of life, and room temperature.
Should data centers use lithium-ion or VRLA batteries?#
Holding period and room constraints decide it, and neither chemistry wins in the abstract. VRLA costs less to buy and carries a mid-life replacement inside the UPS life. Lithium-ion costs more to buy, occupies less floor, tolerates higher ambient temperature, and avoids that replacement — provided the site keeps the machine long enough to collect the difference. A site planning to sell or relocate within a few years often does not.
What is the difference between a UPS and a BESS?#
A UPS is sized for power over seconds to minutes and exists to ride through a supply interruption. A battery energy storage system is sized for energy over hours and exists to shift load, arbitrage price or provide grid services. They can share cell chemistry and share nothing else: discharge rate, control strategy, code pathway and commercial case all differ.
How much does a data center UPS cost?#
Purchase price varies by rating, topology and whether the unit is new or secondary market. Published benchmarks by kVA band are in the data center UPS pricing guide. The store is priced separately and, on a secondary-market purchase, almost always is.
Can you buy a used UPS?#
Yes, and the converter is frequently the better secondary-market buy of the two components. The caveat is the store: batteries age on a calendar as well as a cycle count, so a used UPS should generally be priced as a converter with a new store, and the DC bus window checked against whatever store you intend to fit.
Next step#
Compare DC bus voltages, storage options and published autonomy behavior across current and superseded models in the UPS systems hub.