Specify a data center battery energy storage system by the power and usable energy required at a defined delivery point. Include operating duty, reserve, aging, auxiliaries, recharge and control behavior. A battery container's nameplate capacity does not establish runtime at the critical load or its ability to support an islanded electrical system.
A battery system can serve several commercial and operational purposes. The purchase becomes difficult when an offer combines those purposes without identifying which one controls the equipment selection. Start with the service you need and ask the supplier to show how the complete system delivers it.
SecondWatt's battery and DC-power catalogue is a starting point for equipment research. Treat the intended application as the first selection field.
Key Takeaways
- Define power and usable energy at the same delivery boundary.
- Separate battery nameplate capacity from guaranteed AC energy at the load.
- Specify reserve, degradation, auxiliary demand and recharge conditions.
- Verify the controls and integration needed for each operating mode.
- Compare complete equipment scope, acceptance evidence and warranty obligations.
Define the service the battery must provide#
State whether the system is intended to reduce peak imports, shift energy, support generation, provide outage energy or perform another defined task. Describe the required behavior in normal operation and during a disturbance. Several services may be possible, but they compete for equipment capacity and stored energy.
For example, energy discharged to manage an electricity bill is no longer available in the battery's reserve. The operating policy must decide which objective takes priority, when reserve can be used and how it will be restored. A financial model and an outage plan need the same assumption about available energy.
The Department of Energy's BESS procurement checklist provides a project-development framework for commercial-scale storage. Use it to organize the enquiry while retaining the actual site's requirements and responsibilities.
Define the required battery service#
| Field | Information to establish |
|---|---|
| Primary function | The service that controls equipment selection |
| Protected or served load | Named circuits, buses or meter boundary |
| Power requirement | Continuous and event-specific requirements |
| Duration | Required service period under stated conditions |
| Operating reserve | Minimum energy retained and rules for releasing it |
| Repeated events | Recharge requirement and readiness for the next event |
| Performance period | Requirements at commissioning and later in service |
An energy-storage proposal also needs to identify which loads remain protected through the transition into its operating mode. Do not infer UPS behavior from the presence of a battery and inverter. The complete electrical and control arrangement must address the required interruption and transfer conditions.
Use the UPS and energy-storage overview to distinguish the roles of the systems around the critical load. Then write the BESS requirement at its actual point of connection.
Read the DC and AC specification fields separately#
A storage offer can quote nominal battery energy, usable battery energy, converter apparent power, real-power capability and delivered AC energy. These quantities describe different boundaries. Ask the supplier to name the field rather than reducing the offer to a single capacity number.
A Sungrow PowerTitan 2.0 US datasheet, Version 3 with a 2023 copyright, is preserved in a Connecticut public filing. It illustrates the distinctions without establishing current inventory.
Published reference fields from the identified US datasheet#
| Field | ST5015kWh-2500kW-2h-US | ST5015kWh-1250kW-4h-US |
|---|---|---|
| Nominal DC energy capacity | 5,015 kWh | 5,015 kWh |
| Nominal AC power row, units retained as published | 210 kVA × 12 | 210 kVA × 6 |
| Nominal AC voltage | 690 V | 690 V |
| Nominal frequency | 60 Hz | 60 Hz |
The AC row uses apparent-power units, even though the model names contain kW. Preserve that distinction. Request the offered system's real-power capability, reactive-power requirement and delivery boundary instead of silently relabeling the table as guaranteed MW at the site.
The shared DC capacity also does not establish identical operating capability. Ask for usable AC energy at the required discharge rate, temperature, state-of-charge limits and point in service life. The figure needed for runtime is the energy available to the specified load under those conditions.
Keep the package's conversion, transformer and auxiliary boundaries visible. A system measured at inverter terminals can have a different delivered-energy result from the same equipment measured across the site's connection transformer.
Calculate energy using explicit assumptions#
Start with the load service, then work back through the stated boundaries. For a constant load, energy is power multiplied by time. For a changing load, sum the energy required during each part of the profile rather than multiplying the highest value by an unexplained duration.
Illustrative constant-load energy requirement#
| Input or calculation | Assumed value |
|---|---|
| Required load power | 8 MW |
| Required duration | 2 hours |
| Energy delivered to the load | 8 × 2 = 16 MWh |
| Allowed operating window of retained battery capacity | 90% |
| Retained capacity at the chosen future date | 80% of beginning-of-life nameplate |
| Assumed discharge-path efficiency | 95% |
For this simplified example, the calculated beginning-of-life nameplate requirement is 16 divided by the product of 0.90, 0.80 and 0.95, or approximately 23.39 MWh. The factors are independent editorial assumptions for illustrating the calculation; they are not specifications of the Sungrow references or any offered system.
23.39 MWh — the illustrative nameplate result for delivering 16 MWh under the three stated factors, before adding a separately defined auxiliary-energy allowance or design margin.
This calculation excludes separately supplied auxiliaries and any additional reserve outside the assumed operating window. Define how those items enter the real calculation. Do not deduct an auxiliary load again if the supplier's guaranteed delivered-energy figure already includes it.
Likewise, if an offer already guarantees usable AC energy at the specified later-life condition, do not apply nameplate-to-usable, retention and discharge-efficiency factors to that guarantee again. Ask the supplier to reconcile its guaranteed value to the input fields rather than mixing two calculation methods.
The efficiency assumption is for discharge along the stated path. Round-trip efficiency covers charging and discharging and cannot simply replace that factor. Compare supplier results only after reconciling the measurement boundary and what each efficiency figure includes.
DOE's lithium-ion BESS technical-specification template separately requests usable capacity, round-trip efficiency including auxiliaries and annual degradation information. It is a customizable procurement template; its example requirements are not universal product guarantees.
Check power capability independently of energy. A system can have enough energy for the duration calculation while lacking the required discharge power, reactive capability or transient response.
Specify recharge and the required control behavior#
Ask what happens before, during and after the event. The buyer needs a defined initial state, a dispatch command, the intended electrical response and a path back to readiness. Charging is part of the operating service, particularly when the same system performs daily energy management and retains outage reserve.
Operating-mode schedule#
| Mode or event | Response to define |
|---|---|
| Normal operation | Dispatch objective and protected reserve |
| Planned discharge | Power, duration and applicable operating limits |
| External supply loss | Transition sequence and loads served |
| Islanded operation, where required | Voltage, frequency and protection responsibilities |
| Generator interaction | Coordination, load sharing and operating constraints |
| Recharge | Allowed source, available power and reserve-restoration target |
| Controls or communication fault | Defined local behavior, alarms and recovery |
Sungrow's PowerTitan 2.0 technical white paper describes operating capabilities with configuration-dependent conditions. Treat features such as black start as requirements to confirm for the supplied system and site, rather than capabilities established by the presence of a battery container.
A grid-connected operating mode does not, by itself, demonstrate the intended islanded service. Request the relevant controls, protection, auxiliary-power and restart design. Identify any equipment that must remain available for the storage system to perform its own starting sequence.
For a generation-backed installation, connect the BESS event cases to the gas turbine black-start and load-acceptance guide. Specify which system establishes the required electrical conditions and how the other equipment joins it.
Give the controls integrator an owner-approved priority list. The proposed system should explain which commands prevail when reserve protection, site demand and a commercial dispatch request conflict. Retain the final settings and operating limits in the handover record.
Evaluate condition, degradation and warranty together#
For new, surplus or used equipment, obtain the manufacturing, storage, operating and service history appropriate to its condition. Calendar history and operating duty should remain visible alongside the supplier's condition description.
Request the identity and configuration of battery modules, racks, power-conversion equipment, thermal-management equipment and controls. Record replacements or modifications and ask whether the complete offered combination remains supported. A cell or module specification does not establish the condition or support status of the assembled system.
Condition and warranty comparison#
| Topic | Evidence or term to request |
|---|---|
| Identity | Component inventory and system configuration |
| Storage | Environmental, charging and preservation history |
| Operation | Available duty, throughput, alarm and service records |
| Capacity | Test method, conditions, boundary and measured result |
| Degradation | Stated retention basis and applicable operating limits |
| Warranty | Covered equipment, start event, exclusions and remedy |
| Replacement or augmentation | Responsibility, compatibility and future work scope |
Define the performance period used in the offer. Commissioning capacity, guaranteed later-life capacity and an estimated degradation curve are different forms of evidence. Ask what happens if the relevant acceptance or warranty threshold is missed and how the condition will be measured.
Where a proposal includes future augmentation, separate the initial equipment from the later work. Identify who pays, what remains to be supplied and whether the site has the space and interfaces for it. An assumed future addition should not be counted as equipment delivered today.
For used equipment, establish the support and warranty position for the new buyer and destination. Request written confirmation where transfer or requalification is needed. Keep any seller-provided warranty distinct from manufacturer coverage.
The aim is a documented operating commitment with a practical remedy. A general promise of long life leaves the buyer unable to compare the actual obligations.
Verify installation evidence and acceptance at the agreed boundary#
Request the equipment's certification records and relevant test reports, with the actual configuration and test scope identified. A document for a cell, module or different enclosure should not be treated as proof covering every configuration of the assembled installation.
UL Solutions' energy-storage testing and certification overview distinguishes system certification from thermal-runaway evaluation. It identifies UL 9540A as a “Test Method for Evaluating Thermal Runaway Fire Propagation in Battery Energy Storage Systems.” Ask for the relevant documentation and the installation review required for the actual project.
Avoid converting a manufacturer's list of standards into an independent finding that the receiving site is approved. Give the responsible design team and reviewing authorities the equipment configuration, layout and supporting documents. Record their project-specific requirements in the supply scope.
Acceptance evidence for a BESS installation#
| Acceptance area | Evidence to retain |
|---|---|
| Delivered configuration | Equipment inventory and approved drawings |
| Electrical interfaces | Required installation and protection checks |
| Power and energy | Agreed test rate, duration, conditions and meter boundary |
| Operating modes | Results for the specified dispatch and transition cases |
| Auxiliary systems | Cooling, controls, alarms and required support functions |
| Monitoring | Usable access to operating records and alarm history |
| Final handover | Resolved exceptions, operating limits and support responsibility |
The transformer selection guide helps frame the connection equipment around the BESS. Include that equipment in the energy boundary and commissioning plan rather than leaving it outside both supplier scopes.
DOE's BESS evaluation-method overview describes evaluating deployed systems using charge and discharge metered data. That supports retaining usable operating records after commissioning. Ongoing performance assessment complements the purchase acceptance test; it should not be confused with evidence that a different offered unit has already passed.
Compare the complete BESS supply and submit the enquiry#
Price the same service boundary across proposals. Separate batteries, conversion equipment, transformer and switchgear, thermal management, protection, controls, civil works, installation, commissioning, operating support and any future augmentation.
If an offer excludes an item needed to deliver the stated service, assign that item to another scope and carry its cost and schedule into the comparison. A lower battery-container price can be commercially useful, but the buyer still needs a complete installed requirement.
Data center BESS sourcing brief#
| Requirement | Buyer input |
|---|---|
| Service | Primary application and operating priorities |
| Output | Required MW and usable MWh at a named boundary |
| Duty | Load profile, duration, reserve and repeated-event requirement |
| Electrical system | Connection conditions and required operating modes |
| Performance period | Acceptance and later-life requirements |
| Condition | Acceptable new, surplus or used equipment and required evidence |
| Scope | Included equipment, integration, service and delivery responsibilities |
| Schedule | Design, installation and accepted-operation milestones |
Ask respondents to identify deviations and separate guaranteed values from planning estimates. Compare the required battery service with alternatives on the same basis; storage should be selected because it meets the defined task.
Include the end of service in the commercial scope. Assign responsibility for disconnecting, preserving, transporting and disposing of or recycling the equipment through the appropriate providers. Identify records the buyer will need for that work and distinguish an included obligation from a future service to be purchased. The initial supply price should make those boundaries visible.
SecondWatt acts as an independent intermediary. To request BESS equipment sourcing, provide the required MW, usable MWh, duty, connection conditions, acceptable condition, destination and target date. State whether the requirement includes conversion equipment, electrical infrastructure and operating support.
FAQ: Battery energy storage for data centers#
What is the difference between MW and MWh in a BESS offer?#
MW expresses a real-power rate, while MWh expresses an amount of energy. Specify both at the relevant delivery boundary and operating conditions. A duration calculation divides usable energy by the applicable load, but it still depends on power capability, reserves, auxiliaries and the state of the equipment.
Does nominal battery capacity establish backup duration?#
No. Determine usable energy at the required load boundary after accounting for the stated operating window, equipment condition, discharge path and auxiliary requirements. Ask for the supplier's guarantee and test basis. Keep nameplate energy, usable DC energy and delivered AC energy separate so the runtime calculation uses the appropriate value.
Can a BESS replace a UPS in a data center?#
That requires evaluation of the complete system against the critical load's continuity requirements. Battery energy alone does not establish transfer behavior, power quality or support during every event. Define the allowed interruption and required operating cases, then verify the equipment and controls that deliver them.
Does grid connection prove the BESS can operate independently?#
No. The intended islanded mode needs its own controls, protection, auxiliary-power and operating design. Confirm the supplied equipment's capabilities and the integration scope for that mode. Ask how the system establishes or follows voltage and frequency, which loads it serves and how it returns to its normal arrangement.
How should I assess a used battery system?#
Request component identity, configuration, storage and operating history, condition evidence, measured performance and support terms. Confirm the test boundary and conditions and establish any warranty position for the new transaction. Do not infer remaining capacity or service life solely from the original nameplate or the seller's condition label.
Should I compare BESS offers by price per kWh?#
Use a normalized price only beside its complete scope and energy basis. State whether the denominator is nominal DC capacity or guaranteed usable AC energy, and at what point in service life. Include the power requirement, integration, support and augmentation obligations so a low normalized figure does not conceal missing equipment or a different service.