30-minute IT shutdown
2 kW for 30 min at 48 V—~24.5 Ah planning floor before margin.
UPS Battery Calculator
Estimate required battery Ah from load, target runtime, voltage, and efficiency. Use this for UPS battery backup sizing (Ah for your minutes)—not brand replacement shopping lists. Start from the UPS calculator hub if you still need load or kVA.
Quick answer
UPS battery Ah sizes the DC plant for your target backup minutes. Energy need is approximately load (kW) times runtime (hours), adjusted for inverter efficiency and depth-of-discharge margin, then divided by nominal string voltage (higher V reduces Ah for the same energy). Example: 2 kW for 30 minutes at 48 V with 0.85 efficiency needs about 24.5 Ah before aging, temperature, and parallel-string layout—field banks are usually larger. For online (double-conversion) UPS battery backup calculations, use a lower planning efficiency (often 0.85–0.92) than standby units at the same kW—see the Quick Example below. Use measured kW from the load calculator, confirm voltage with your OEM, cross-check minutes in the runtime calculator, or start from how long will UPS last if backup time is your primary question.
Quick UPS Battery Calculator
Enter load and target runtime only. Required Ah, warnings, and recommended next steps below use the same load and minutes. Default 48 V string and 0.85 efficiency (adjust in Advanced).
Estimated battery: 24.5 Ah @ 48 V · 1176 Wh
Same load and target minutes as full results below. Quick defaults: 48 V, efficiency 0.85—change voltage or efficiency in Advanced.
Advanced UPS Battery Calculator
Quick Examples
DC-path / inverter efficiency for planning (default 0.85).
UPS Battery Results
Engineering disclaimer
Estimates only. Verify with manufacturer battery tables, discharge-rate limits, and review by a qualified professional for binding designs.
Results
Energy (DC-equivalent): 1176 Wh (1.18 kWh)
Required battery: 24.5 Ah @ 48 V
Default: 2 kW, 30 min, efficiency 0.85.
Explain this result (summary)
- Energy need: About 1.18 kWh DC-equivalent for 30 min at 2.00 kW.
- Ah result: 24.5 Ah at 48 V with efficiency 0.85 (1176 Wh).
- Cross-check: Compare minutes in the runtime calculator; lay out blocks in the bank calculator.
Operational guidance
Typical IT ride-through
30 minutes at 2 kW is a common planning anchor—confirm discharge rate limits with the OEM.
Required Ah vs runtime
| Runtime (min) | Ah @ 48 V |
|---|---|
| 10 | 8.2 |
| 20 | 16.3 |
| 30 (your target) | 24.5 |
| 45 | 36.8 |
| 60 | 49.0 |
Planning note: For VRLA UPS strings, many projects size on usable energy (DoD policy) rather than day-one nameplate. A common planning window is roughly 50–80% usable fraction and 0.85–0.92 DC-path/inverter efficiency (depends on topology and loading). If the battery room runs hot (often 25°C+), add Ah margin or reduce the target runtime before you lock string count.
People also ask
- How many Ah for 30 minutes? At 2 kW and 48 V with ~0.85 efficiency, the planning floor is about 25 Ah before aging margin.
- Is Ah enough to order? You also need max discharge current, terminal layout, and rack constraints.
- Ah vs runtime tool? This page sizes Ah from target minutes; runtime estimates minutes from known Ah and strings.
- 48 V vs 110 V string? Higher nominal voltage lowers Ah for the same energy—confirm parallel string count and OEM discharge limits.
- Parallel strings and Ah? Field banks often use multiple strings; total installed Ah must still meet C-rate and end-of-discharge rules per vendor tables.
- How do Wh and Ah relate? Wh = Ah × nominal string voltage; kWh = Wh ÷ 1000. Results show both for OEM kWh comparisons.
- What C-rate should I plan for? Discharge current ÷ Ah rating—high C-rate loads need more installed Ah than energy math alone.
- What depth of discharge (DoD) should I use? Plan shallow DoD for longer life—often 50–80% of nameplate for VRLA UPS strings.
- Does round-trip efficiency change Ah? Lower DC-path efficiency increases Wh and Ah for the same AC load minutes.
- How do you calculate online UPS battery backup? Use Ah ≈ (kW × 1000 × minutes ÷ 60) ÷ (V × efficiency). Online double-conversion units often plan at η 0.85–0.92—then cross-check minutes in the runtime calculator.
- Is this the same as a battery backup calculator? This page sizes Ah when you know target minutes and load. The runtime calculator estimates minutes from known Ah, V, and strings—use both before ordering.
UPS battery planning guidance
- Vendor tables: Final string count and C-rate limits come from battery OEM data (and UPS vendor packs from APC, Eaton, Vertiv class frames)—use this page for planning Ah only.
- Aging margin: Size for end-of-life capacity at the design horizon, not day-one nameplate alone.
- Discharge rate: High-rate bursts may need more Ah than energy math alone—check OEM C-rate limits.
- Depth of discharge: Shallow cycling extends life; planning efficiency should reflect DOD policy, not 100% use of nameplate.
- Runtime cross-check: Enter the same kW, V, and efficiency in the runtime calculator and reconcile minutes ↔ Ah before procurement.
- Online UPS: Double-conversion units often need a lower efficiency input (0.85–0.92) than standby—use the Online UPS Quick Example or see online vs offline UPS.
- Temperature derating: Hot battery rooms shorten VRLA life and reduce effective Ah—add margin or lower assumed runtime when ambient exceeds ~25°C; see battery maintenance.
- State of health (SoH): Trend impedance and load-test minutes against design kW before trusting day-one Ah—replace strings when OEM thresholds trip, not only when alarms flash.
Upstream: load, capacity. Cross-check: runtime. Scenario: how long will UPS last. Neighboring: cable size, voltage drop, breaker size.
Full four-step path: UPS calculator hub (load → capacity → runtime → battery).
UPS battery for common scenarios
15-minute generator bridge
5 kW for 15 min—higher discharge rate; verify OEM limits.
60-minute NAS ride-through
0.5 kW for 60 min at 48 V—light load, longer Ah at the same voltage.
Online UPS 1.5 kVA / 30 min
~0.9 kW at 48 V with η 0.88 (double-conversion)—typical retail online frame; OEM charts override spreadsheet Ah.
2 kW on 110 V string
30 min at 110 V cuts Ah versus 48 V for the same energy—confirm string count with the OEM.
UPS battery formula (quick reference)
Ah ≈ (kW × 1000 × runtime_min ÷ 60) ÷ (V × efficiency). Wh = Ah × V · kWh = Wh ÷ 1000. See formula notes and worked examples below in the depth section.
Battery strings and parallel banks
Field UPS plants rarely use a single cell string. Series strings raise nominal DC voltage (more V × fewer Ah for the same energy); parallel strings add amp-hour capacity and share discharge current. The calculator above gives a planning Ah at your entered voltage—multiply by parallel string count only when each string meets OEM C-rate and end-of-discharge limits.
Example: 2 kW for 30 minutes at 48 V needs ~25 Ah before margin. Two parallel 48 V strings of 50 Ah each may meet energy, but each string must still tolerate the discharge rate at your load step. Confirm string count, fuse/breaker layout, and rack weight with the battery OEM before ordering.
Deep dive: How to calculate UPS battery size (strings, margin, and vendor tables). Lay out blocks in the UPS battery bank calculator. Cross-check minutes in the UPS runtime calculator.
How to size UPS battery Ah
- Confirm load kW and target runtime minutes.
- Enter string voltage and planning efficiency.
- Read Ah; cross-check in runtime.
Frequently Asked Questions
Why does my UPS vendor software disagree slightly with this page?
Vendors embed chemistry-specific curves, temperature coefficients, and minimum cell voltages. Use this calculator for directional planning, then finalize strings with OEM tools and stamped project documentation where required.
Should I add margin for battery aging?
Yes. End-of-life capacity is lower than day-one ratings; prudent designs reserve Ah so degraded strings still meet the runtime contract at the design horizon.
Does higher inverter efficiency always reduce Ah?
Higher efficiency reduces DC energy required for the same AC load, which lowers Ah at a fixed voltage. Efficiency varies with load level, so use vendor curves near your true operating point.
Is amp-hour the only figure I need to order batteries?
No. You also need maximum discharge current, terminal layout, breaker coordination, recharge current limits, and physical rack constraints—Ah is necessary but not sufficient.
How do series and parallel strings change Ah?
Series connections increase nominal voltage; parallel connections add amp-hours and share current. Total installed Ah must satisfy both energy math and per-string C-rate limits—see the strings and banks section above.
When should I use the battery size guide instead of this calculator?
Use this page for quick Ah from kW and minutes. For string layout, aging margin, and OEM discharge tables, follow How to calculate UPS battery size.
How does this tie to the UPS runtime calculator?
Runtime estimates minutes from known battery parameters; this step estimates Ah when minutes and load drive procurement. Move between the tools as your knowns change.
How do I convert Ah to Wh or kWh?
Multiply Ah by nominal string voltage for Wh; divide by 1000 for kWh. The results card shows both—use kWh when comparing to OEM energy ratings.
What is battery C-rate in UPS planning?
C-rate is discharge current divided by Ah rating. A 50 A draw on a 100 Ah string is 0.5C. Exceeding vendor C-rate limits requires more parallel strings or a larger bank.
What depth of discharge should I assume?
Many VRLA UPS designs plan 50–80% usable energy, not 100% of nameplate. Shallow DoD extends calendar and cycle life—align with your maintenance policy.
How do you calculate online UPS battery backup?
Ah ≈ (kW × 1000 × minutes ÷ 60) ÷ (V × efficiency). Online double-conversion UPS typically plans at 0.85–0.92 inverter efficiency at partial load—lower η increases required Ah versus standby at the same kW. Example: 0.9 kW for 30 min at 48 V with η 0.88 needs about 10.7 Ah before aging margin. Cross-check minutes in the runtime calculator and topology notes in the online vs offline UPS guide.
Is this the same as a battery backup calculator?
Not exactly. This page answers how many Ah do I need when load and target backup time are known. Tools titled “battery backup calculator” often estimate how long existing batteries will last—use the UPS runtime calculator for that direction, then return here for procurement Ah.
How does battery temperature affect UPS Ah sizing?
High ambient temperature accelerates VRLA aging and lowers usable capacity under load. A common planning rule: every 10°C above ~20°C can roughly halve calendar life—add 10–20% Ah margin or reduce assumed runtime when the battery room runs hot. Confirm on OEM temperature derating curves.
How do I estimate UPS battery state of health (SoH)?
Combine semi-annual internal ohmic/impedance trending against baseline, periodic load tests at design kW, and float-voltage logs. When impedance rises past OEM limits (often +30–35% vs baseline) or a load test misses target minutes, plan string replacement—even if float voltage looks normal.
How it works
Battery amp-hour (Ah) sizing answers whether the DC plant can deliver enough energy for the target minutes at the protected AC load. Conceptually, you convert load power (kW) and required backup time into an energy demand (kWh), translate that to DC watt-hours using practical inverter and cable efficiency assumptions, then divide by the string voltage to obtain an amp-hour requirement before manufacturer derating curves.
Higher DC bus voltage reduces amp-hour for the same energy because each amp-hour carries more watt-hours when multiplied by a larger voltage. Temperature, end-of-discharge voltage, aging, and desired depth of discharge all increase the installed Ah relative to a naive arithmetic estimate—your battery vendor tables remain authoritative for final cell selection.
This calculator is positioned after load and runtime intent are understood. Treat its output as a planning anchor, then validate against UPS manufacturer software, battery tables, and local codes governing ventilated battery rooms and maintenance access.
Formula and sources
Planning anchor: DC energy (Wh) ≈ (Load kW × Runtime hours) ÷ Overall DC-path efficiency; Ah ≈ DC energy (Wh) ÷ Nominal battery voltage (V)
Overall efficiency bundles inverter conversion, cable loss, and conservative headroom; exact factors vary by topology and state of charge.
Always round up to the next commercial block or string count and apply aging margin recommended by the battery OEM.
Worked examples
- Two kW for thirty minutes at forty-eight volts
Half an hour at 2 kW implies about 1 kWh of AC-side energy before efficiencies. At 48 V, the naive amp-hour floor is roughly 21 Ah before inverter loss, temperature derating, and end-of-life margin—field designs normally select a materially larger bank.
- Raising string voltage from forty-eight to one hundred ten volts
For the same energy demand, moving from 48 V to 110 V cuts the amp-hour requirement roughly in proportion to the voltage ratio because watt-hours per string increase with voltage for the same current profile.
- Short five-minute bridge versus thirty-minute ride-through
Five minutes at the same kW needs one-sixth of the energy of thirty minutes, but very short windows still require attention to discharge rate limits—some chemistries prefer longer, gentler discharges than aggressive high-rate bursts.