UPS Battery Sizing Calculator for Selecting Discharge Ratings
Selecting a UPS battery bank means checking whether it can deliver the required power for the full backup duration. A battery’s Ah label gives useful capacity information, but the manufacturer’s constant-power discharge table is more directly applicable to many UPS sizing decisions.
The AKCP UPS Battery Sizing Calculator calculates the required watts per battery block and per cell. You can then compare that requirement with a datasheet rating for the selected duration and cutoff voltage.
Use the UPS battery sizing calculator
Start with the real load, required backup time and battery-mode inverter efficiency. Add future load growth if the system must accommodate expansion. The additional runtime reserve extends the duration you need to find in the discharge table; it does not change the instantaneous electrical load.
Enter the approved series count and the number of parallel strings to check. Series count is part of the UPS’s voltage configuration. It should not be changed simply to make the sizing result look better.
UPS Battery Sizing Calculator
Find the constant-power rating your batteries need, then compare it with a manufacturer's discharge table.
Starting values are illustrative. Use specifications for your UPS and batteries. This widget calculates in your browser and sends no inputs.
The calculator is starting. If inputs do not appear, its JavaScript may have been blocked.
1. Load & runtime
2. Efficiency & design allowances
UPS output limits & fixed losses
3. Battery configuration
Cutoff voltage & configuration limits
Describe a manufacturer-approved configuration. Increasing battery count does not establish UPS compatibility.
4. Compare a battery datasheet
Optional. Use the constant-power table for your exact battery model. Do not enter Ah or a nominal Wh rating here.
Required rating per battery block
Bank & UPS checks
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Output ratings and battery ratings are separate checks. Extra batteries cannot increase a UPS's W or VA limit.
Datasheet comparison
Enter a rating to compare.
—A watts-based comparison does not predict an exact runtime or convert a battery rating into Ah.
Compare parallel strings
| Strings | Blocks | Required W/block | Rating margin | Check |
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Method, assumptions & references
Planned W = connected W × (1 + growth / 100). VA = W / load power factor. Design duration = required runtime + additional reserve minutes. Runtime determines which discharge-table column you use; it does not change the instantaneous load.
Actual DC demand = planned W / battery-mode efficiency + fixed DC overhead. Design battery power = DC demand × (100 / remaining-capacity %) × temperature correction × (1 + additional design margin / 100).
Required W/block = design battery power / (blocks in series × parallel strings). Required W/cell = W/block / cells per block. A datasheet W/cell rating is multiplied by cells per block. Minimum strings = design battery power / (series blocks × datasheet W/block), rounded up to a whole string.
Manufacturer ratings must cover at least the design duration and must be taken at an end voltage no lower than the required cutoff. A longer-duration or higher-cutoff rating is treated as conservative. If the entered rating does not cover these conditions, no capability or minimum-string comparison is shown.
Aging, temperature and design allowances are separate multipliers. For example, 80% remaining capacity produces a 1.25 aging multiplier. Apply temperature correction from the manufacturer's rating temperature to your design conditions; no universal temperature curve is assumed. Do not apply an allowance twice if it is already included in the datasheet rating. The aging multiplier is a planning approximation, not a guarantee of aged high-rate performance.
Assumes fully charged, matching batteries and ideal load sharing. Constant-power tables already account for discharge rate; no additional Peukert correction is applied. No lithium BMS limits, charging voltage limits, charger sizing, recharge time, transient load, cable losses or weakest-cell behavior are modeled. Values are for planning; confirm the complete system with the UPS and battery manufacturers.
References: EnerSys battery application manual · C&D UPS battery sizing
Read the correct battery discharge table
Battery tables may list W per cell or W per battery block. The calculator accepts either. For a typical 12 V lead-acid block containing six cells, a 250 W-per-cell rating represents 1,500 W per block at the table’s specified conditions.
Check the duration, end voltage and reference temperature before entering a rating. A rating at ten minutes cannot establish suitability for a fifteen-minute requirement. A rating that allows the battery voltage to fall below the required UPS cutoff can also overstate usable capability. The calculator flags those mismatches. Longer-duration or higher-cutoff ratings are treated as conservative comparisons.
Keep sizing allowances separate
The remaining-capacity input provides an aging allowance. At 80% remaining capacity, the model applies a multiplier of 1.25. Temperature correction should come from the battery supplier for the difference between its table conditions and your design conditions.
Additional design margin is separate from aging, temperature and future load growth. Avoid applying any allowance twice if it is already included in the rating or design basis. Constant-power discharge tables already account for discharge rate, so this calculator does not add a separate Peukert correction.
Worked example for a fifteen minute requirement
The following values are illustrative and do not specify a real battery model.
Input | Value |
Real load and backup duration | 40 kW for 15 minutes |
Battery-mode efficiency | 95% |
Design remaining capacity | 80% |
Temperature correction | 1.00 |
Additional design margin | 10% |
Configuration to check | 40 blocks in series and one string |
Cells per block | Six |
The load requires approximately 42.11 kW from the battery before allowances. Applying the aging multiplier and 10% margin raises the design battery power to 57.89 kW. Dividing that requirement across 40 blocks gives approximately 1,447.37 W per block, or 241.23 W per cell.
Suppose a hypothetical datasheet specifies 1,500 W per block for at least fifteen minutes at a compatible cutoff. One string has a 60 kW rating, giving about 3.64% margin over the factored requirement. If the rating is only 1,200 W per block, one string provides 48 kW and falls short. The mathematical minimum becomes two whole strings, subject to the UPS’s approved configuration limits.
Check voltage and output capacity separately
With 40 twelve-volt blocks, nominal bank voltage is 480 V. At six cells per block and an end voltage of 1.75 V per cell, the end voltage is 420 V. If the UPS requires a higher minimum DC voltage, use an appropriate higher-cutoff table and review the configuration.
The UPS must also support the planned load within its W and VA output ratings. Extra batteries extend energy availability; they do not increase inverter output capacity. Check manufacturer limits for strings, charging, protection and installation before selecting a bank.
When monitoring a high-voltage string, use measurement equipment rated for the intended measurement point. A sensor designed for an individual block must not be assumed suitable for the complete string. AKCP battery and UPS monitoring explains how suitable sensing and available UPS telemetry can support ongoing checks after the sizing work is complete.
Frequently Asked Questions (FAQ)
Why does the required watts figure stay the same when I change runtime
For a constant load, instantaneous power stays the same. A longer duration changes the datasheet rating the battery must meet, so a different table column is needed.
Can I enter an Ah rating in the datasheet comparison
No. That field requires a constant-power rating in W per block or W per cell at the relevant duration and end voltage.
Does the minimum string count confirm compatibility
No. It is a mathematical result. The UPS manufacturer must permit the configuration and provide appropriate charging and protection.
Does an 80 percent aging input guarantee end of life performance
No. It is a sizing approximation. Aged high-rate performance, weak cells and real operating conditions still need validation.
