Calculation sequence
Step-by-step method
- Calculate required load energy.
- Divide by user-entered efficiency and usable depth.
- Convert nominal energy to ideal amp-hour capacity.
ElectricalIntermediateStandby Power and Batteries
Battery Capacity Screening Calculator
Calculate ideal minimum amp-hour capacity for a constant AC load and required runtime.
Why it matters: The ideal inverse energy balance is useful for early planning but is not a selected battery string.
Calculated result
Engineering evaluation
Ideal required battery capacity (Cʳ)57.87 Ah
- Required nominal energy (Eʳ)
- 13.889 kWh
Engineering interpretation
The ideal screen requires 13.88889 kWh nominal energy or 57.87037 Ah at 240 V DC.
Important limitations
- Ideal screening only. Exact manufacturer discharge data are mandatory for final use; do not round down or treat this value as a selected battery string.
Engineering knowledge center
Understand the calculation—not just the answer
Governing relationships
Eʳ = Pᴸ·t/[(η/100)(DOD/100)]
Cʳ = 1000Eʳ/Vᴰᶜ
Current substitution
Eʳ = 5×2/[0.9×0.8] = 13.88889 kWh; Cʳ = 1000×13.88889/240 = 57.87037 Ah.
Equation legend
Inputs and calculated quantities
- Pᴸ
- AC load — kW
- t
- Required runtime — h
- Vᴰᶜ
- Nominal DC voltage — V
- η
- UPS/inverter efficiency — %
- DOD
- Usable depth of discharge — %
- Cʳ
- Ideal required battery capacity — Ah
- Eʳ
- Required nominal energy — kWh
Applications
How this calculation is used
Building design
Screen required nominal energy and Ah.
Construction
Check load, runtime, voltage, and efficiency assumptions.
Commissioning
Compare the ideal requirement with manufacturer selection and test data.
Quality controls
Assumptions and limitations
Assumptions
- Constant load and nominal voltage for the ideal screen.
- Entered efficiency and usable depth apply over the full interval.
Limitations
- Ideal screening only; exact manufacturer discharge data are mandatory for final use.
- Cell count, end voltage, temperature, aging margin, duty cycle, recharge, redundancy, and selection rules are omitted.
Field use
Verification procedure
- Follow the approved electrical-safety program and use correctly rated instruments.
- Confirm wiring, ratios, units, operating mode, and measurement boundaries.
- Document simultaneous readings and compare with approved project and manufacturer data.
Common engineering mistakes
- Combining values from different time intervals, phases, or measurement boundaries.
- Treating a screening result as equipment selection, protective-device coordination, or code compliance.
Technical basis
References and source standards
- IEEE Standards AssociationPower quantity, harmonic, transformer, motor, battery, and metering standards; verify the current applicable edition during professional review.
- NIST Guide for the Use of the International System of UnitsElectrical quantity, unit, symbol, and conversion context.
Engineering calculation disclaimer
These engineering tools are provided for educational, preliminary evaluation, field verification, and commissioning support. Results depend on the accuracy of user-entered information and the assumptions stated for each calculation. Every colored status and comparison is a screening indicator only; project criteria, contract requirements, applicable codes, manufacturer instructions, measurement uncertainty, and authorized engineering judgment govern. The tools do not replace project-specific engineering analysis or the judgment of a licensed professional engineer.
Read the full use and limitation statement