Battery Sizing Calculator – Practical Explanation
Battery sizing is often misunderstood, especially by first-time system builders. The goal is not simply to add more batteries, but to ensure that the system can supply power for the required duration without pushing the batteries beyond reasonable operating limits. This page explains how the SolarMathLab Battery Sizing Calculator arrives at its results and what those numbers actually mean.
1. What Battery Sizing Really Means
At its core, battery sizing is about energy. Any electrical load consumes a certain amount of power over time, and the battery bank must store enough usable energy to meet that demand. The calculation depends on four main inputs: load power, runtime, system voltage, and usable battery capacity.
Problems usually arise when one of these factors is ignored. For example, selecting batteries based only on amp-hour ratings without considering voltage or depth of discharge often leads to systems that appear large on paper but fall short in real use.
2. Electrical Relationships Used in the Calculator
The calculator relies on basic electrical relationships that are taught in introductory electrical and energy storage courses. These relationships are simple, but they are consistent across battery technologies and system sizes.
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Energy usage:
Electrical energy consumption is calculated by multiplying power by time.
Energy (Wh) = Power (W) × Time (hours) -
Battery capacity:
Battery capacity expressed in amp-hours is derived from total energy and system voltage.
Capacity (Ah) = Energy (Wh) ÷ Voltage (V) -
Usable capacity:
Only a portion of a battery’s nominal capacity should be used during normal operation. This portion is defined by the recommended depth of discharge.
Usable capacity = Nominal capacity × DoD -
Bank configuration:
Series connections raise voltage. Parallel connections increase capacity. Both are combined to match system requirements.
3. Depth of Discharge in Real Systems
Depth of Discharge (DoD) represents how much of a battery’s stored energy is used during a cycle. Operating batteries too deeply, too often, generally shortens their service life. For this reason, different battery chemistries are typically operated within different DoD ranges.
- Lead-acid batteries are commonly limited to around 50% usable capacity.
- Lithium-ion batteries are often operated at roughly 80% DoD.
- LiFePO₄ batteries are designed to tolerate deeper discharge, commonly up to 90%.
The calculator applies these values as usability limits rather than absolute maximums. This approach reflects common design practice rather than laboratory test conditions.
4. Series and Parallel Battery Layout
Most battery banks are built using a combination of series and parallel connections. Series strings are used to reach the required system voltage, while parallel strings are added to increase total storage capacity.
The calculator determines how many batteries are required in series to match the selected system voltage, then calculates how many parallel strings are needed to supply the required usable capacity.
5. Using the Calculator Inputs
To use the calculator effectively, input values should reflect realistic operating conditions rather than ideal or peak ratings.
- Load power: Continuous or average power draw, not short surge values.
- Runtime: The number of hours the load must be supported without recharge.
- System voltage: Nominal battery bank voltage.
- Depth of discharge: Based on the battery chemistry being used.
- Battery rating: Manufacturer-specified nominal voltage and capacity.
The output includes total required capacity, estimated battery count, and a suggested series-parallel arrangement.
6. Formula Sources and Technical References
The equations used here are not proprietary. They are the same relationships found in standard electrical engineering references, battery manufacturer documentation, and energy storage application notes.
- Energy and power relationships commonly documented in IEC and IEEE educational material.
- Amp-hour capacity definitions used across battery datasheets.
- Depth of discharge guidelines published by battery manufacturers.
- DC series and parallel configuration principles used in low-voltage systems.
7. Assumptions and Practical Limits
This calculator is intended for planning and comparison purposes. It does not account for every real-world variable. Factors such as inverter losses, temperature effects, cable resistance, surge currents, and long-term battery aging are not included in the calculation.
8. About SolarMathLab
SolarMathLab develops calculation tools focused on solar power systems, batteries, and electrical fundamentals. The emphasis is on transparent formulas and explainable results rather than product recommendations or brand-specific guidance.
Tools are reviewed internally for consistency and alignment with commonly accepted electrical principles before publication.
9. Disclaimer
Results provided by this calculator are estimates only and are intended for educational use. They do not replace professional electrical design, site-specific evaluation, or compliance with local codes and standards.
10. Closing Notes
Battery sizing becomes much clearer once the relationship between energy, voltage, and usable capacity is understood. This calculator is designed to support that understanding by showing how each input contributes to the final result, rather than presenting a single unexplained number.