Battery Capacity Calculator – Estimate Runtime, Energy and Pack Design
Whether you are sizing a small electronics project, building an e-bike pack, planning a backup power system or analyzing a UPS, battery math always comes back to the same ideas: voltage, capacity and power. This Battery Capacity Calculator brings those pieces together with five focused tools so you can move from ratings on a label to realistic expectations for runtime and energy.
Enter battery voltage and capacity in Ah, mAh or Wh, specify your load and efficiency and the calculator estimates runtime in hours and hours:minutes. Use the pack builder to scale up from individual cells to full packs, convert between energy units and quickly check how long a UPS or inverter bank might support a given load.
1. Battery Runtime – How Long Will My Battery Last?
The runtime tab answers the classic question: “If my device draws this much power, how long will my battery last?” You provide:
- Battery voltage (V)
- Battery capacity and unit (Ah, mAh or Wh)
- Load power in watts or load current in amps
- Usable capacity percentage (to avoid deep discharge)
- System efficiency (to account for conversion and wiring losses)
Internally, the calculator converts the battery capacity into watt-hours, applies the usable capacity and efficiency and divides by your effective load in watts. The result is an estimated runtime in hours and an easy-to-read hours:minutes format.
2. Pack Builder – Series and Parallel Cell Combinations
Many real-world battery packs combine cells in series and parallel to reach a desired voltage and capacity. The pack builder tab lets you enter:
- Cell nominal voltage
- Cell capacity (Ah or mAh)
- Series count (S)
- Parallel count (P)
- Optional load power and efficiency to estimate runtime
From there, the calculator computes pack voltage (cell voltage × series count), total capacity (cell capacity × parallel count), total energy in watt-hours and total cell count. If you specify a load, it also estimates how long that pack could run the load under ideal conditions.
3. Charging Time – How Long to Charge This Battery?
The charging tab gives a rough estimate of how long it will take to charge a battery between two states of charge. You specify:
- Battery capacity and unit (Ah or mAh)
- Charge current in amps
- Start and end state of charge (SOC) in percent
- Charging efficiency (to account for heat and taper)
The calculator determines how many ampere-hours you need to add and divides by the effective charge current after efficiency losses. It returns time in hours and hours:minutes. Real-world CC/CV profiles and BMS behavior can add time, especially near full charge, so this is a planning estimate rather than a guarantee.
4. Wh ↔ Ah ↔ mAh Conversion – Quickly Move Between Units
Battery datasheets and product listings may quote capacity in Ah, mAh or Wh. The conversion tab helps you translate between these formats by using:
- Wh = V × Ah
- Ah = Wh ÷ V
- mAh = 1000 × Ah
You enter voltage and whichever capacity value you know (Wh, Ah or mAh). The calculator detects the most likely starting point, performs the conversions and displays all three values with the precision you choose.
5. UPS / Inverter Runtime – Backup Power Planning
For backup power systems, the most important question is how long your battery bank can support a given load. The UPS/inverter tab asks for:
- Total battery bank voltage
- Battery bank capacity and unit (Ah, mAh or Wh)
- Inverter efficiency
- Continuous load power in watts
- Target runtime to estimate maximum safe load
It converts capacity and voltage into total stored energy, applies the inverter efficiency and then estimates runtime for your specified load. It also calculates how large a continuous load you could support for at least the target number of hours, which is helpful when sizing systems for critical equipment.
What Affects Real-World Battery Runtime?
The math in this calculator assumes ideal conditions: constant voltage, constant load, fresh batteries and moderate temperature. In real use, several factors reduce runtime compared with the theoretical estimate:
- Battery age and cycle count: Capacity drops as cells age and are cycled.
- Discharge rate: Very high load currents reduce usable capacity, especially for lead-acid batteries.
- Temperature: Cold temperatures reduce available capacity; extreme heat accelerates aging.
- Inverter and converter losses: DC/DC and DC/AC stages always waste some energy as heat.
- Cutoff voltages and BMS limits: Protection circuits may stop discharge before every last bit of energy is used.
Using conservative values for usable capacity and efficiency helps you design systems that perform reliably under real-world conditions.
How to Use This Battery Capacity Calculator Effectively
- Start with the Battery Runtime tab to understand how long a given battery can power your specific load.
- Use the Pack Builder tab to design packs from cells and check pack voltage and energy.
- Switch to the Charging Time tab when choosing charger current or sizing charge circuits.
- Use the Wh ↔ Ah ↔ mAh tab when comparing datasheets or products with different units.
- Finish with the UPS / Inverter Runtime tab when planning backup systems or off-grid setups.
Always treat the results as estimates and cross-check them with manufacturer datasheets and safety guidelines when designing real systems, especially with high-energy lithium packs or grid-connected inverters.
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Battery & Power FAQs
Frequently Asked Questions About Battery Capacity and Runtime
Quick answers to common questions about battery sizing, runtime and energy units.
Real batteries rarely behave like ideal energy tanks. High load currents, low temperatures, aging cells, inverter losses and conservative BMS cutoffs all reduce runtime compared to the theoretical calculation. Treat the estimate as a best case, and include a safety margin in critical designs.
Generally no. Many chemistries suffer if repeatedly discharged to 0% SOC. Using a usable capacity below 100% (for example, 70–90%) helps maintain cycle life and keeps your estimate realistic. The runtime and UPS tabs let you set this via usable capacity and efficiency settings.
Mixing batteries of different capacity, age or chemistry in the same series or parallel string is not recommended. It can lead to imbalance, premature failure or safety issues. Packs should be built from closely matched cells and managed with an appropriate BMS or charge controller.
For the same total energy in watt-hours, higher voltage systems can reduce current and wiring losses, but runtime depends mostly on total Wh and load power. Voltage choice also affects safety, insulation, component ratings and regulations, so it should be chosen carefully for each application.
Yes, as long as you use correct nominal voltage and realistic capacity values, the math applies to any chemistry. However, each chemistry has different recommended depth of discharge, charge profiles and safety rules, so always follow the manufacturer’s guidance when building or modifying real systems.