Battery Capacity Calculator
Leave blank to auto-calculate C-Rate from load and voltage
Enter Battery Specs
Select a tab, enter your battery capacity, voltage, and load to see runtime, charge time, or bank configuration results.
How to Use the Battery Capacity Calculator
Choose a Calculator Tab
Select Runtime Calculator to find how long your battery will last, Charge Time to estimate how long recharging takes, or Battery Bank to design a series/parallel configuration. Each tab has its own set of inputs tailored to that calculation.
Select Battery Chemistry and Enter Specs
Click a chemistry preset (Li-ion, LiFePO4, Lead Acid, NiMH, or NiCd) to automatically fill in the recommended Depth of Discharge and efficiency values for that technology. Then enter your battery capacity in Ah or Wh, and the nominal voltage.
Enter Your Load or Charge Current
For the Runtime tab, choose a device preset or enter your load in watts. For the Charge tab, enter the charge current from your charger's label. Fine-tune the DoD and efficiency sliders if you have specific values from the battery datasheet.
Review Results and Export
The calculator shows runtime in hours and minutes with an energy breakdown chart — usable, efficiency loss, and reserved capacity. Use the Export CSV button to download your calculation for documentation or comparison across multiple battery options.
Frequently Asked Questions
What is the difference between Ah and Wh for battery capacity?
Ampere-hours (Ah) measures charge — how many amps a battery can deliver over time. Watt-hours (Wh) measures energy — how many watts a battery can deliver over time, accounting for voltage. To convert: Wh = Ah × Voltage. A 100Ah battery at 12V stores 1,200 Wh, while a 100Ah battery at 3.7V stores only 370 Wh. For comparing batteries of different voltages — such as a 12V lead-acid versus a 48V lithium pack — Wh is the correct comparison metric. Most portable power stations advertise in Wh for this reason. This calculator accepts both Ah and Wh input for the Runtime tab.
Why does DoD matter for battery runtime?
Depth of Discharge defines how much of the rated capacity you can safely use before recharging. Using 100% of a lead-acid battery's capacity can reduce its cycle life from 300 cycles to under 50 cycles. By limiting DoD to 50%, the same battery may last 500+ cycles. LiFePO4 batteries tolerate 90% DoD without significant cycle life reduction, giving them a practical energy advantage far beyond what their nominal Ah rating suggests. Always use the manufacturer's recommended DoD for your chemistry — it is not just a safety guideline but the key to long battery life. Our presets use industry-standard DoD values for each chemistry type.
How is C-Rate used in battery calculations?
C-Rate expresses charge or discharge current as a fraction of capacity. A 1C discharge rate for a 100Ah battery is 100A — it would theoretically drain the battery in one hour. A 0.5C rate is 50A, expected runtime about two hours. C-Rate matters because many batteries have maximum continuous discharge ratings (e.g., 2C for a lithium pack) and optimal charge rates (typically 0.5C for lead-acid, 0.5–1C for Li-ion). Exceeding the maximum C-rate can cause overheating, voltage sag, and reduced capacity. The runtime calculator computes the effective C-rate from your load and battery specs, helping you verify you are within safe operating limits.
What is the best battery chemistry for solar storage?
LiFePO4 (lithium iron phosphate) is widely considered the best chemistry for stationary solar storage as of 2025. It supports 90% DoD, 3,000–5,000 cycles at that depth, has excellent thermal stability with no thermal runaway risk, works in temperatures from -20°C to 60°C, and has high round-trip efficiency of 95–98%. Lead-acid (AGM or gel) is a lower-cost alternative but requires 50% DoD and lasts only 300–500 cycles, making the lifetime cost higher. NMC/NCA lithium is used in some portable systems for its higher energy density, but it is less stable thermally and requires a more sophisticated battery management system.
How do I calculate the charge time for my battery?
Charge time is calculated by dividing battery capacity by charge current: Time (h) = Capacity_Ah / Current_A. For a 100Ah battery charged at 20A, that is 5 hours at 100% efficiency. In practice, charger efficiency is 85–95%, so the actual time is slightly longer. The calculator adjusts for charge efficiency automatically. Additionally, most chargers use a constant-current / constant-voltage (CC/CV) profile — the last 20% of charge takes proportionally longer as the charger switches to CV mode and current tapers. For planning purposes, add about 10–20% to the calculated time for a complete, balanced charge.
How do series and parallel battery connections work?
Series connections (S) increase voltage while keeping capacity (Ah) the same. Two 12V 100Ah batteries in series give 24V at 100Ah — useful for 24V or 48V systems. Parallel connections (P) increase capacity while keeping voltage constant. Two 12V 100Ah batteries in parallel give 12V at 200Ah — useful for extending runtime. Combined configurations like 4S2P mean 4 batteries in series, then 2 of those series strings in parallel, giving 4× voltage and 2× capacity. Total energy in Wh is always V_total × Ah_total. For safety, always use identical batteries of the same age, capacity, and chemistry when connecting in parallel to avoid imbalanced current sharing.