Battery Capacity Calculator - Free Online Tool

Calculate runtime, charge time, and battery bank configurations

A battery capacity calculator is an essential tool for anyone working with portable power — whether you are designing an off-grid solar system, building a DIY electric vehicle, specifying a backup power supply, or simply trying to figure out how long your portable speaker will last on a single charge. Understanding battery capacity requires more than just the Ah (ampere-hour) number printed on the label — it also depends on voltage, depth of discharge, efficiency losses, and the actual power demand of your load. Battery capacity is typically measured in ampere-hours (Ah) or watt-hours (Wh). A 100Ah battery at 12V contains 1,200 Wh of energy, but that does not mean you can use all 1,200 Wh every time you discharge. The useful capacity depends on two critical factors: depth of discharge (DoD) and round-trip efficiency. Lead-acid batteries, for example, should only be discharged to 50% DoD to preserve cycle life, whereas lithium iron phosphate (LiFePO4) batteries can comfortably reach 90% DoD. Efficiency losses during discharge — due to internal resistance and heat — further reduce the energy that actually reaches your load. The runtime formula combines all these factors: Runtime (hours) = (Capacity_Ah × Voltage × DoD × Efficiency) / Load_Watts. For a 100Ah, 12V lithium battery with 80% DoD, 90% efficiency, powering a 50-watt load, the runtime would be approximately 17.3 hours. The same battery with a lead-acid chemistry profile (50% DoD, 80% efficiency) would only deliver about 9.6 hours — nearly half as long — even though the nominal capacity is identical. This distinction is crucial when comparing battery technologies by cost or physical size. Charge time is equally important for planning. The basic formula is: Charge Time (hours) = Capacity_Ah / Charge_Current_A. A 100Ah battery charged at 10A takes about 10 hours, but in practice, charging efficiency means you need to supply slightly more energy than the battery actually stores. The C-rate concept describes current relative to capacity — a 1C rate for a 100Ah battery is 100A of current, charging or discharging in roughly one hour. Most lithium cells are rated for 0.5C to 1C continuous charge, while lead-acid batteries prefer 0.1C to 0.2C to avoid overheating and plate damage. For larger power systems, batteries are connected in series and parallel configurations. Series connections increase voltage while keeping capacity the same — two 12V 100Ah batteries in series give 24V at 100Ah. Parallel connections increase capacity while keeping voltage constant — two 12V 100Ah batteries in parallel give 12V at 200Ah. Combined configurations like 2S4P (2 series, 4 parallel) allow you to hit any voltage and capacity target. Understanding the total energy in watt-hours helps compare configurations regardless of voltage. Common applications for this calculator include: dimensioning solar battery banks to cover overnight loads, calculating how many power banks you need for a camping trip, specifying UPS (uninterruptible power supply) run times for servers, designing EV conversion battery packs, and estimating how many charges a portable battery pack can provide to a smartphone or laptop. Each application has different DoD requirements, temperature operating ranges, and cycle life considerations, making it important to match the battery chemistry to the use case. Battery technology continues to evolve rapidly. LiFePO4 (lithium iron phosphate) has become the dominant choice for stationary storage due to its exceptional cycle life (3,000–5,000 cycles), thermal stability, and tolerance for full discharge. Li-ion NMC and NCA cells offer higher energy density and are preferred for mobile applications where weight matters. Lead-acid retains a role in automotive starting and low-cost backup applications. Understanding the efficiency and DoD characteristics of each chemistry is the foundation for accurate capacity planning.

Understanding Battery Capacity

What Is Battery Capacity?

Battery capacity is the total amount of electrical energy a battery can store and deliver. It is commonly expressed in ampere-hours (Ah), which describes how many amps a battery can supply for how many hours. A 100Ah battery can theoretically supply 10A for 10 hours, or 1A for 100 hours. In practice, Watt-hours (Wh) is the more universal measure because it accounts for voltage — a 100Ah, 12V battery stores 1,200 Wh, whereas a 100Ah, 3.7V lithium cell stores only 370 Wh. Capacity ratings are typically given for a specific discharge rate (e.g., C/20), meaning the battery is discharged to empty over 20 hours. Higher discharge rates reduce the usable capacity due to the Peukert effect, particularly in lead-acid chemistry.

How Is Runtime Calculated?

Runtime is calculated by dividing available energy by load power. The full formula is: Runtime (h) = (Capacity_Ah × Voltage × DoD × Efficiency) / Load_Watts. Depth of Discharge (DoD) specifies the fraction of capacity that can be safely used — 80% DoD means you can use 80% of the rated capacity before recharging to protect battery longevity. Efficiency accounts for energy lost as heat during discharge, typically 80–95% depending on chemistry. If you know the battery capacity in Wh directly, the formula simplifies to: Runtime (h) = (Capacity_Wh × DoD × Efficiency) / Load_Watts. The C-Rate describes the discharge current as a multiple of capacity — at 1C, a 100Ah battery discharges in about one hour.

Why Does Battery Chemistry Matter?

Different battery chemistries have significantly different performance profiles. Lead-acid batteries are inexpensive and reliable but should only be discharged to 50% DoD to achieve their rated 200–300 cycle life. Li-ion NMC/NCA cells offer high energy density and 80% DoD, making them ideal for mobile devices and EVs. LiFePO4 (lithium iron phosphate) supports 90% DoD with 3,000–5,000 cycles and excellent thermal stability, making it the top choice for solar and backup systems. NiMH and NiCd are largely legacy technologies but still appear in power tools and older electronics. Choosing the wrong chemistry for your application can result in premature battery failure or fire risk.

Einschränkungen und reale Faktoren

Battery capacity calculations are theoretical estimates. Real-world performance is affected by several factors. Temperature significantly impacts capacity — most lithium batteries lose 20–30% capacity at 0°C and can be damaged by charging below freezing. Battery age reduces capacity over time — expect 80% of original capacity after the rated cycle count. Self-discharge rates mean batteries lose charge even when not in use. The Peukert effect causes lead-acid batteries to lose usable capacity at high discharge rates. Voltage sag under load means actual terminal voltage is lower than nominal. Always add a 20–30% safety margin to your calculations for real-world deployments.

Key Formulas

Energy in Watt-Hours

Wh = V × Ah

Total energy stored equals nominal voltage times capacity in ampere-hours. A 12V, 100Ah battery stores 1,200 Wh.

Battery Runtime

Runtime (h) = (Ah × V × DoD × η) / Load_W

Usable runtime accounts for depth of discharge (DoD) and discharge efficiency (η). This gives the real-world hours, not the theoretical maximum.

Energy Density

Wh/kg = Wh / Weight (kg)

Gravimetric energy density measures how much energy a battery stores per unit of weight. Li-ion NMC achieves ~250 Wh/kg; lead-acid only ~35 Wh/kg.

C-Rate

C-rate = I / Capacity (Ah)

Expresses charge or discharge current relative to battery capacity. 1C for a 100Ah battery is 100A (full discharge in ~1 hour). Most Li-ion cells are rated for 0.5C–1C continuous charge.

Reference Tables

Battery Chemistry Comparison

Key performance characteristics of common rechargeable battery chemistries for capacity planning and technology selection.

Chemistry	Nominal Voltage	Energy Density (Wh/kg)	Recommended DoD	Cycle Life (at rated DoD)	Round-Trip Efficiency
Li-ion NMC/NCA	3.6–3.7 V	150–260	80%	500–1,000	92–98%
LiFePO4 (LFP)	3.2 V	90–160	90%	3,000–5,000	95–98%
Lead-Acid (FLA)	2.0 V / 12V pack	30–40	50%	200–300	75–85%
Lead-Acid (AGM)	2.0 V / 12V pack	30–40	50%	300–500	80–85%
NiMH	1.2 V	60–120	80%	500–1,000	66–80%
NiCd	1.2 V	40–60	80%	1,000–2,000	70–80%

Common Battery Sizes and Capacities

Typical capacity and voltage for standard battery form factors used in consumer electronics and DIY projects.

Form Factor	Chemistry	Nominal Voltage	Typical Capacity	Energy (Wh)
AA	NiMH	1.2 V	2,000–2,800 mAh	2.4–3.4
AA	Alkaline (primary)	1.5 V	2,000–3,000 mAh	3.0–4.5
18650	Li-ion	3.6 V	2,500–3,500 mAh	9.0–12.6
21700	Li-ion	3.6 V	4,000–5,000 mAh	14.4–18.0
26650	LiFePO4	3.2 V	3,000–3,600 mAh	9.6–11.5
12V Group 24	Lead-Acid	12 V	70–85 Ah	840–1,020
12V Group 31	Lead-Acid / AGM	12 V	95–125 Ah	1,140–1,500
48V Server Rack	LiFePO4	51.2 V (16S)	100–200 Ah	5,120–10,240

Worked Examples

Energy in a 3.7V 3000mAh Li-ion Cell

A standard 18650 Li-ion cell is rated at 3.7V nominal and 3,000 mAh. Calculate its energy in watt-hours.

Convert mAh to Ah: 3,000 mAh = 3.0 Ah

Apply energy formula: Wh = V × Ah = 3.7 × 3.0 = 11.1 Wh

For comparison, a phone battery (3.85V, 4,500 mAh) stores 3.85 × 4.5 = 17.3 Wh

The 18650 cell stores 11.1 Wh of energy. A 4-cell pack (4S1P at 14.8V) would store 44.4 Wh — enough to charge a typical smartphone about 2.5 times accounting for conversion losses.

Runtime of a 100Wh Battery at 20W Load

A portable power station is rated at 100Wh. You connect a 20W LED light. Estimate the runtime assuming 90% inverter efficiency.

Usable energy with 90% efficiency: 100 × 0.90 = 90 Wh

Runtime: 90 Wh / 20 W = 4.5 hours

If the power station uses LiFePO4 with 90% DoD, it may already account for DoD in the 100Wh rating

If the 100Wh is total (not usable): Runtime = (100 × 0.90 × 0.90) / 20 = 4.05 hours

The LED light runs for approximately 4 to 4.5 hours depending on whether the rated capacity includes DoD. Always check if the manufacturer quotes total or usable energy.

Sizing a Solar Battery Bank for Overnight Load

An off-grid cabin uses 1,500 Wh overnight (10 hours). Design a 12V LiFePO4 battery bank with 90% DoD and 95% efficiency.

Required usable energy: 1,500 Wh

Account for efficiency: 1,500 / 0.95 = 1,579 Wh total energy needed

Account for DoD: 1,579 / 0.90 = 1,754 Wh total battery capacity needed

Convert to Ah at 12V: 1,754 / 12 = 146.2 Ah

Choose standard batteries: Two 12V 100Ah LiFePO4 batteries in parallel = 200Ah (2,400 Wh)

Safety margin: 2,400 / 1,500 = 1.6× — provides 60% margin for cloudy days

Two 12V 100Ah LiFePO4 batteries in parallel (2,400 Wh total) provide 1,500 Wh usable energy with a healthy 60% margin for reduced solar charging on overcast days.

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.

Ergebnisse überprüfen und exportieren

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.

Häufig gestellte Fragen

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.