Battery Capacity Calculator

Battery Capacity Explained: Ah, Wh, and What They Really Tell You

Battery capacity is one of those specs that looks simple until you try to use it for planning. A battery label might say “12V 100Ah” or a power bank might say “20,000mAh.” Those numbers are useful, but they are not the same kind of measurement. Amp-hours (Ah) and milliamp-hours (mAh) describe charge capacity, while watt-hours (Wh) describe energy capacity. Energy is what you actually spend when you run a device, power a light, drive an inverter, or keep equipment running during a blackout.

The easiest way to keep it straight is this: current is “how fast charge flows,” voltage is “how much push the charge has,” and watts are “how fast energy is being used.” Battery capacity becomes meaningful for runtime only when you connect the battery’s capacity to its voltage and your device’s power demand. This calculator brings those pieces together so you can convert between units, estimate runtime, size a battery bank, and estimate charging time with practical adjustments.

Why Ah Alone Can Be Misleading Without Voltage

Ah is a measure of charge. Two batteries can have the same Ah rating but store different amounts of energy if their voltages differ. A 12V 100Ah battery stores about twice the energy of a 6V 100Ah battery, because energy scales with voltage. That is why the conversion formula is so important:

Wh = Ah × V

If you only remember one formula for battery capacity planning, it should be this one. Once you know Wh, you can compare batteries across voltages, compare chemistry options more fairly, and estimate runtime against a device load in watts. You can also convert a power bank’s mAh rating into Wh if you know the nominal cell voltage, which is usually around 3.6–3.7V for lithium-ion cells (many USB power banks step that voltage up internally).

mAh vs Ah: The Same Measurement at Different Scales

mAh is simply Ah multiplied by 1000. Manufacturers often use mAh for smaller batteries because it produces a bigger-looking number. A 5000mAh phone battery is 5Ah. That does not mean it can deliver 5 amps for one hour at any voltage; it means the battery can deliver 5 amp-hours at its cell voltage. To translate that into energy you can use, convert to Wh using the battery’s nominal voltage.

This calculator’s conversion tab helps you quickly move between mAh, Ah, and Wh, and it can scale totals when you have multiple identical batteries in a pack.

Nominal Voltage vs Real Voltage: Why Labels Don’t Stay Constant

Batteries are not perfect voltage sources. Their voltage changes depending on state of charge, current draw, temperature, and chemistry. A “12V” lead-acid battery might be about 12.7V when full at rest, closer to 12.0V under moderate load, and lower as it approaches empty. A “12V” lithium iron phosphate (LiFePO₄) pack is often labeled 12.8V nominal because the cells behave differently. For capacity planning, using the nameplate nominal voltage keeps estimates consistent.

When you want more precision, you can adjust voltage to match your real operating conditions. The formulas still work; you are just choosing a voltage that better reflects your setup.

Usable Energy: Depth of Discharge and Efficiency

A battery’s total stored energy is not always the energy you should plan to use. Two practical settings shape “usable energy”:

• Depth of Discharge (DoD): how much of the battery you intend to use before recharging. Using 100% DoD means draining the battery fully. Using 80% DoD means using only 80% of its energy and leaving a reserve.
• System efficiency: energy lost in wiring, converters, inverters, and the battery itself. For DC loads connected directly to the battery, efficiency may be high. For AC loads powered through an inverter, efficiency is typically lower.

The runtime estimator in this tool applies DoD and efficiency to the total battery energy before estimating runtime. That turns a “best case” nameplate capacity into a more realistic planning number.

Estimating Runtime from Battery Capacity

Runtime is fundamentally an energy division problem:

Runtime (hours) ≈ usable Wh ÷ load W

If you have 1000Wh usable energy and your device uses 100W, you get roughly 10 hours. If your device uses 250W, you get about 4 hours. This is why converting to Wh is so helpful: it lets you compare load and battery on the same energy scale.

In the Runtime Estimator tab, you can enter your battery as Ah + voltage or as Wh directly. You can also model series and parallel wiring so the calculator can compute total bank voltage, total energy, and usable energy. Then you can enter the load as watts (common for appliances) or as amps at the system voltage (common for DC equipment).

Why Series and Parallel Wiring Matters

Battery banks are often built by combining multiple batteries. Understanding what changes in series vs parallel prevents common sizing mistakes:

• Series increases voltage and keeps Ah the same. Two 12V 100Ah batteries in series become 24V 100Ah.
• Parallel increases Ah and keeps voltage the same. Two 12V 100Ah batteries in parallel become 12V 200Ah.

In both cases, total energy in Wh adds up. Two identical batteries always store roughly double the energy of one battery, regardless of whether you wire them in series or parallel. The choice is mostly about matching your system voltage, inverter requirements, and current levels in wiring.

Battery Bank Sizing: From Load and Hours to Number of Batteries

Battery bank sizing starts with a simple question: “How many watts will I use, and for how many hours?” Multiply watts by hours to get energy in Wh. Then adjust for DoD and efficiency to avoid sizing a bank that only meets the goal in ideal conditions.

The Battery Bank Sizing tab computes:

• Required energy = load W × hours
• Adjusted energy = required energy ÷ (efficiency × DoD)
• Recommended series count based on your target system voltage and your battery voltage
• Recommended parallel strings based on energy per series string

This approach produces a practical “how many batteries” estimate and gives you a configuration that makes sense for 12V, 24V, or 48V systems, while still allowing a custom target voltage if needed.

Planning for Surges, Peaks, and Real-World Loads

Many loads are not steady. Refrigerators cycle, pumps surge at startup, power tools draw bursts, and inverters have idle consumption. These behaviors can reduce runtime more than expected. A watt meter, smart plug, or inverter telemetry can help you learn an appliance’s average power over time. Once you know the realistic average watts, runtime estimates become much more accurate.

If your system is powering motor loads, you should also consider that short peak currents can stress batteries and reduce effective capacity. This is especially important with lead-acid batteries under high draw, where the usable capacity can drop noticeably compared with the “20-hour rate” Ah rating printed on the label.

Lead-Acid vs Lithium: Why Chemistry Affects Your Results

Chemistry changes how a battery behaves under load, how deeply you can cycle it, and how efficient it is:

• Lead-acid batteries often have lower usable DoD for longevity and can lose effective capacity at high discharge currents. Voltage sag is more pronounced, and recharging can take longer near full due to absorption phases.
• Lithium batteries often maintain voltage more steadily during discharge and typically have higher charge/discharge efficiency. Many lithium systems also allow deeper cycling depending on the battery management system (BMS) and the manufacturer’s recommendations.

This calculator lets you set DoD and efficiency explicitly so you can model different chemistries. For planning, a conservative DoD and realistic efficiency often produce better decisions than aiming for best-case numbers.

Charging Time: Why It’s Not Just Capacity ÷ Charger Amps

A common rule of thumb says charging time is “Ah divided by charger amps.” That is a starting point, but real charging is not perfectly linear. Many chargers deliver constant current early on and then taper the current as the battery approaches full. Heat, internal resistance, and charger limits also matter. That’s why practical estimates often include a multiplier.

The Charging Time tab estimates how many Ah you need to replace based on start and end state of charge, divides by charger current, and then applies a charge factor. A charge factor around 1.10–1.30 is often used to account for taper and inefficiency, but the correct value depends on battery chemistry and charger type.

Energy Cost Estimation: Turning kWh into Currency

If you want to estimate how much electricity costs to run a device or to recharge energy, you need kWh. One kWh is 1000Wh. This calculator includes an optional cost estimate in the runtime tab: you can choose to base cost on the load’s energy usage over a chosen number of hours, or on battery energy used. Then it multiplies kWh by your tariff and formats the result in your chosen currency.

Cost estimates are most useful when you use realistic average power and a realistic time window. They are less useful when you use maximum nameplate values that only occur briefly.

How to Use This Calculator for Common Scenarios

If you are converting a battery label into energy, use the Capacity & Conversion tab. Enter Ah (or mAh) and voltage to get Wh and kWh. If you have multiple identical batteries, set the count to see total energy.

If you are trying to estimate how long a battery will run a load, use the Runtime Estimator tab. Enter your bank configuration (series/parallel), set DoD and efficiency, and enter your load as watts or amps. The calculator returns total and usable energy and the estimated runtime.

If you are designing a bank for a target number of hours, use the Battery Bank Sizing tab. Enter load watts, desired hours, DoD, efficiency, and your battery’s specs. Select your target system voltage and the calculator suggests series and parallel counts.

If you are planning charging logistics, use the Charging Time tab. Enter capacity, start and end state of charge, charger amps, and a charge factor to get a practical time estimate.

Limitations and How to Get Closer to Your Real System

This tool is designed for clear planning and quick estimates. It does not simulate every battery curve, temperature effect, or inverter behavior. If your results need to match a specific system closely, refine inputs using real measurements: measure average watts, confirm inverter efficiency at your load level, and adjust DoD to match your battery’s recommended limits. Also remember that old batteries often deliver less than rated capacity.

The most reliable approach is iterative: start with conservative settings, compare with real usage, and then adjust efficiency and load assumptions until your estimates match your actual experience.