Battery Capacity & Runtime Calculator
Determine your battery’s energy in Watt-hours and estimate its runtime.
The standard voltage of your battery (e.g., 3.7V, 12V, 24V).
The Amp-hour rating, found on the battery’s label.
The total power consumption of the device(s) you will connect.
Total Battery Capacity (Energy)
Estimated Runtime
Capacity in mAh
Chart: Estimated runtime changes based on appliance load.
| Appliance Load (Watts) | Estimated Runtime (Hours) |
|---|---|
| 25 W | 48.0 Hours |
| 50 W | 24.0 Hours |
| 100 W | 12.0 Hours |
| 250 W | 4.8 Hours |
| 500 W | 2.4 Hours |
What is Battery Capacity?
When you need to calculate battery capacity using voltage, you’re really asking “how much energy can my battery store?” While Amp-hours (Ah) tell you about the charge a battery holds, the true measure of its energy capacity is Watt-hours (Wh). Watt-hours account for both the charge (Ah) and the electrical pressure (Voltage), giving a complete picture of the battery’s potential.
This is crucial for comparing different batteries. For example, a 12V, 100Ah battery and a 24V, 50Ah battery both have different voltage and Ah ratings, but they hold the exact same amount of energy (1200 Wh). This calculator helps you understand this total energy and what it means for powering your devices.
Battery Capacity Formula and Explanation
The core principle to calculate battery capacity is a simple multiplication. The formula combines the amp-hour rating with the battery’s nominal voltage.
Primary Formula:
Watt-hours (Wh) = Voltage (V) × Amp-hours (Ah)
To estimate how long the battery will last, you divide its total energy by the power draw of your appliance:
Runtime (Hours) = Watt-hours (Wh) / Load (Watts)
| Variable | Meaning | Unit | Typical Range |
|---|---|---|---|
| Wh | Watt-hours | Energy | 5 – 100,000+ |
| V | Voltage | Electrical Potential | 3.7 – 48 |
| Ah | Amp-hours | Electrical Charge | 1 – 400+ |
| Watts | Load Power | Power Consumption | 1 – 5000+ |
Practical Examples
Example 1: Off-Grid Camper Van Setup
You have a deep cycle battery for your van’s lighting and fridge.
- Inputs:
- Battery Voltage: 12 V
- Battery Capacity: 200 Ah
- Combined Load (fridge, lights, fan): 80 Watts
- Results:
- Total Capacity: 12 V × 200 Ah = 2400 Wh
- Estimated Runtime: 2400 Wh / 80 W = 30 Hours
For a setup like this, you might need an inverter size calculator to ensure you can safely power your AC appliances.
Example 2: Portable Power Station for Electronics
You are charging a laptop and a phone with a portable power station.
- Inputs:
- Battery Voltage (internal): 21.6 V
- Battery Capacity: 24 Ah
- Combined Load (laptop and phone): 75 Watts
- Results:
- Total Capacity: 21.6 V × 24 Ah = 518.4 Wh
- Estimated Runtime: 518.4 Wh / 75 W = ~6.9 Hours
How to Use This Battery Capacity Calculator
- Enter Nominal Voltage: Find the voltage (V) on your battery’s label and enter it.
- Enter Capacity in Ah: Input the Amp-hour (Ah) or milliamp-hour (mAh) rating. If you have mAh, divide by 1000 to get Ah (e.g., 5000 mAh = 5 Ah).
- Enter Appliance Load: Add up the wattage of all devices you’ll be running simultaneously and enter the total in the “Appliance Load” field. If a device lists Amps instead of Watts, use our Ohm’s Law calculator to find the wattage (Watts = Volts x Amps).
- Interpret the Results: The calculator instantly shows the total energy (Watt-hours) and the estimated runtime in hours.
Key Factors That Affect Battery Capacity
- Discharge Rate (C-Rate): Drawing power slowly will yield more energy than drawing power quickly. A battery rated at 100 Ah (at a C/20 rate) might only provide 80 Ah if drained in one hour (1C rate).
- Temperature: Batteries perform best at room temperature (around 25°C or 77°F). Extreme cold can reduce effective capacity by up to 50%, while high heat can increase it temporarily but will permanently damage the battery over time.
- Battery Age and Health: As a battery cycles through charges and discharges, its internal chemistry degrades. An older battery will not hold as much energy as it did when it was new.
- Depth of Discharge (DoD): Regularly draining a battery to 0% will shorten its lifespan. For example, lead-acid batteries should ideally not be discharged below 50%. A proper battery life calculator considers DoD for long-term health.
- Battery Chemistry: Different types like Lithium-ion, Lead-Acid, and LiFePO4 have different voltage curves and are affected by the factors above in different ways. LiFePO4 batteries, for instance, can often be safely discharged to 80-90% without significant lifespan reduction.
- Self-Discharge: All batteries slowly lose charge even when not in use. This rate depends on the chemistry and temperature, but it means a battery will have less than its full capacity after sitting on a shelf for a few months.
Frequently Asked Questions (FAQ)
How do I calculate battery capacity from mAh and voltage?
Use the formula: (mAh * V) / 1000 = Wh. For example, a 3000 mAh, 3.7V battery has (3000 * 3.7) / 1000 = 11.1 Wh.
Is Watt-hours or Amp-hours a better measure of capacity?
Watt-hours (Wh) is a better measure of total energy because it includes voltage. Amp-hours (Ah) is only a measure of charge and can be misleading when comparing batteries with different voltages.
Why is my actual runtime less than the calculated estimate?
This calculator provides an ideal estimate. Real-world factors like inverter inefficiency (which can waste 10-15% of the power), temperature, battery age, and a high discharge rate will reduce your actual runtime.
Can I calculate battery capacity with just voltage?
No, voltage alone is not enough. You must also know the Amp-hour (Ah) rating to calculate the total energy in Watt-hours (Wh). Voltage tells you the pressure, but Ah tells you how much charge is available.
What does the C-Rate of a battery mean?
The C-Rate describes how quickly a battery is discharged relative to its maximum capacity. 1C means the battery is fully discharged in one hour. 0.5C (or C/2) means it is discharged in two hours.
How does a deep cycle battery differ from a car battery?
A deep cycle battery is designed to provide steady power over a long period and be deeply discharged, making it ideal for campers or solar systems. A car battery is designed to provide a large burst of current for a short time to start an engine.
Does this calculator work for all battery types (Li-ion, Lead-Acid)?
Yes, the fundamental formula (Wh = V x Ah) is the same for all battery chemistries. However, the usable capacity and lifespan considerations differ significantly between types. For example, it is recommended not to discharge a lead-acid battery below 50% of its capacity.
How can I estimate the power consumption of my house?
Estimating total power consumption requires checking the wattage of all your appliances and considering how long you use each one. Our power consumption guide explains this in detail.
Related Tools and Internal Resources
Explore our other calculators and guides to build a complete understanding of your power needs.
- Solar Panel Calculator: Determine how many solar panels you need to charge your batteries.
- Inverter Size Calculator: Find the right inverter to power your AC devices from a DC battery.
- Battery Life Calculator: Estimate the lifespan of your battery in charge cycles based on usage.
- Ohm’s Law Calculator: A fundamental tool for understanding the relationship between voltage, current, and resistance.
- Power Consumption Guide: Learn how to estimate the energy usage of your appliances.
- Deep Cycle Batteries: Explore options for robust, long-lasting power storage.