Battery Life Calculator: How Long Will It Last?
The calculator for this page is switched off while it is checked. The formula is below, and the worked example in the article shows how to use it by hand.
A battery life calculator estimates how long a battery will power a device based on its capacity, the device’s power draw, and efficiency. You enter the battery capacity (in mAh or Wh), the average power consumption (in watts or mA), and optionally the device’s efficiency, and the calculator returns the expected runtime in hours and minutes. This page explains the formula, how to use it, and common pitfalls.
Formula
The core formula converts battery capacity from milliamp-hours (mAh) to watt-hours (Wh), then divides by the device’s power draw, adjusting for efficiency. In symbols:
Runtime (hours) = (Capacity_mAh × Voltage_V / 1000) × (Efficiency_% / 100) / Power_W
- Capacity_mAh – battery charge, in milliamp-hours (mAh). Common for phone batteries: 3000–5000 mAh.
- Voltage_V – nominal battery voltage, in volts (V). Lithium-ion cells are typically 3.7 V.
- Power_W – average power the device consumes, in watts (W).
- Efficiency_% – fraction of stored energy actually delivered to the device (0–100%). 100% means no loss.
The product (Capacity × Voltage / 1000) gives the total energy in watt-hours (Wh). Dividing by power gives hours, and multiplying by efficiency adjusts for real-world losses.
Worked Example
Using the default values from the calculator: a 3000 mAh battery at 3.7 V powers a device drawing 1.5 W with 90% efficiency.
- Convert capacity to watt-hours: 3000 mAh × 3.7 V / 1000 = 11.1 Wh.
- Adjust for efficiency: 11.1 Wh × 0.90 = 9.99 Wh usable.
- Divide by power draw: 9.99 Wh / 1.5 W = 6.66 hours.
The calculator outputs 6.66 hours (rounded to 2 decimals). The energy capacity is 11.10 Wh, and the current draw is 405 mA (1.5 W / 3.7 V × 1000).
When to Use This Calculator
Use it to estimate runtime for a wide range of devices, from smartphones and laptops to LED lights, RC cars, or DIY electronics projects. It is especially handy when you are choosing a battery for a project or comparing two batteries with different voltages and capacities. The calculator works best for devices with a roughly constant power draw. For devices that vary (like a motor that speeds up and slows down), use the average power over a typical cycle.
How to Read the Result
The result is the theoretical runtime under ideal conditions. Real-world runtime is often shorter because of factors like:
- Battery age – capacity fades over time and with charge cycles.
- Temperature – extreme cold or heat reduces effective capacity.
- Discharge rate – drawing high current can lower the usable capacity (Peukert’s law for lead-acid).
- Device efficiency – voltage converters and regulators waste some energy.
If the result seems too long or too short, double-check the units. A common mistake is entering capacity in Ah instead of mAh, or power in mW instead of W. Always verify that the voltage matches the battery’s nominal voltage—a 3.7 V lithium cell is not the same as a 1.5 V AA battery.
Choosing the Right Inputs
To get a useful estimate, you need accurate numbers. Here’s where to find them:
- Battery capacity – printed on the battery or in the device manual. For phones, it’s often in the specs; for laptops, it may be in the battery report.
- Voltage – for single-cell lithium batteries, use 3.7 V (nominal). For multi-cell packs, multiply by the number of cells in series (e.g., 2S = 7.4 V).
- Power draw – measure it with a USB power meter or multimeter, or use the device’s rated wattage. For example, a typical smartphone in active use draws about 1–2 W, while a laptop might draw 30–60 W.
- Efficiency – if you don’t know, use 85–95% for most electronics. For simple resistive loads (like an incandescent bulb), use 100%.
Limitations of the Formula
The linear formula assumes that the battery delivers its full rated capacity at any current. In reality, high discharge rates reduce usable capacity, especially for lead-acid and alkaline batteries. For lithium-ion, the effect is smaller but still present. Also, the battery’s voltage drops as it discharges, so the actual power delivered changes over time. The formula gives a good approximation, but not a guarantee.
Another limitation: the efficiency factor lumps together all losses, but those losses vary with load. A device that draws 10 W might have a different efficiency than one drawing 1 W. For the most accurate estimate, measure the actual current draw with a multimeter and use the measured voltage under load.
Alternatives and Related Calculations
If you know the current draw in milliamps (mA) instead of power in watts, you can use a simpler formula: Runtime (hours) = Capacity (mAh) / Current (mA). This works when the voltage is the same on both sides, which is often true for small devices. For example, a 3000 mAh battery powering a device that draws 200 mA would last 15 hours.
For solar or rechargeable systems, you might also want to calculate charging time: Charging time ≈ Battery capacity (Wh) / Charging power (W) × 1.3 (to account for losses). But that’s a separate topic.
Frequently Asked Questions
Why does my phone’s battery drain faster than the calculator says?
Phones have variable power draw—screen brightness, network usage, and apps all affect it. The calculator uses average power, but if you’re gaming or streaming, the actual draw is higher. Also, battery capacity degrades with use, so an old battery won’t hold the rated mAh.
Can I use this calculator for a laptop?
Yes, but you need to enter the laptop’s battery capacity in mAh or Wh and the average power draw. Laptop batteries often have voltage around 11.1 V (3 cells) or 14.8 V (4 cells). Check the battery label for the Wh rating; if you have Wh, you can skip the voltage and just divide by power draw.
What is the difference between mAh and Wh?
mAh measures charge, while Wh measures energy. Wh = mAh × V / 1000. Two batteries can have the same mAh but different Wh if they have different voltages. For example, a 3000 mAh at 3.7 V has 11.1 Wh, while a 3000 mAh at 7.4 V has 22.2 Wh—twice the energy.
How accurate is this calculator?
It’s accurate for steady loads and fresh batteries. Real-world accuracy is typically within 10–20% if you use correct inputs. For critical applications (like medical devices), always test with a real battery and measure runtime.
What if my device uses AC power?
If the device runs on AC, it’s not directly powered by a battery. But if you’re using an inverter, you can still estimate runtime: first, find the device’s wattage (on the label), then add inverter losses (typically 10–20% efficiency loss). For example, a 100 W device with a 90% efficient inverter draws about 111 W from the battery.