How to Calculate Battery Life
This estimator supports two modes: mAh/mA for smartphones and power banks, and Wh/W for laptops and larger electronics. An adjustable efficiency factor accounts for real-world losses from heat, DC conversion, and battery aging.
Formulas
| Mode | Formula | Best for |
|---|---|---|
| mAh / mA | Capacity (mAh) ÷ Current (mA) × Efficiency | Phones, power banks |
| Wh / W | Capacity (Wh) ÷ Power (W) × Efficiency | Laptops, tablets, appliances |
How to Find Current Draw or Power Consumption
A smartphone draws around 200–400 mA during normal use and 800–1,200 mA during gaming. Laptops consume 10–20 W in low-power mode and 45–80 W under load. Check the product spec sheet for "Typical Current" or "Power Consumption." A USB power meter is another quick option for real-world measurement.
When to Lower the Efficiency Value
Fresh batteries in mild temperatures perform close to rated capacity. Reduce efficiency to 70–80% if the battery has more than 300 charge cycles, the temperature is below 50°F (10°C), or you notice noticeably shorter runtime than expected. The default 85% works well for most everyday scenarios.
Frequently Asked Questions
With an 85% efficiency and a 4,000 mAh phone, it provides 10,000 × 0.85 ÷ 4,000 ≈ 2.1 full charges. Conversion losses always reduce the actual number below the theoretical value.
At 15 W average draw and 85% efficiency: 56 ÷ 15 × 0.85 ≈ 3.2 hours. Heavy workloads that push the CPU/GPU to full load will significantly cut this time.
Wh = mAh × Voltage (V) ÷ 1,000. For a typical 3.7 V lithium cell: 5,000 mAh × 3.7 ÷ 1,000 = 18.5 Wh.