🔋 Battery Life & Charging Time: Calculator

Estimate device battery runtime, or full charge time from your charger wattage.

mAh
V
W
ESTIMATED RESULT
10.3 hours
Estimated runtime at 1.5W average draw
Battery Energy
15.4 Wh

How Battery Life & Charging Time Are Calculated

Batteries store energy measured in watt-hours (Wh), calculated from their capacity in milliamp-hours (mAh) and voltage. Once you know a battery's energy capacity, you can estimate how long it will last under a given power draw, or how long it will take to charge from a given charger.

Battery Energy (Wh) = Capacity (mAh) × Voltage (V) ÷ 1000
Battery Life (hours) = Battery Energy (Wh) ÷ Average Power Draw (W)
Charging Time (hours) = Battery Energy (Wh) ÷ (Charger Power (W) × Efficiency)

Why Charging Isn't 100% Efficient

Some energy is always lost as heat during charging due to resistance in the battery and charging circuitry. Typical charging efficiency for consumer lithium-ion devices ranges from about 80-90%, which is why this calculator includes an adjustable efficiency factor.

Typical Power Draw Reference

📎 Related Tools: Electricity Cost Calculator Ohm's Law Calculator Voltage Drop Calculator Capacitance Calculator

❓ Frequently Asked Questions

Why do I need to enter battery voltage?
Battery capacity is often listed only in mAh, but mAh alone doesn't tell you the actual energy stored — you need voltage too, since Energy (Wh) = mAh × Voltage ÷ 1000. Most phone batteries use around 3.7-3.85V nominal.
What charging efficiency should I use?
80-90% is typical for modern lithium-ion charging circuits. Fast chargers and wireless charging can sometimes have lower effective efficiency due to additional heat generated during rapid power transfer.
Why does my actual battery life differ from the estimate?
Real-world power draw varies constantly based on screen brightness, active apps, signal strength, and background processes — this calculator uses a single average draw figure, while actual usage fluctuates throughout the day.
Does fast charging affect these calculations?
Fast charging simply means a higher charger wattage, which this calculator already accounts for — however, many devices intentionally slow down charging speed above 80% battery to protect long-term battery health, which isn't modeled here.
Can I use this for laptop or power bank batteries too?
Yes — as long as you know the capacity (mAh or convert Wh directly) and voltage, the same math applies to laptops, power banks, and other lithium-ion battery devices.

🏆 About This Tool — Accuracy & Trust

🔒 Data Privacy: All calculations run entirely within your browser using JavaScript. Nothing you enter here is ever transmitted to our servers, stored, sold, or shared.

📐 Accuracy Note: This tool uses standard, widely published formulas and guidelines. Results are estimates for informational purposes; for financial, medical, or engineering decisions, consult a licensed professional.

📅 Last Updated: September 2026.