Battery Life Calculator
Enter the battery capacity and the device's power consumption to estimate how many hours the battery will last. Supports both milliamp-hour (mAh) with voltage, and watt-hour (Wh) capacity.
Battery Life Calculator
Estimate battery runtime
Battery Life: - hours
Equivalent days: - days
Battery life (hours) = battery capacity (mAh) × voltage (V) ÷ device power consumption (mW), or directly battery life (hours) = watt-hour (Wh) ÷ device power consumption (W).
How Battery Life Calculation Works
Battery life estimation is based on the fundamental relationship between energy, capacity, and power:
**Energy (Watt-hours)**: Wh = capacity_mAh × voltage_V ÷ 1000. This is the total energy stored in the battery.
**Power Consumption**: The device's typical electrical power draw in watts (W).
**Battery Life Formula**: - If using mAh + voltage: life_hours = (capacity_mAh × voltage_V) ÷ power_mW Note: power_mW = power_W × 1000, so this simplifies to life_hours = (capacity_mAh × voltage_V) ÷ (power_W × 1000) = (capacity_mAh × voltage_V ÷ 1000) ÷ power_W = Wh ÷ power_W - If using Wh directly: life_hours = Wh ÷ power_W
**Real-World Considerations**: - Battery capacity degrades over time (age, charge cycles). - Power consumption varies with usage (screen on/off, cellular signal strength, background apps). - Temperature affects battery chemistry - extreme heat or cold reduces effective capacity. - Manufacturers' mAh ratings are typically at a standard discharge rate; real-world use may differ. - This calculator provides theoretical estimates under constant power draw.
**Typical mAh & Wh values**: - Smartphone battery: 3000-5000 mAh at 3.8 V ≈ 11-19 Wh - Tablet battery: 7000-10000 mAh at 3.8 V ≈ 27-38 Wh - Laptop battery: 40-100 Wh - LED light strip (5m): ~1-2 W, runs 50-100+ hours on a 12 V car battery - Wi-Fi router: ~3-5 W, runs 10-20 hours on a 20 Wh power bank
**Example**: A 3000 mAh battery at 3.7 V (≈ 11.1 Wh) powering a 5 W device: life = 11.1 ÷ 5 = 2.22 hours ≈ 2 hours 13 minutes.
- Formulas for both mAh×V and Wh inputs
- Real-world caveats and variability
- Typical capacity references
- Example calculation
Worked Example
Example: Estimating battery life for a smartphone:
Battery capacity: 4000 mAh Voltage: 3.8 V (typical lithium-ion) Device power consumption: 6 W (screen on, cellular data, moderate usage)
Step 1: Calculate watt-hours: Wh = 4000 mAh × 3.8 V ÷ 1000 = 15.2 Wh Step 2: Calculate battery life: life = 15.2 Wh ÷ 6 W = 2.53 hours
**Result**: Approximately 2.5 hours of runtime with screen on and moderate usage.
**If the same phone is in standby mode** (perhaps 0.5 W power draw): life = 15.2 ÷ 0.5 = 30.4 hours ≈ 1.3 days.
**If the battery is 3 months old** (mild capacity degradation, perhaps 10% less): effective capacity ≈ 3600 mAh, Wh ≈ 13.3, life at 6 W ≈ 2.2 hours.
**The calculator shows the Wh result and the step-by-step formula so you can verify the math independently.**
Battery Life vs. Usage Patterns
The calculator assumes constant power draw, but real-world runtime depends heavily on what you're doing:
**Heavy usage** (gaming, video streaming, GPS): High power draw (5-10+ W for smartphones), shorter runtime. **Moderate usage** (web browsing, social media, calls): Medium power draw (3-5 W), moderate runtime. **Light usage** (idle, background tasks): Low power draw (0.5-2 W), longer runtime. **Standby/sleep**: Very low power draw (0.1-0.5 W), can last days or weeks.
**Tips to extend battery life**: - Reduce screen brightness. - Disable unnecessary connections (Bluetooth, GPS, 5G when not needed). - Close background apps. - Use power-saving mode. - Keep the battery between 20-80% charge for longest lifespan.
**Battery health over time**: Lithium-ion batteries typically retain 80% of original capacity after 300-500 charge cycles. After 2-3 years, actual runtime may be noticeably less than the calculator estimate.
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Open calculator →Frequently Asked Questions
Why do estimated battery lives differ from what I experience?
The calculator assumes constant power draw, but real usage varies. Screen-on time, signal strength, background apps, age, and temperature all affect actual battery life. The estimate is a theoretical baseline under constant power draw.
Can I use this for car batteries (12 V, 6 V, etc.)?
Yes. Enter the battery's mAh rating and its voltage (e.g., 12 V for a car battery). The calculator will give you watt-hours and estimated life based on the device power you enter. For a 12 V car battery rated at 50 Ah (50,000 mAh), Wh = 50,000 × 12 ÷ 1000 = 600 Wh. A 50 W device would run for 600 ÷ 50 = 12 hours.
Is this calculator accurate for laptop batteries?
It provides a reasonable estimate, but laptop power draw varies wildly depending on workload, screen brightness, and power-management settings. For best results, use the manufacturer's rated mWh and your typical observed power draw (often shown in the OS battery menu).
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