CalcSutra

Battery Life Calculator

Estimate battery runtime: runtime = capacity (Ah) × voltage / load (W).

Enter Values

Fill in the fields and press Calculate to see instant results.

What is the Battery Life Calculator?

The Battery Life Calculator estimates how long a battery can power a device based on battery capacity, voltage, and the device's power consumption. Battery runtime is critical information for portable electronics, backup power systems, emergency equipment, and off-grid applications. Understanding battery life helps users plan charging schedules, design backup systems, and select appropriate batteries for their applications.

Formula

Battery runtime is calculated using:

Runtime (hours) = (Capacity × Voltage) / Load Power

Where:

  • Runtime = Estimated battery duration (hours)
  • Capacity = Battery capacity (measured in amp-hours, Ah)
  • Voltage = Battery nominal voltage (volts, V)
  • Load Power = Device power consumption (watts, W)

Energy formula: Runtime = (C × V) / P = Energy / Power

How to Use

  1. Enter the Battery Capacity (Ah) in amp-hours
  2. Enter the Battery Voltage in volts (V)
  3. Enter the Device Load in watts (W)
  4. Click Calculate
  5. The calculator displays Estimated Runtime in hours

Worked Example

Given:

  • Battery capacity = 100 Ah (typical car battery reserve)
  • Battery voltage = 12 V
  • Device load = 120 W (laptop power supply)

Calculation:

Energy = Capacity × Voltage = 100 Ah × 12 V = 1,200 Wh

Runtime = Energy / Power = 1,200 Wh / 120 W = 10 hours

Result: Battery runtime is approximately 10 hours

Note: Actual runtime may be 15-25% less due to inefficiencies

Real-World Applications

  • Backup Power: Size UPS batteries to ensure adequate runtime for critical equipment shutdown
  • Portable Devices: Estimate smartphone, tablet, and laptop battery life based on usage
  • Off-Grid Systems: Design solar systems with adequate battery storage for multi-day autonomy
  • Emergency Equipment: Ensure emergency lights and communication systems have sufficient runtime
  • RV and Marine: Plan battery capacity for various appliances and overnight usage

Common Battery Specifications

  • Car Battery: 12V, 40-100 Ah, 480-1200 Wh typical
  • Deep Cycle Battery (solar): 12V, 100-500 Ah, 1200-6000 Wh typical
  • Laptop Battery: 11-15V, 40-100 Wh typical
  • Smartphone Battery: 3.7-5V, 10-20 Wh typical
  • UPS Battery: 12V or 24V, 7-200 Ah, 84-4800 Wh typical

Key Definitions

  • Battery Capacity (Ah): Amp-hours; current × time (100 Ah = 10A for 10 hours)
  • Battery Voltage: Nominal voltage (12V car battery, 1.5V AA cell, 3.7V lithium-ion cell)
  • Energy (Wh): Watt-hours; capacity × voltage (100 Ah × 12V = 1,200 Wh)
  • Load Power (W): Device power consumption in watts
  • Deep Cycle Battery: Designed for repeated charge/discharge, not car starting
  • Runtime: Time battery can power a device at continuous load

Frequently Asked Questions

How do I calculate battery runtime?

Runtime = (Battery Capacity in Ah × Battery Voltage in V) / Device Load in W. For example, a 100Ah 12V battery powering a 120W device runs for (100 × 12) / 120 = 10 hours.

Why is my battery runtime less than calculated?

Batteries have efficiency losses (typically 15-25%) due to internal resistance, temperature effects, and inverter losses. Conservative estimates subtract 20% from theoretical runtime.

What is Ah (amp-hours)?

Amp-hours measure battery capacity. One amp-hour (1 Ah) provides 1 ampere of current for 1 hour. A 100 Ah battery can provide 100A for 1 hour, 10A for 10 hours, or 1A for 100 hours.

What is the difference between Wh and Ah?

Wh (watt-hours) = Ah × V. Wh is more accurate for energy comparison because it accounts for voltage. Two 12V batteries with different capacities may have different Wh ratings.

How does temperature affect battery life?

Cold temperatures reduce battery capacity (fewer available amp-hours). Hot temperatures increase self-discharge. Optimal performance is at room temperature (20-25°C). Plan for 20% capacity loss in cold weather.

Why do I need different battery types for different applications?

Car batteries are optimized for high starting current; deep-cycle batteries are for repeated charge/discharge. Using wrong battery type causes poor performance or damage.