Utility July 13, 2026 · 8 Min Read

Battery Life Calculator – Guide & Formulas

Calculate the estimated runtime of a battery based on battery capacity, discharge safety margin, and continuous load current.

Estimate battery runtime for your electronics, UPS, or solar backup. Input the battery capacity (mAh or Ah), voltage, load current (mA or Amps), and battery chemistry type to compute operational hours under load.

Key Takeaway

Use the free Battery Life Calculator to calculate the estimated runtime of a battery based on battery capacity, discharge safety margin, and continuous load current. Get instant results with step-by-step explanations.

How to Use the Battery Life Calculator

  1. Select the battery capacity unit (milliamp-hours mAh or Amp-hours Ah).
  2. Enter the battery capacity value (e.g., 2000 mAh for AA rechargeable, or 100 Ah for marine deep-cycle).
  3. Input the average continuous load current drawn by the device.
  4. Select the battery chemistry (Lithium-Ion, Alkaline, Lead-Acid, etc.) which sets default safe discharge depths.
  5. Adjust the safety/derating factor (standard is 15-20% losses to account for internal resistance, heat, and aging).
  6. Instantly read the estimated run duration in hours, minutes, or days.

The Formula

Runtime (hours) = (Capacity in Ah * Depth of Discharge) / Load Current in Amps * (1 - Losses%)

Variable Definitions

  • Capacity: The total electric charge a battery can deliver, typically in milliamp-hours (mAh) or Amp-hours (Ah)
  • Depth of Discharge (DoD): The percentage of battery capacity that can be safely drained without damaging the cells (e.g., 50% for Lead-Acid, 80-90% for Lithium)
  • Safety/Loss Factor: Efficiency penalty covering self-discharge, wiring heat, and ambient temperature impacts

Estimating UPS Backup Battery Runtime

Calculate the run duration of a 12V 100 Ah AGM Lead-Acid battery powering a continuous load of 8 Amps, with a safe 50% Depth of Discharge and a 10% thermal loss factor.

  1. Step 1: Check capacity: 100 Ah.
  2. Step 2: Factor in safe Depth of Discharge: AGM Lead-Acid shouldn’t exceed 50% discharge, giving 100 Ah × 0.50 = 50 Ah usable capacity.
  3. Step 3: Apply the safety/loss factor: 50 Ah × (1 - 0.10) = 45 Ah effective capacity.
  4. Step 4: Compute runtime: 45 Ah / 8 Amps = 5.625 hours (which is 5 hours and 37 minutes).

Frequently Asked Questions

Why shouldn’t I discharge a battery to 0%?

Deep discharging can cause irreversible chemical changes and physical stress inside the cells, dramatically reducing the battery’s overall lifecycle. This is especially critical for Lead-Acid batteries, which shouldn’t go below 50% state of charge.

How do I convert Wh (Watt-hours) to Ah (Amp-hours)?

Multiply or divide by Voltage: Amp-hours (Ah) = Watt-hours (Wh) / Voltage (V). For example, a 12V 480Wh battery has a capacity of 40 Ah (480 / 12).

How does cold temperature affect battery capacity?

Cold temperatures slow down the internal chemical reactions in batteries, temporarily reducing effective capacity and voltage output. At 32°F (0°C), standard batteries can lose up to 20% of their rated runtime.