Battery Runtime Calculator 🔋

Calculate how long your boat's battery bank will last based on capacity, battery type, and electrical load. Supports lead-acid, AGM, and lithium batteries.

How to Use the Battery Runtime Calculator

Select battery type, enter capacity (Ah), system voltage, number of batteries wired in parallel, and total electrical load in watts. The calculator estimates how long your battery bank will last.

Depth of Discharge (DoD) varies by battery type: Lead-acid and AGM should not be discharged below 50%, while lithium (LiFePO4) can safely discharge to 80%.

For marine use, always maintain a reserve. The 'rule of thirds' applies: use 1/3 of capacity, keep 2/3 in reserve. Consider solar panels or alternator charging for extended trips.

Boat & Marine Calculator - Introduction

Estimate how long a 12 V house bank will run your boat's electronics, lights, and refrigeration before the safe depth-of-discharge limit is reached. The model uses Peukert's equation, so it correctly shows that a flooded lead-acid bank delivers less runtime under heavy current draw than its nameplate amp-hours would suggest — and that LiFePO4 holds its rating almost flat across loads.

How It Works

Usable amp-hours = total Ah × DoD% (lead-acid 50%, AGM 80%, lithium 90%). Load amps = total load watts ÷ 12 V. Runtime (h) = (Usable ÷ load) × (Usable ÷ (load × 20))^(k−1), where Peukert exponent k is 1.25 for lead-acid, 1.15 for AGM/gel, 1.05 for LiFePO4. A 200 Ah AGM bank with a 10 A load yields ≈ 16 h vs ≈ 19 h for LiFePO4 of the same nameplate Ah.

Usage Scenarios

  • Overnight at anchor on house loads: A 200 Ah AGM bank (160 Ah usable at 80% DoD) running a 60 W fridge + 25 W lights + 20 W instruments (~8.75 A) lasts roughly 17 hours under Peukert — long enough for sunset-to-sunrise without dropping below 50% state of charge.
  • Sizing a lithium upgrade: Replacing 2 × 100 Ah lead-acid (100 Ah usable) with 1 × 100 Ah LiFePO4 (90 Ah usable) under a 10 A load gives ~7.4 h on lead-acid vs ~9.1 h on lithium — and the lithium bank still has cycle life left after years of daily discharge.
  • Verifying you can run a windlass: A 1000 W windlass at 12 V pulls 83 A. Five minutes of cycling pulls roughly 7 Ah from the bank; combined with the night's house load, you can confirm the morning departure won't leave you below 60% state of charge with no engine started.

Frequently Asked Questions

How long will my boat batteries last?

Runtime depends on total battery capacity (Ah), depth of discharge (DoD), and electrical load. Formula: Runtime = (Capacity × Batteries × DoD%) / (Load watts / Voltage). For example: 2× 100Ah AGM at 12V with 120W load = (200 × 50%) / (120/12) = 10 hours.

What is depth of discharge (DoD) and why does it matter?

DoD is how much of a battery's capacity you can safely use. Lead-acid and AGM batteries should not be discharged below 50% (50% DoD) to preserve lifespan. Lithium (LiFePO4) batteries can be safely discharged to 80% DoD, providing more usable capacity.

Should I choose lithium or AGM batteries for my boat?

Lithium (LiFePO4) batteries offer 80% usable capacity vs 50% for AGM, last 3-5x longer, weigh 60% less, and charge faster. However, they cost 3-4x more upfront and require a compatible charging system. For frequent use and long-term savings, lithium is better.

How do I calculate my boat's total electrical load?

List all electrical devices and their wattage: navigation lights (~25W), chart plotter (~20W), VHF radio (~25W), refrigerator (~50W), cabin lights (~20W), etc. Add up all devices that run simultaneously. Include a 10-20% buffer for efficiency losses.