Battery sizing
Boat Battery Size Calculator
Work out the right house bank capacity for your boat in minutes. Enter your daily energy use, the autonomy you need at anchor, and your chemistry — Marine Power Designer returns the required nominal and usable Ah at 12V, 24V or 48V.
What this calculator does
Marine battery banks are sized around three things: daily energy use (Wh/day), days of autonomy between charges, and the usable depth of discharge (DoD) for your chemistry. The calculator turns those inputs into a recommended bank size in amp-hours.
It also flags the realistic charging current your bank can accept and warns when your inverter, charger or cable run is the limiting factor.
How marine battery sizing works
The core formula:
Bank Ah = (Daily Wh × Autonomy days) / (System V × Usable DoD)
Add a margin (typically 10–20%) for cold weather, ageing and the inevitable load you forgot. For LiFePO4, also confirm the BMS continuous current rating supports your inverter's peak draw.
Inputs you'll need
- Daily energy consumption in Wh or kWh (fridge, autopilot, lights, electronics, inverter loads)
- Days of autonomy at anchor without charging
- System voltage: 12V, 24V or 48V
- Battery chemistry: LiFePO4, AGM, Gel or Flooded lead-acid
- Inverter size (optional, for peak load checks)
Frequently asked questions
How many amp-hours do I need for my boat?
Multiply your daily energy use (Wh) by the days of autonomy you want, divide by the system voltage (12V / 24V / 48V), then divide by the usable depth of discharge for your chemistry (≈0.8 for LiFePO4, ≈0.5 for AGM/Gel, ≈0.5 for Flooded).
Is LiFePO4 worth it on a boat?
For most cruisers, yes. LiFePO4 gives you ~80% usable capacity vs ~50% for lead-acid, higher cycle life, faster charging, and lower weight per usable Wh — at a higher upfront cost.
What depth of discharge should I plan for?
Plan around 80% DoD for LiFePO4 to maximise cycle life, 50% DoD for AGM/Gel/Flooded. Going deeper shortens battery life dramatically on lead chemistries.
12V, 24V or 48V house bank?
12V is fine up to ~2kW loads. Move to 24V above that and 48V for large inverters (≥5kW) or long cable runs — higher voltage means smaller cables and lower I²R losses.
Ready to size your system?
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