Marine Battery Capacity Calculator
Amp-Hours, Watts, and How Long the Lights Stay On
A battery's amp-hour rating alone doesn't answer "how long will this run my equipment" — that depends on the load's power draw, the battery's voltage, and how deeply it's safe to discharge before recharging. This calculator connects those pieces in both directions: estimating runtime from a battery bank and load, or sizing the battery capacity needed to hit a target runtime.
The Formulas
Usable Energy (Wh) = Capacity (Ah) × Voltage × (DoD / 100)
Runtime = Usable Energy / Load Power
Required Capacity:
Required Energy (Wh) = Desired Runtime × Load Power
Required Capacity (Ah) = Required Energy / (Voltage × (DoD / 100))
Depth of Discharge (DoD) is the fraction of total capacity considered safe to use before recharging — commonly 50% for flooded lead-acid batteries, and higher for AGM or lithium chemistries.
Worked Examples
| Mode | Inputs | Result |
|---|---|---|
| Estimate Runtime | 200 Ah battery, 12 V, 100 W load | 1,200 Wh usable, 12.0 hours runtime |
| Required Capacity | 8 hours desired, 150 W load, 12 V | 1,200 Wh required, 200.0 Ah needed |
Depth of discharge has a direct, proportional effect on both results: cutting DoD from 50% to 25% (common for extending flooded lead-acid battery lifespan) would double the required battery capacity for the same runtime.
Where This Matters
- House bank sizing — matching battery capacity to overnight or multi-day loads (lighting, refrigeration, electronics) without a shore or generator connection.
- Anchoring out — estimating how many nights a battery bank can support essential loads before needing to run the engine or a generator to recharge.
- Adding new equipment — checking whether an existing battery bank has headroom for a new load, like a windlass, inverter, or additional electronics.
How to Use This Calculator
- Choose the mode: Estimate Runtime or Required Battery Capacity.
- For Estimate Runtime: enter Battery Capacity (Ah), Voltage (V), Depth of Discharge (%), and Load Power (W).
- For Required Capacity: enter Desired Runtime (hours), Load Power (W), Voltage (V), and Depth of Discharge (%).
- Select Calculate to see the runtime or required capacity.
Related Calculations
Check anchoring depth needs alongside your power planning with the Anchor Chain Length Calculator, or plan fuel for the generator run with the Boat Fuel Consumption Calculator.