Boat Battery Bank and Runtime Calculator

Calculate series-parallel bank size, usable energy, estimated steady-load runtime, and capacity needed for a target runtime. All vessel and battery inputs stay in this browser tab.

Do not use a runtime estimate as the only power reserveKeep an independently determined reserve for bilge pumps, navigation lights, communications, alarms, and other essential equipment. Confirm battery, BMS, inverter, cable, fuse, charger, and installation limits with their manufacturers and a qualified marine electrician.

Battery bank and load

Matching batteries and wiring

Presets only change reserve floor and Peukert fields.

Use the data-sheet nominal voltage, not charging voltage.

Use capacity at the rated-hour basis entered below.

Series adds voltage, not Ah.

Parallel strings add Ah.

Usable capacity assumptions

Use a synchronized battery monitor where available.

Keep this below starting SOC and follow battery/BMS limits.

100% means the bank still delivers its rated capacity.

Continuous load

Use continuous running watts; this model does not check surge power.

Use efficiency at the expected load from the inverter data sheet.

Discharge-rate and sizing assumptions

1.00 disables rate compensation; use the battery data sheet where possible.

C20 capacity is rated over 20 hours.

Used to estimate the bank Ah and matching batteries required.

Inputs are processed only in your browser and are not added to analytics events.

Battery bank result

Enter the bank, reserve, load, and discharge assumptions to calculate.

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Battery bank formulas

For matching batteries, each series string raises voltage. Equal strings in parallel raise amp-hour capacity.

Bank voltage = one-battery voltage × batteries in series

Bank Ah = one-battery Ah × parallel strings

Nameplate energy (Wh) = bank voltage × bank Ah

Do not assume batteries can be combined merely because the arithmetic works. Follow manufacturer limits for chemistry, age, model, BMS, series/parallel count, fusing, and balancing.

Runtime and Peukert formulas

The simple estimate is usable amp-hours divided by battery-side current. The rate-adjusted estimate uses the entered capacity-rating time and Peukert exponent.

Usable fraction = (start SOC − reserve SOC) × health

Simple runtime = bank Ah × usable fraction ÷ battery current

Peukert runtime = usable SOC fraction × rated hours × (rated current ÷ load current)exponent

Set the exponent to 1.00 for the simple amp-hour result. A higher exponent changes effective capacity as discharge rate moves away from the rating current.

How to use this calculator

  1. Enter the nominal voltage and rated Ah of one battery, then describe the number in each series string and the number of equal parallel strings.
  2. Enter starting state of charge, a reserve floor, and remaining capacity based on testing or defensible manufacturer information.
  3. Choose battery-side DC amps, DC equipment watts, or AC watts through an inverter. Enter the complete steady load, including continuously running electronics.
  4. Use the battery data sheet for the capacity rating period and Peukert exponent. Treat chemistry presets only as editable starting points.
  5. Compare the result with measured power, an installed shunt monitor, temperature and battery/BMS limits. Preserve a separately assessed essential-load reserve.

Example: two 12.8 V batteries in parallel

Two matching 12.8 V, 100 Ah batteries in parallel form a 12.8 V, 200 Ah bank with 2,560 Wh of nameplate energy. From 100% to a 20% reserve floor at 100% capacity health, the simple usable capacity is 160 Ah or 2,048 Wh before conversion losses.

At a constant 300 W AC load and 90% inverter efficiency, the modeled battery draw is about 26.04 A. With a 20-hour rating and a 1.05 exponent, the calculator estimates approximately 5 hours 51 minutes to the reserve floor. This is an illustration, not a promised operating time.

Frequently asked questions

How do series and parallel connections affect a boat battery bank?

Series-connected matching batteries add voltage while the string Ah remains equal to one battery. Equal strings in parallel add Ah while voltage remains equal to one string. Multiply series count by parallel-string count for total batteries.

How is boat battery runtime calculated?

The simple calculation divides usable capacity by battery-side current. This tool also offers an editable Peukert model, which uses the entered capacity rating period and exponent to approximate the effect of discharge rate.

What reserve state of charge should I use?

There is no universal value. Use the battery and BMS manufacturer's operating limits, then preserve additional capacity for essential equipment and uncertainty. The chemistry buttons are starting points, not recommendations for a specific battery or voyage.

Why does inverter efficiency reduce runtime?

An inverter must draw more DC power from the bank than it supplies to an AC load. The calculator divides AC load watts by the entered efficiency to estimate battery-side power; inverter idle consumption should be included if it is not already represented by that value.

Why can actual marine battery runtime be shorter?

Loads cycle, voltage changes during discharge, and real capacity varies with temperature, age, balance, discharge rate, wiring, BMS cutoffs, and battery condition. Rated Ah is not a guarantee of usable energy in every installation.

Does this calculator size cables, fuses, chargers, or a BMS?

No. It does not check starting surge, fault current, conductor ampacity, voltage drop, overcurrent protection, charger compatibility, ventilation, ignition protection, grounding, or BMS limits.

Are my boat battery inputs uploaded or tracked?

No. Calculations run locally in the browser, and this tool does not send, store, or attach input values to analytics events.

Limits and electrical safety disclaimer

  • The result is a steady-load estimate. It does not simulate intermittent duty cycles, motor starting, inverter surge, changing voltage, alternator or solar input, self-discharge, BMS balancing, or shutdown behavior.
  • Peukert's law is an approximation. It is especially sensitive to the chosen exponent and rated-hour capacity, and a fixed exponent becomes less reliable at extreme current.
  • Battery health is a user-entered derating factor, not a diagnostic test. Temperature and individual weak batteries can reduce available capacity further.
  • Parallel banks need balanced current paths and manufacturer-compatible protection. A numerically valid configuration is not proof of a safe or supported installation.

Safety disclaimer: Battery banks can deliver destructive fault current and may create fire, explosion, corrosive-electrolyte, and electric-shock hazards. This educational estimate does not replace battery/BMS/inverter manuals, applicable marine electrical standards, overcurrent protection, an installed monitor, or design and inspection by a qualified marine electrician.

Methodology and sources

Last reviewed: August 1, 2026. Bank topology and discharge-rate assumptions were checked against current manufacturer documentation:

Manufacturer examples and defaults are explanatory inputs, not universal operating limits. Use the documentation for the exact batteries and equipment installed aboard.

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