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Sizing and power

Battery Bank Sizing for Marine Air Conditioning

The short answer

Estimate the energy for the intended cooling period, add other onboard loads and allow for inverter losses where applicable. Divide by the battery capacity you can actually use while retaining reserve. Check continuous current, compressor starting demand and the charging plan separately: enough stored kWh does not guarantee the system can start.

Battery Bank Sizing for Boat AC — Requirements and Limitations

Worked example, not a product rating: a 1.5 kW compressor load operating for 60% of eight hours uses 7.2 kWh. If a separate 0.09 kW pump runs throughout, add 0.72 kWh. At 90% inverter efficiency, those loads require about 8.8 kWh from the battery before other loads or reserve.

A nominal 12.8 V, 400 Ah battery stores 5.12 kWh. Its permitted discharge depth, state of charge, temperature and reserved capacity determine how much is usable. Direct-DC systems avoid an inverter for that load, but still need correctly designed cables, protection and a battery capable of the required current.

Record actual consumption overnight before relying on an estimate for cruising. A recharge plan must cover cooling and the rest of the boat; battery capacity without sufficient charging simply postpones a shortfall.

Steps for Battery Bank Sizing for Boat AC

  1. Work out the system draw in kilowatts: about 1.1 kW per 12,000 BTU plus 90 W for the pump.
  2. Multiply by the hours you want it running.
  3. Multiply by 0.6 — a fixed-speed compressor cycles roughly sixty per cent of the time once the cabin is cold.
  4. Add about fifteen per cent if the unit runs through an inverter.
  5. Divide by the usable depth of discharge of your chemistry — lithium gives you most of its capacity, lead-acid about half.

Each of these answers a different part of the same problem.

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