By Boat Battery editorial team · 2 minute read · Editorial policy
Record energy used across a representative day and night, together with charging and weather. A useful energy log separates appliance demand from the energy returned by solar or the engine.
Choose a representative cruising weekend
Choose a weekend that resembles the way you normally cruise. A quiet afternoon at the marina will miss overnight refrigeration, anchor lighting and the morning routine. Record when you leave, the starting state of charge and whether the battery monitor has been set up correctly. A percentage display without a known capacity or reliable synchronisation can mislead you.
Record battery and charging readings
Take readings at repeatable points: departure, sunset, bedtime and the next morning. Note watt-hours or amp-hours consumed and charged if the monitor provides separate counters. A single net reading can hide a large daytime load behind a large solar contribution. Write down engine running time, shore connection and any unusual appliance use beside the readings.
Log weather and appliance use
Keep the context with the numbers. Fridge settings, ambient heat, guests aboard and shaded solar panels can change the result. For equipment that cycles, an instantaneous current reading is not a daily energy measurement. Observe it over enough time to include on and off periods, or use a suitable energy meter installed safely for that circuit.
Turn measured consumption into a sizing scenario
Use the log to build a normal-day estimate and a less favourable scenario. The latter might mean another night out, a cloudy day or additional navigation hours. Feed those scenarios into the calculator separately. Preserve a reserve for essential loads rather than treating the battery protection cutoff as your ordinary end point. Repeat the log after a major appliance change.
A useful log entry
- Time and monitor reading
- Energy charged and consumed where available
- Engine and shore charging periods
- Weather, shade and fridge setting
- Appliances and hours used
Frequently asked questions
Can I size the bank from battery voltage alone?
Voltage alone is a poor measure of remaining capacity across much of a LiFePO4 discharge cycle. Use a correctly configured monitor and measured loads.
Related installation planning resources
Use these notes to prepare your own measurements and questions. Check the exact equipment manuals and have the proposed system assessed for your vessel before buying or changing electrical components.
Calculate your boat battery capacity
The house bank calculator takes your loads, your nights on the hook, your solar and your alternator, and returns the bank, the DC-DC charger, the AGM equivalent and the weight you take off the boat.
Open the calculator