How to Size a Lithium Battery Bank for a Yacht
8 min read · Updated
Correctly sizing a lithium (LiFePO4) house bank is the single decision that most affects onboard comfort, generator hours and long-term reliability. This guide sets out the practical method we use at MS Ocean when advising owners and captains on lithium conversions and new-build specifications.
Start with a realistic daily energy budget
Every sizing exercise begins with an honest 24-hour load list. Measure or estimate each load in watts, multiply by hours of daily use, and total in kilowatt-hours. Include galley, HVAC, entertainment, navigation, lighting and standby loads. For superyachts, split the budget by cruising, at-anchor and marina modes — the profiles are very different.
Choose an autonomy target
Autonomy is how long the bank should support the daily budget without recharging. Typical targets sit between 8 and 24 hours at anchor. Multiply your at-anchor daily kWh by the autonomy in days, then divide by a safe depth of discharge (usually 0.8 for LiFePO4).
Match charging to the bank
A well-sized bank fails in service if charging is under-scoped. Aim to recharge the used capacity in a single reasonable session — alternator, shore charger and solar working together. Under-charging silently reduces cycle life and pushes generator hours up.
Design the DC architecture safely
Lithium banks need a proper BMS strategy, correctly rated fusing at the battery terminals, protection against alternator load-dump, and clear isolation for service. This is where retrofits most often go wrong.
Need advice on your own vessel? Read more about our services, explore MarinePower.app or request a consultation.