Step 1: Calculate daily energy
Example:
Daily use = 3,000Wh
Step 2: Decide how many low-generation days you want to cover
If you want 1.5 days:
3,000Wh x 1.5 = 4,500Wh
Step 3: Add your operating reserve
Do not plan to hit a BMS low-voltage disconnect every day.
If you decide that 85% of nominal battery energy is your planned working window:
Required nominal energy = 4,500 / 0.85 = 5,294Wh
A practical design target would therefore be around 5.3kWh or more, before considering ageing, cold conditions, future loads and system-specific margins.
Convert energy to Ah
At 12.8V:
5,294 / 12.8 = 414Ah
At 25.6V:
5,294 / 25.6 = 207Ah
At 51.2V:
5,294 / 51.2 = 103Ah
Do not oversize the bank without sizing charging
A huge bank with insufficient charging can create a different problem: the boat spends days operating at low state of charge because generation cannot replace consumption.
A balanced design asks:
- How much do I use?
- How much do I store?
- How much can I generate on a good day?
- How much can I generate on a poor day?
- What is my backup when solar is weak?
- How quickly can I recover after two poor days?
Apply this guidance to your boat
Use this page as one input to a vessel-specific plan. Record the actual equipment, cable runs, charging sources and operating pattern aboard before choosing hardware or changing the installation.
- Measure the real load and its operating time instead of relying on a label or a generic boat profile.
- Check every proposed current against the battery, BMS, protection, cable and connection limits.
- Confirm the result against current manufacturer instructions and the rules that apply to your vessel.