Sizing the controller to the array and nothing more.
The calculated figure is a floor. Undersizing clips your best days.
Sizing
How to Size an MPPT Solar Controller for a Boat
Divide your array's watts by the bank's nominal voltage to get charge current, add headroom for a cold bright day, and buy a controller at or above that figure. 400 W on a 12.8 V bank is about 31 A, so a 30 A controller is marginal and a 40 A is not.
- Steps
- 5
- Last reviewed
- 4 September 2026
- Independence
- Nobody paid for this page.
Equipment and information required
- Total array watts
- Bank nominal voltage
- Panel open-circuit voltage and your wiring layout
How to Size an MPPT Solar Controller for a Boat: steps
Divide array watts by bank nominal voltage
Use 12.8 V for a 12 V LiFePO4 bank and 25.6 V for a 24 V one. 400 W ÷ 12.8 V is about 31 A. This is the current the controller has to pass.
Add headroom for a cold, bright day
Panels exceed their rated output when they are cold and the light is strong. A controller sized exactly to the calculation will clip or shut down at the moment it is producing most. This figure is a floor, so buy at or above it.
Check the maximum PV input voltage
Work this out from your panels in series, not in parallel, and use open-circuit voltage on a cold morning rather than the rated operating voltage. Exceeding the controller's PV input limit destroys it immediately.
Confirm a true lithium profile
Check absorption, float or storage settings against the exact battery manual. Some lithium manufacturers specify a float voltage. Disable equalisation and inappropriate temperature compensation, and confirm BMS and low-temperature charging controls.
Prefer a controller you can read
Bluetooth or a display is worth paying for here. A controller you can read is a controller you will notice failing, and solar failures are silent until the bank is flat.
Common installation and calculation errors
Using rated panel voltage for the input limit.
Use open-circuit voltage at the coldest temperature you will see.
Related equipment and battery guides
More on sizing
How to Calculate Boat Electrical Loads in Amp-HoursMultiply each load's current in amps by the hours it genuinely runs in a day, then add them up. A 5 A fridge cycling at 45% duty is 5 × 10.8 = 54 Ah, not 5 × 24.How to Size a Boat Lithium House Battery BankTake your daily amp-hour draw, subtract half of what your solar returns on an average day, multiply by the nights you spend away from shore power, then divide by 0.8 because you cycle lithium to 80%. Round up to a bank you can buy.How to Size a Marine DC-DC Battery ChargerTake the smaller of a quarter of your bank's capacity and half your alternator's rating, then round down to a charger you can buy. A 300 Ah bank behind a 90 A alternator gives min(75, 45) = 45, so a 40 A charger.How Many Solar Panels Does a Boat Need? Sizing StepsDivide the amp-hours you want solar to replace by 4.25 sun-hours and 0.75 for real-world losses, then multiply by the bank's nominal voltage to get watts. Replacing 100 Ah a day on a 12 V bank needs roughly 400 W.