Calculator
Boat Solar Panel Size Calculator
Panel watts against your daily draw, returning the average daily return and whether the array covers your consumption.
From the house bank calculator, or your own shunt readings.
4.25 peak sun-hours as an Australian annual average, derated 25% for heat, dirt, shading, angle and controller losses. We credit only half of this when sizing a bank.
Your numbers
100 Ah
Average daily return from this array
| Panel rating | 400 W |
| Average return | 100 Ah/day |
| Credited for sizing (half) | 50 Ah/day |
| Covers your draw | 68% |
| Watts to cover the draw | 600 W |
Calculator assumptions and limits
- 4.25 peak sun-hours as an Australian annual average
- 25% derating for heat, dirt, shading, angle and controller losses
- Average day, not best day: size the bank for the grey ones
Every assumption here is versioned and dated on the methodology page. If you disagree with one, the working is there to argue with.
Battery sizing calculation guides
The same arithmetic this calculator runs, written out step by step.
How to Size an MPPT Solar Controller for a BoatDivide 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.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.How to Connect Boat Solar Panels to Lithium BatteriesWire the panels to an MPPT controller with a true lithium profile, land the controller's output on the bank through its own fuse, and connect the battery side before the panel side when commissioning.
The other calculators
House bank calculatorYour loads, your nights away from shore power, your solar and your alternator, returning the bank, the DC-DC charger, the AGM equivalent and the weight off the boat.Trolling motor runtimeThrust rating, the speed you actually troll at and your bank size, returning hours on the water and the bank you need for a full day.DC-DC and alternator sizingAlternator rating and bank capacity, returning the charger to fit and the time to refill from your engine.AGM to lithium swapYour existing AGM capacity, returning the lithium bank that delivers the same usable energy and the weight you take off the boat.