How Many Solar Panels Does a Boat Need? Sizing Steps

Equipment and information required

Decide how much of the day's draw solar should carry
Covering the whole daily draw from solar alone needs an array most boats cannot fit. Covering most of it, and running the engine or shore power occasionally, is the usual compromise.
Convert amp-hours back into watts
Target amp-hours × nominal voltage ÷ 4.25 sun-hours ÷ 0.75 derating gives panel watts. To replace 100 Ah a day on a 12.8 V bank: 100 × 12.8 ÷ 4.25 ÷ 0.75, about 400 W.
Derate honestly for an Australian deck
The 0.75 covers heat, salt, dirt and the shading a mast and boom throw across a deck all day. Panels on a boat rarely see their rated output, and a rig that shades one panel can pull down a whole series string.
Check the array physically fits
Measure the arch, bimini or coachroof before committing. Rigid panels need flat, ventilated mounting; flexible panels that sit directly on a deck run hotter and lose output, and they fail sooner.
Plan the wiring layout before you buy
Series raises voltage and suits long runs but suffers badly from partial shading. Parallel tolerates shading but needs heavier cable. On a boat with a rig overhead, parallel or multiple controllers usually wins.
Size the controller to match
Array watts divided by bank nominal voltage gives the controller current you need, and that figure is a floor rather than a target.

More on sizing