Solar is sized from daily energy, not from panel watts, and not from the size of the arch you happen to have. The calculation is simple. Getting honest inputs is the hard part.
The three inputs
Daily energy use. Measured over a realistic period, not estimated from nameplates. A shunt-based battery monitor will tell you this in a week of normal use, and the number is almost always higher than owners expect.
Peak sun hours. Not hours of daylight — the equivalent hours at full rated irradiance. This varies enormously across Australia and across the year. Hobart in winter and Darwin in winter are not the same problem, and sizing an array from an annual average will leave you short in exactly the season you notice.
System efficiency. Everything between the panel's rating and the energy that reaches the battery: controller losses, temperature derating, soiling, cable loss and shading. A planning figure around 75% is common; a heavily shaded rig deserves less.
Size for the worst month you actually cruise
An array that covers your consumption in December and falls 40% short in June is not a 12-month system. Decide which months matter, then size for the worst of them. If the boat is only used in summer, size for summer and say so.
Shading costs more than any other factor
On a monohull, boom, rigging, antenna and sail shadows move across the array all day. Because of how panels are wired internally, a shadow across part of one panel can cost far more than its share of the array's output.
Two practical responses: split the array across multiple MPPT controllers so a shaded group does not drag down an unshaded one, and place panels where the rig does not sweep over them, even at the cost of a smaller total array.
Panel watts are measured in a laboratory
Rated output is measured at a standard cell temperature that an Australian deck rarely delivers. Output falls as panel temperature rises, so a panel bolted flat to a hot deck produces meaningfully less than the same panel with air moving under it. Mounting for airflow is free performance.
Then check the rest of the chain
An array that can replace your daily energy is necessary, not sufficient. The controller must be sized for the array, the cabling for the current, and the battery must be able to accept the charge current the array can deliver on a good day. Check the battery's charge-current limit and the BMS charge rating before finalising an array.