Deciding on a system size involves several considerations, and the goal is a sensible match between generation and your needs — not a bigger system for its own sake.
Understand your electricity use
The first step is understanding how much electricity you consume, and when. Your utility bills and meter readings show monthly and seasonal totals. Look at how use changes across the year: heating or cooling seasons, an electric vehicle, or a growing household all shape the picture. Past consumption patterns are the best available guide to future needs.
Assess your site
Next, look at the physical side: the unshaded roof area available and its orientation toward the sun. In the northern hemisphere, south-facing surfaces capture the most direct sunlight; in the southern hemisphere, north-facing surfaces do. East- and west-facing roofs can still work well, often with a different layout. Shade from trees and neighboring buildings shrinks the usable area, so the assessment is about real, unshaded space.
Clarify your generation goals
Your goals also shape the size. Are you aiming to offset most of your consumption, cover only daytime use, or pair panels with a battery for backup? Each goal points toward a different size. Offsetting everything requires more panels than covering daytime use alone.
Get a local assessment
Research on your own is useful preparation, but a professional assessment by someone who knows your region, its rules, and its utility programs is the step that turns rough ideas into a workable design. Professionals model the site, the shading, and your usage together — that modeling is what sizing really is.