How Panel Count Is Determined
The number of panels you need depends on two things: the total system size in kilowatts and the wattage of each individual panel. A 6 kW system using 400W panels needs 15 panels. Using higher-wattage panels means fewer panels for the same output, which matters if roof space is limited. This calculator estimates your ideal system size first, then converts that into a panel count using standard 400W residential panels.
Factors That Affect How Many Panels You Need
Your electricity bill is the biggest factor—higher consumption means a larger system. But roof direction also matters: a south-facing roof in the northern hemisphere (or north-facing in Australia and New Zealand) produces more per panel, so you may need fewer panels. Shading reduces output per panel, meaning you might need more to compensate. The calculator accounts for all of these factors.
Panel Count vs. Roof Space: A Practical Guide
A standard 400W solar panel is roughly 1.7m × 1.0m (about 18 sq ft). That means a 10-panel system needs approximately 17 square metres (180 sq ft) of usable roof space. "Usable" is key—you need to subtract areas around vents, skylights, chimneys, and edges. Most installers recommend leaving at least 30cm from roof edges. If your roof is tight on space, consider higher-efficiency panels (which cost more but produce more per square metre) or focus on the most sun-exposed section.