Efficiency describes power from a given module area
Module efficiency compares electrical output with the solar energy striking the panel under standardized test conditions. Higher efficiency generally means more rated watts can fit into a given module area.
Higher efficiency matters most when roof area is scarce
If usable roof space is limited, a higher-efficiency module can help fit more DC capacity into the available area. On a large uncomplicated roof, the economic difference may matter more than the efficiency percentage alone.
Panel wattage and efficiency are related but not identical
A physically larger panel can have a high watt rating without being the most efficient module. Compare wattage, dimensions and efficiency together when roof density matters.
Annual production depends on the whole system
Roof direction, shade, local solar resource, inverter design, temperature, clipping and downtime influence annual kWh. A high-efficiency panel on a poor roof can still produce less energy than a lower-efficiency module in a better location.
Compare warranties and architecture too
Efficiency is only one specification. Product warranty, performance warranty, degradation terms, inverter compatibility, dimensions and installer design can be equally important to the final project.