Optimal year-round tilt angle ≈ your latitude. In winter, add 15°. In summer, subtract 15°.
At 40°N latitude: year-round optimal = 40°. Winter: 55°. Summer: 25°.
A fixed tilt at latitude × 0.87 minimizes annual energy loss vs. a single optimal fixed angle.
Equal to your latitude for best year-round output. Adjust +15° in winter, −15° in summer for seasonal optimization.