The most efficient tilt for photovoltaic panels in Saint Louis, Missouri at a latitude of 38.64°:
Your photovoltaic panels need to be angled facing south.
If you're mounting the photovoltaic panels at a stationary angle, such as on your roof, the most efficient angle is 32.5°.
If you're planning to change the angle of your photovoltaic panels twice per year, the most efficient angle is 14.9° in summer months and 53° in winter months.
If you're planning to change the angle of your photovoltaic panels each season, the most efficient angle is 11.2° in summer months and 58.4° in winter months, and 35.6° in autumn and spring months.