The most efficient tilt for photovoltaic panels in Seattle, Washington at a latitude of 47.62°:
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 39.3°.
If you're planning to change the angle of your photovoltaic panels twice per year, the most efficient angle is 23.3° in summer months and 60.9° in winter months.
If you're planning to change the angle of your photovoltaic panels each season, the most efficient angle is 19.5° in summer months and 66.4° in winter months, and 44.4° in autumn and spring months.