Weather data calculations in monthly values: Order of precision

With respect to an hourly computation, monthly calculations for Geneva show that for the transposition on tilted plane induces a yearly error of the order of:

South plane, tilt 0..90°: < 1.3%

SE or SW planes: tilt 45°: < 1.1%, tilt 90° : < 4.1%,

E or W planes: tilt 45°: < 2.7%, tilt 90° : < 11 %,

But if only the global monthly values are known, the uncertainty on the monthly diffuse estimation correlation model (about 5%) may induce 1 to 4 % error more.