Pneumaticraft has a Universal Sensor that can give at least a rough idea of how soon the next rainstorm is, so I assume it's more predictable than "roll the dice every tick". All I can say from my personal experience, though, is it seemed to rain way more often if I used TooManyItems (back in the days before MC 1.2.5, when I first encountered NEI) to kill the rail as soon as it started than if I let it rain itself out.
So it rains no less than 6.25% of the time. As for an upper bound, I'll guess that it doesn't usually rain more than about 25% of the time, so you can count on your basic IC2 solar panels probably giving you at least 0.3 EU/t apiece on average, and mo more than 0.46875 EU/t, factoring in how they don't work at night.
Or, you could put them in an eternal-day, no-weather (or desert-biome) Mystcraft age and get 1.0 EU/t from each.