Question about correct winter tilt, which is latitude +15°. Let’s say April for my location is latitude 57°, would the correct angle be 42° or 72°? I would think 42° is correct with the sun less over head in winter?
The latitude +15 tilt angle for winter optimization is not an exact science.
If we were setting a PV system up on the North Pole, then we would have to tilt the system facing the ground. If we were at the equator, what 15 would we go towards, north or south.
It was a general "rule of thumb" that was most commonly used in the pre-computer age (where we came from).
Another thing that gets me here is the definition of winter. I don't like it from a solar perspective. Winter starts on the shortest day of the year. Shouldn't the shortest day of the year be the middle of winter? Should the end of winter (about 3/20) be half way between the shortest and the longest day of the year?
For an off-grid system, where you want to optimize for those shorter days when the sun is lower in places where most people here live (latitudes between the tropic of cancer 23.5 degrees and 40 or so degrees) I think that the latitude plus 15 tilt is a good practice for the most part. Best is to check it out on software and see how to maximize your production with some software, such as PV watts. Even though PV Watts is not for batteries, it is a simple way of giving us some good info about the insolation of different months at different tilt angles.
There are many more variables than what the latitude plus 15 rule of thumb gives us, but it is a good place to start. Most people in the world live between the tropic of cancer and the arctic circle, so not a bad place to go with latitude plus 15 if you don't have a supercomputer from NREL working for you.