I don’t know how we’d pull this off but wouldn’t it be slick if you could enter the model numbers of all of your gear, and then it would spit out a recommendation?
In general, we add up the power draw from all of your gear and then recommend the regen that will best accommodate that. There are a few assumptions that can be made. Source gear and pre amps usually pull about 30-45W each. The amp is usually what dictates what regen you need. This is a generalization, but if you’re running class A A/B amps, I don’t recommend anything smaller than a P15.
At the end of the day, the more headroom the regen has for your system, the better.
If you get me a list of your gear, I’ll pass along my recommendation.