So I can't speak to trail/enduro riding, but at an MX track, I have a setup detailed
here that works quite well. It does not have the ability to fill back up on-site via solar. It's just an extra "gas can" that I refill once I get back home. I've got ~85 hours on the bike now. Here's what I can tell you:
With 240v / 3.3kw charging, I've never had an issue where I ran out of juice before I was done riding. I get a touch less than 1% a minute recharging. My track does 20 minutes on then 20 minutes off. (Sometimes 40 minutes off if it's very busy.). That's been more than enough to keep the battery happy and me exhausted. Even when guest riders take the thing out for a session on top of my own sessions. For me and my particular use case, swappable batteries are just not needed. I obviously cannot speak for anyone else.
Taking a full on-board battery, plus another 5kwh is sufficient on most days. I have a second 5 kwh battery and when it's
very very muddy, that second battery comes into play. (I bought the second battery before I had the bike out of my living room. There was a very long lead time to get them from China.) That second battery may be handy if I ever road trip to another track and camp there for a weekend. But it's really a luxury and not a requirement. Most of the time, I only bring one with me.
A lot of this will depend on how advanced a rider you are. I ride at 48hp and I go through about 30% of the battery in a 20 minute session on a normal day. But if you're a 60-80hp kinda guy and you're always going full send, then you'll deplete the battery faster and that 1% a minute recharge rate may be insufficient.
I would
not bother with 120v track-side charging unless you have a very long time between sessions. It's just too slow. It's definitely a "over night once you get home" kind of charge.
All told, I love this bike. A lot. I'm trying to sell my KTM 250 4-stroke. Besides how much I love riding it, I also have no interest anymore in cleaning air filters or changing oil all the time.