The new, sleeker collars are a big improvment over the old clunky ones. These hold their charge for much longer, several weeks sometimes without low battery signals. And the low battery signal is great. And the separate charger is great. Mine isn't solar, haven't seen that. I love our Innotek system.
I have the in-ground, wired version. I absolutely love it. Its the greatest thing. He's so well trained to it that it would take 3-5 days for him to know if the system went off.
We dog-sit a golden retreiver 2-3 days a week. She stays around because he stays within his boundary, and we often leave them 12-14 hours unattended. Bikes, cars, ATVs, walkers, deer, elk, other dogs all have tested his training and he's passed.