Individual manufacturers websites often have conduit fill charts, even for LBs and T sections. You could start there. From what I have seen on those charts, it is often the same or less than the conduit it attaches to. There are options though. A T fitting in conduit can suck for a variety of reasons, but you could consider substituting a junction box, or something with more room to work in. That way you can pull wires to that point, then do a separate pull to get them the rest of the way to your destination.
If dealing with a solar install, a good method is to have a larger wiring raceway below inverters, or between batteries and inverters. The solar can also pass through such a box. As long as all your wiring is rated to the highest voltage in the box or higher.
Sometimes when it gets complicated, it's a good time to step back and think "Is there another way entirely?". I had a similar situation when re-routing 4/0 to a panel, then to an inverter, which went to another panel. The larger wiring boxes helped a lot.
