Why 2 speed in your case? Why not just wire in the pump so that it kicks on for solar demand, then kicks off when no-demand. You are probably going to want the full 3/4 hp to run solar anyway.

My solar controller has a relay for a booster (110V only, unfortunately) that could do easily handle the 3/4 hp pump (its a Goldline). If your pump is 220v, you could use that relay to trip a 220v relay to run the pump, or get a controller that has a built in 220v relay (I'm pretty sure some of the more expensive pumps do this).

Or, even cheaper, just set your pump timer to run for the 4 or 5 peak sun hours and then at night. No need for a relay!