quick Q, why is the gray sch40 direct burial "electrical" PVC pipe about 40% less $$ than white "plumbing" sch40 PVC? any reason why the gray PVC cant carry pressurized water (residential for backyard water)...?
If the gray PVC is also Schedule 40 (and for some purposes the NEC requires that) then it could also withstand the pressure and temperature needs for domestic water, both hot and cold.
There may be an issue with the adhesive used. Electrical PVC joints can be made using only the adhesive, without using primer first, but pipe joints subject to high internal pressure (more than just a few PSI) requires the use of primer or a combined primer/adhesive product.
There might also be an issue with the chemical composition of the electrical PVC for domestic water use.
Around here Sch40 electrical PVC costs more than the Sch40 white stuff. But you can also get Category 200 white PVC which is thinner and cannot be used for pressurized water either.