As others have recommended, I think it is better to save your coal for its other uses.
Lately I've been powering my miners with 1 wire, 1 BatBox, and 5 Solar. Use a regular drill with an OV scanner. I just set up several and let them run. Come back a few days later, collect the loot, and move them 9 squares. I have used crystals to power diamond drill mines in the past. IIRC it takes about 500,000 EU / mine. I estimate I average about 1 diamond per 9x9 mine.
Quarries are not very efficient but are good for scrap and clearing up the "abandoned mine" mazes. I usually just use a BC Filler to put a cap on them when done.