I just went through calculations which show that for where I live, the direction of my house, the pitch of my roof, etc... 6,750 Watts worth of solar panels and their ancillary equipment will yield an annual output of 7,300 KWH of electricity.

In my opinion, the easiest way to compare electricity directly to coal is via BTU's.

7,300 KWH x 3,412 BTU's/KWH = 24,970,000 BTU's

Why, that is just about exactly the number of BTU's in a single ton of coal (if burned at 100% efficiency). Allowing for the inherent inefficiencies of coal, lets say 1.5 tons are required for equivalence, to grant as much favor to the solar panes as is possible. Therefore I will state that 1.5 tons of coal = 7,300 KWH of electricity, useable BTU for useable BTU.

Now a solar panel system yielding 7,300 KWH annually will run about $28,000 installed, and the system should last 25 years. But 1.5 tons of coal will cost (at $250 per ton) $375 per year.

$28,000/$375 = 75 years. The solar panels will begin showing their advantage over coal in 75 years. But by then, you would likely have replaced the solar panels and much of the other associated equipment 2 to 3 times.

Just thinking....