[Google Maps API v2] Creating Custom Tiles - 10x10 mile grid for the US

I want to create a custom tile layer for the continental US that shows
the impact of power plants (ex. coal).

So I'm going to calculate a value for each grid area (probably using
php - since that is my skill set). I'm going to develop a formula
based on power plant output in MWs and distance from the power plant
to the center point of the grid.

Then I'll have a table with the corners of each grid area, and an
impact value. How would I convert this into a custom tile layer?

I'd like to do a heat map.
If possible, I'd also like opacity.

The one tool I found is a converter from shapefiles to tiles:
http://www.casa.ucl.ac.uk/software/gmapcreator.asp

I don't have ArcGis or any other commercial GIS tools. I'm looking for
a free solution.

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