In my current job I manage development at an environmental software provider. We have a couple dozen chemical and petrochemical customers who are using our software to calculate and report there GHG emissions to the EPA. We are actually in the middle of the first year of reporting for 2010 this month. The plants who are reporting make chemicals that are used directly or indirectly be each of us everyday including ethylene, glycols, nylon precursors...the list goes on. To the point, a single ethylene cracker I just ran calculations for has about 750,000 metric tons CO2 emissions per year . At one particular plant there are 8 of these crackers operating year round. This one plant represents less than 2% of the worldwide capacity of ethylene. I will not even go into the refinery calculations and the emissions generated by the products they make (which we all consume).
Simply put, the stated 21,700 tons of CO2 saved by this solar project is trivial. It is great if Wal-Mart can save money or be more efficient with this project, but to even try selling it as any sort of significant environmental project is ludicrous. I would venture to guess that Wal-Mart could make a much larger impact, from an environmental standpoint, by looking at their suppliers and demanding responsible operation at the manufacturing level.