If I install solar panels on a 50K sq.ft. roof in N.Calif (PGE cust.), how do I sell the energy?

Most likely you'll set up a deal with the utility company to have them install a grid-tie meter that can monitor power you put out as well as take in.

As for a roofing material, they just recently came out with solar roof tiles. Here's an example: http://www.premierpower.com/solar_energy_residenti...

I don't think they generate as much electricity as other panels because they're limited in their orientation to the sun. That may be ok for you though since the savings from the roof install may make up for this.

I don't know about using a solar roofing material that covers the roof and serves as the collector but in order to sell the energy you just need to look up "net metering laws" in California. You'll have to pay PGE to install a separate meter and hook up your new "off the grid" building to the grid but it's doable and you might be able to make money off of your idea.

Goos Luck!

It is rare to be able to generate energy in excess of what commercial tenants will use. If you have A/C in the building, that alone will be hard to keep up with.

You'll need to call PG&E to start a conversation. At the power levels you're contemplating, you might be required to be classified as a small generator, meaning you actually get paid for every kWh you produce, although a paltry sum (the wholesale rate, minus distribution costs). And you might not be able to net meter. PG&E may also require that your power drop be updated. There is a classic story of one remote house that wanted to have a large array, but PG&E initially insisting on a $600,000 upgrade to their power lines. That was eventually resolved in favor of the solar producer, who only had to pay $20,000 in the end.

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Call PG&E. Your meter will run backwards and they will credit you for your generation.