Here is justification for installing solar to provide my transportation needs.

My small EV has 8 kW-Hr of batteries. Assume I drive 50 miles and deplete the batteries. I need enough panels to generate 8 kw-hr. According to a solar calculator for Houston, I need 2.3 kWatts of panels. The data for Houston: 2.3 kwh, 13.7 cents per kwh, and a tracking collector. The result is you harvest 500$/ year worth of energy.
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Here are the panels that I may use. You can sort by different categories. Click at the top of the $/watt column, and one with the best value is the Suntech panel = 175 watt, $ 541.
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Add to each panel, an inverter to invert the dc power to AC. I will effectively slow down my meter, assuming I am using more than I generate.
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The inverter costs 192$. Panels + inverter + interconnect cost is $541 + $192 + $47= 780$. 12 panels costs 780 * 12 = $9360. Add $500 for misc costs. The cost is now $9860. The federal rebate is 30%. Now, the cost is now $6902 for 2.1 kWatt.
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Go to an amortization calculator here. The data: Principal investment $6902, Payments per year is 1, Annual interest rate = blank, Number of regular payments = 20, No balloon payment, payment amount is 500. The annual interest rate is 3.8%. The return could be better. But interest for savings accounts are pretty bad, and the stock market has had a poor return lately.

You want to take bets on what the future holds? Cost of energy going down? Cost of solar panels going up? Improved $ per watt for solar panels - for sure.

The case for electric cars can be made as follows: Assume an EV and a gas car are the same initial cost. That the gasoline cost is 13 cents per mile. Assume an EV costs 5 cents per mile. This includes battery replacement costs played off against gas engine repairs. 50 miles with gas is $6.50. 50 miles of EV cost is $2.50. You are saving 4$ per day. Or $120 per month. Or $1440 per year. The rate of return of purchasing solar panels is 20%. This is at today's prices. The cost of gasoline and elecrtricity is likely to increase.

Note that I did not include the cost of a tracking mount for the panels. I plan to do that design when I get going, and post a set of plans online. It will look like this, but with more panels. See the one axis tracker.