Mike, you illustrated what I meant. The example you cited of land and a 401K have intrinsic value for an infinite period. Machinery, cars, and solar panels don't. One way to look at it is you are prepaying the electric bill until the system breaks even, then you are in the profit. Using a quick google of some government site
http://www.eia.gov/tools/faqs/faq.cfm?id=97&t=3
my state uses an average of 1029 kWk/month, costing $95. This puts breakeven on a 900 kWh/month system at 17.54 years as calculated on cost. If it runs for another 2.5 years (20 year life) you make $2,850, or if it goes to 30 years, you make $14,250, assuming no breakdowns.
Now, if you had invested $20,000 at 3% for 20 years, you would get back $33,090, 30 years gets you $42,570. For my simple mind, investing in this scenario nets you $10,240 more, or for a 30 year, $8,320 more.
At some point, it looks like the solar panels beat the return of the investment, but it would be in year 70 or so I think. Another thought, sure the panels may last 30 years, but what if technology changes between now and then, and in year 15 the "new improved panel" comes out and you want to replace yours. Then you've lost money for the first 15 years.