Solar thermal projects (space heating, water heating, passive solar homes...) emphasized on this site tend to have good economic payoffs. Many of the projects listed here have simple payback periods under of 3 years.
Building a solar project protects you from future increases in energy prices.
The "dividends" earned from solar DIY projects are inflation protected and tax exempt.
State and federal governments offer economic incentives to encourage development of renewable energy projects. The new federal energy bill offers tax rebates for most solar energy projects. I have completed projects in which the full cost of materials was paid for by these incentives.
Households consume a large part of the energy pie. Solar projects to reduce energy consumption have a significant beneficial effect on fossil fuel emissions.
Solar thermal projects emphasized on this site make good Do It Yourself projects. They are within the skill level of typical DIYers, and offer large savings over using commercial products.
Good projects to get the kids involved in, illustrating how the sun works, and how best to take advantage of it.
This is something that our great-grandparents understood, but we have lost over the last century of extreme dependence on fossil fuels.
Being at the mercy of (or in the center of) middle east turmoil, and paying out $15+ billion a month for foreign oil is no fun! So, put on your tool-belt and do something about it!
For a brief and excellent rundown on the energy challenges we face in the US, look here. Its and eye opener.
Also, "The Energy Guy" provides very readable information on energy issues and climate change Energy Issues Summary.