Besides being more environmentally friendly is it worth the investment in solar cell panels and wind mills for home use? Is it possible to be "off the grid" completely?
These are two rather distinct questions; only one of which is relevant to a Money web site.
In general, the investment is questionable. Leaving aside the green feel-good factor, you need to look at a total cost of ownership (TCO) and payback on the asset. Neither is cheap as such. There are DIY windmill plans, but you likely still need a commercial battery charger/inverter/controller setup.
Government incentives, depending on where you live, may change the story considerably. Many jurisdictions around the globe have both incentives to install and then power-feed-in tariffs if you sell back excess power.
Your latitude also has an impact on your total available solar energy, along with regular weather patterns for both cloud cover and wind.
One of the cheapest solar projects can be hot-water. Particularly if you have a pool, or even for domestic use, if you use a lot.
All that said, given the green feel-good factor, if you want a small set of solar panels and have the space/budget, go ahead! You can add more later.
For the second question: it is indeed possible to live off grid. Some remote houses do just this, and the methods to accomplish it vary. The number one thing you need to do is work on a power budget; and be both ruthless as well as realistic. Fridge, freezer, AC, furnace, plasma TV, etc. Depending on your climate and preferences, these may not all be possible for an off-grid lifestyle. (Of course, if you get a propane fridge and have a truck come by once a month, does that count as off-grid?)
To answer the investment aspect takes a bit of math. First, solar insolation numbers:
This represents the average sun-hours per day for a given area. You can see the range from 4 to 6, or 1460 hrs to about 2190 hrs of sun per year depending on location. I believe electricity also has a range of cost, but 15 cents per KWH is a good average.
So, a 1KW panel will produce as much as $328 per year of electricity in a high sun-hrs area, but only $219 in a lower sun-hrs area.
If we agree to ignore the government subsidies and look for the stable price unaffected by outside influence, an installed price of even $2500 would produce a return of 13% and a reasonable full payback over an 8 year period.
I call this installed price a tipping point, the price where this purchase provides a decent return. Some would accept a lower return, and therefore a higher price. As duff points out, this should be treated as the post rebate/tax credit price. Those help to push the price below this point. At the price point where the energy cost per panel is below, the government intervention may be unnecessary. The power companies may find consumer owned panels are the cheapest way to clip the peak consumption which tends to be the most expensive power demand.)
One can take the insolation numbers and cost of local power to produce a grid showing the return for a 1KW panel in $$/year. (At this point the cost of money kick in. The present value of $100/yr is far higher today than if short term rates were say, 8%) Once panels drop to where they are compelling for the higher return areas, I'd expect volume to drive continued improvements in cost and better economies of scale. Initially, the need for storage isn't there, as the infrastructure is in place to drive your meter backwards if you produce more than you use. The peek sun coincides with peek demand and the electric companies are happy to have your demand go negative during those times.
Update - the conversation with Duff led me to research 'demand charge' a bit more. You see, the utility company has to have equipment to generate the peak demand, usually occurring in the early afternoon, say 12N-2PM as the sun is brightest and AC use in particular, highest. I found that Austin energy has a PDF describing the fee for this. Simply put, the last kW of demand will cost you $14.03 in summer months and $12.65 in winter. This adds to $160/yr that a 1kW panel might save the owner. Even if one does capture the full power at peak every month, $100 is still non-trivial. This factor alone justifies $1000 worth of panel cost, and as Duff points out, the government may find it cheaper to use this method to clip peak demand than by funding bigger power generators.
To summarize, the question isn't so much "are they worth it" as "what is a xKW panel worth?" (A function of annual savings and time value of money.) The ever decreasing installed cost for a given system makes solar an inevitable part of the future power technology. I am not a green tree hugging guy, but I do like to breathe fresh air as much as anyone. I'm happy with whatever role solar plays in cutting down pollution.
I have personally known a family in the hills of Southern Oregon, US who lived off the electricity grid.
As far as being "possible" yes, but easy is a certain no. This family was very dedicated to the point of living without grid electricity. A special built home of native field stones, careful alignment with the sun, location within the valley. I would assume that making a normal home be off the electric grid is much more difficult. Not impossible, but pretty darn hard.
Although this isn't related to homes directly, as an IT professional I know that wind power tends to be cost effective to the point that many data centers (the massive buildings holding the servers that are the backbone of the internet) actually invest in their own wind turbines to slash costs since servers tend to be power hogs.
As far as going "off the grid" that ultimately depends on how much wind/sun you're getting at your residence, but if you look at places like Dallas, PA, CA, and other areas where the major hosts place centers, they're typically in areas where there's plenty of sunlight or wind.
Going back to small scale thinking however, one of my contacts actually leases a colocation building in PA where he has a few server racks, and while he currently has electric there, he also owns a couple of turbines which have been powering <60% of the demand, and he's actually planning to add solar and also feed that back to the grid at a profit.
So overall wind/solar definitely has the potential for a decent ROI, at both large and small levels, but performance will vary greatly from area to area. I know that Lowes actually started advertising about carrying solar panels, so going in and asking about the performance and if you can arrange an audit of your home might be a good place to start.
If you Google "green audits" I'm sure you can find a trillion companies "specializing" in green power, but as with any sales rep (including at Lowe's) I'd do some due-diligence so you don't get taken for a ride, and also to check references because I don't think "green audit" companies have any official certifications/standards.
Solar water heaters are definitely worth the money (if you live in sunny states like South-South-West or Hawaii, at least). In some countries (like Greece, Cyprus and Israel, to name a few) most people use hot water from the solar heaters almost exclusively. I pay $30-$40 a month to PG&E for the privilege.
Unfortunately, in the US these heaters are much more expensive than they are in the more advanced European countries, so all the savings go to drain because of the vast price difference ($300 for a gas heater vs $2000 for a solar heater).
Solar water heaters are definitely questionable in the Northeast -- the season when you most need them is also the season when they are least effective.
Solar electric isn't a huge moneymaker, but with rebates on installation and carbon-reduction credits (SRECs) -- and a group purchase discount if you can get one, either at a town level or through organizations like One Block Off The Grid -- it can definitely turn a profit. Early estimate was that my setup would pay its initial costs back in 4 years, and the panels are generally considered to be good for a decade before the cells have degraded enough that the panels should be replaced. I haven't had a negative electric bill yet, but I've gotten close, and my setup is a relatively small one (eight panels facing SSE on a 45-degree roof). Admittedly I've also been working to reduce electricity use; I don't think I have an incandescent bulb left in the house.