As any kind of renewable energy professional and especially as one who deals with people who want to go off-grid, the top question we get, naturally, is: how big a system do I need, and how much will it cost?
If you want to go off-grid, meaning, you tell the electricity company to stuff it (minus the few cents per kilowatt they'll start paying YOU, depending on your state regulations) and just live off what you produce yourself, the answer to how big a system you need is: power output must equal your top consumption on the winter solstice if it's been cloudy for a few days.
If you have no idea what you want aside from just "going solar, baby", this is the answer I gave a customer today:
The short answer to your question is: it depends on how much energy you currently use, and how much of that you want to produce with solar. Your home size and how efficient is your energy consumption are key variables. Many times folks find that going complete off-grid with an "average" or larger home is just too expensive. Energy efficiency becomes very important when you are trying to be unconnected from a utility and every light bulb matters, so it is always, always important to invest money in energy efficiency first if there are opportunities to do so.
What many folks do is choose to offset a certain percentage of their energy with solar. To do this they remain connected to a utility, typically purchasing their energy but also getting credit for what they produce. This makes the cost cheaper (the batteries used to store energy are a significant expense), allows for wiggle room (you don't have to meticulously plan your energy so as not to go over your capacity) and yet still allows you to reduce your emissions and electric bill--the goals of most of us who want to go solar.
To get your first ballpark idea of how much it might cost you, there are some very helpful on-line calculators. Here is a very basic one; also worthwhile is this one, which is more involved and technical but lets you compare two cost scenarios side by side. To use the calculators you'll need to know your monthly energy costs, and looking at two scenarios side by side can help you see how much energy efficiency can reduce the cost of your system.
Federal and state tax credits are a very important component of what makes solar affordable; dsire.org has a comprehensive list of all of these available in all areas of the United States. Some of the calculators try to account for these incentives. If you find yourself interested enough in solar to move forward I would research your local incentives and talk to a good solar professional who can help you design a system that is the right size for your energy needs--and who can help you work with your energy consumption increase efficiency and make any solar you do install that much more cost effective.
That's probably too wordy and technical for a first pass, so if you have suggestions for clarity, let me know, because you know, it really is a complicated, individualized-design kind of a thing*. Which fits in with some of the environmental ideals of holistic understanding of one's living as it relates to energy and the environment. I'm sure it can also frustrate people who like easy, simple answers--but the truth is that to go off-grid with the typical energy usage of a 2500 square foot home you'd have to have a garage-sized room over re-enforced concrete to house the size and weight of your batteries. (Each is roughly one square foot of volume, eighty pounds of weight!) But with all the energy incentives up in some areas, like this one, more and more people are throwing up grid-connected systems as a form of investment, and you know what? That's some progress.
*"Solar in a box" kits do exist--and you're not going to get the most efficient performance out of them, compared to a well-designed system for your individual needs, potentially perpetuating the "solar doesn't work" mindset which is quite far from the truth.