The cost of solar photovoltaics keep dropping. In fact, solar cells have been getting cheaper every year for so long that they now have their own equivalent of Moore's Law: Swanson's law:
Swanson's law is an observation that the price of solar photovoltaic modules tends to drop 20 percent for every doubling of cumulative shipped volume. At present rates, costs halve about every 10 years.
Unfortunately, a complete photovoltaic system consists of a lot more than just solar cells, and the other components of the system are not getting as cheap so quickly.
However, this does introduce an interesting possibility: It may become cost effective to install solar panels on vertical surfaces even though they won't be as effective as a rooftop installation. You'll need more solar panels to generate the same amount of power as a rooftop installation with a better angle. This added cost might be entirely offset by much lower installation costs, however. As an additional bonus, vertically mounted panels will be much easier to keep clean, especially if they're installed on first floor walls.
I've been thinking about this idea for a while, but today I started wondering how far you could take the idea, and I came up with an even more absurd idea: the solar garage door. I'm imagining a mass-produced, standard-sized garage door with integrated solar PV panels, which is a drop in replacement for many of the garage doors currently installed in houses all across the U.S. Replacing a garage door is by far the easiest thing I can imagining doing to the outside of a house, and is well within the capabilities of the average do-it-yourselfer. I would expect a couple of professionals to be able to replace a typical garage door in a matter of minutes.
Obviously not every garage door is a candidate, since many will be facing the wrong direction, and there are many that are too small, or oddly-sized. I don't know how many garages there are in the United States, but I'm guessing on the order of tens of millions. And probably about a quarter of them are south facing. In addition, the typical garage door will not be obstructed by trees or other foliage since there will almost always be a driveway in front of it.
OK, so realistically, a standard American two-car garage door is only about 10 or 12 square meters, and you can only expect to get 100 or 200 watts per square meter even in the best circumstances. So the solar garage door might only produce 5 or 10 kilowatt hours over the course of a day. That's not enough to run a house or charge an electric car. But it might be enough to pay for itself in a couple of years, especially since the greatest power production will occur during the times when electricity is the most expensive. And certainly this idea makes more sense for Dallas than Seattle.
Five or ten kilowatt hours is hard to get excited about in most circumstances. But there are times when it is very exciting indeed -- when the power grid is down. Keep in mind, as I write this Florida is recovering from Hurricane Irma, Puerto Rico has just been hammered by Hurricane Maria, and Houston is still drying out from Hurricane Harvey.
Many people keep generators for dealing with prolonged power outages. Solar PV has some potential advantages. It can pay for itself over time since it produces electricity for free once it's installed, and it doesn't require fuel to run. In fact it doesn't have moving parts, so it's very reliable compared to a generator which may or may not run if you haven't used it in a long time -- or possibly ever. And a solar system doesn't preclude buying a generator as well. In fact, after the solar system pays for itself, it can pay for the generator too.
A solar garage door will only produce a little bit of power during the day, but even a small installation can produce enough power to charge a cell phone or a laptop, and possibly even run a freezer enough to keep it from thawing out overnight. A refrigerator may require a battery to keep it cold enough overnight, but batteries are getting cheaper too.
So anyway, my argument for the solar garage door is that it could be an insanely cheap way to retrofit solar to an existing house, its incremental savings in the electricty bill will allow it to pay for itself, and then it provides some added security in the event of a prolonged power outage.
It also provides a way for prospective solar customers to get their feet wet with minimal risk, expense, or even hassle. And it might be a good way for a new solar company to bootstrap its way into the emerging market (well, maybe) of vertically installed solar panels.