An important role in any sales job is being honest. If you're not honest, you cannot easily earn people's trust, so given that I normally sell solar panels on a day-to-day basis, here are some of the downsides of using them to generate power or electricity.
Firstly, by far the biggest downside is the initial cost of equipment necessary to generate electricity or hot water from solar technology. Solar energy technologies have a much higher initial necessary cost than just buying electricity from the national grid. Certainly, there is constant research into this area to try to bring down the cost of solar energy. This research is very important. Companies want the technology to manufacture solar panels as cheaply as possible so that they can out-compete other solar panel companies on price. Consumers want the cheapest solar panels possible because that means, environmental concerns aside, they will have the maximum amount of money to spend on other things, like food! The government want the technology to bring down the prices so that solar panels become more widely adopted. This is so that the government can meet its emission targets, and because the green vote is constantly becoming more important - there is a paradigm shift occurring in that the population are really starting to care about the environment, with phrases such as your "carbon footprint" coming into common conversation.
The government, however, do provide incentives to help reduce the costs to home-owners of installing solar systems, such as the Feed In Tariff available in the UK, which means that solar panels will pay themselves off over about 10 years, and the payments last for 25 years, so a large profit can be made over time. However, there is still that large upfront cost with which to come to terms.
Solar panel installations also need quite a large are of roof space. A system requires at least 3 to 4 square metres of roof space as a bare minimum. This is about the area needed to park a small car. However, typically, a much large system will be installed, because then the cost per unit of electricity generated will fall enough that it is a worthwhile investment. The reason why the return will improve with a larger system is that there is still a very similar amount of labour required to install the solar panels, but they will generate much more electricity, obviously!
Air pollution can be a problem for making solar power, and often one that isn't mentioned. It can degrade the efficiency of photovoltaic cells, in the same way that cloud cover can. This is because less energy from the sun will get through. None of these effects are long-lasting, and are really only an issue in very heavily polluted areas - such as cities in less economically developed countries.