To get solar panels installed for free, partner with a company looking to provide free installation in exchange for the government incentives that come with owning and operating a solar array. Companies such as Sunrun install solar panels for free and charge a monthly fee for the power they provide.
Some people buy solar panels thinking that they are eliminating their power bill altogether. In some places this is the case, as the solar cells generate enough power to provide the home with electricity and even add some power to the larger grid. However, companies such as Sunrun don't provide solar cells for free with that sort of arrangement.
Instead of charging up front for the solar array, companies such as Sunrun install the panels at no cost and then take the financial benefits. They've put the money into the solar panels, not the homeowner, and so they benefit from much of the electric savings and from the government incentives for the installation. The homeowner likely has an electric bill that is lower than before, but the rate gives Sunrun a profit to help it recoup the costs of the installation and then some. So while the initial installation is free, the homeowner doesn't get as much of a cost reduction on his power bill as he would have if he'd paid for the solar panels himself.