Elon Musk claims Tesla can make solar-panel roofs cheaper than an ordinary shingle roof, even before taking into effect the energy costs. But there’s a big catch to his claim.

The company recently unveiled a design for glass solar tiles that are transparent when viewed directly, but have a special coating that, when seen from an angle on the ground, has the appearance of a traditional material.

The problem is that the price comparison appears to be to more expensive roofing materials such as slate and that the solar panel tiles will likely still cost more than cheaper materials such as asphalt. (Of course, in the long run the energy bill savings could overcome that.)

While it might seem unlikely that solar panels could cost less even that slate or terra cotta, Musk argues there are two main differences. One is that traditional roofing materials are comparatively heavy and have high shipping costs compared to the glass used in the Tesla tiles. Another is that the tempered glass tiles are much less prone to breakage in transport, something that makes up a decent chunk of the total costs of a traditional roof.

Musk believes Tesla can win people over to solar by producing individual solar tiles that are laid on a roof in much the same way as ordinary roofing. He argues that the more common method of large solar panels is offputting to many homeowners.

The downside is that it’s an approach much more suited to new home construction. With existing homes, people may be in fact be more willing to install a solar panel on top of existing tiles than to completely replace a perfectly functional roof.