The main difference between mono-crystalline and poly-crystalline panels is that of efficiency. In lay terms, it means that a typical poly-crystalline panel will occupy a little more space than mono-cystalline panel as mono-crystalline panels are usually more efficient. As it takes less space, a more efficient panel can lower cost of structure, wiring, and other components in an installation. This means at a system, level mono-crystalline panels often can be as competitve as poly-crystalline panels but it depends on the premium you are paying for mono-crystalline panels.