Monocrystalline silicon (mono-silicon or single silicon). Right now, these are the most efficient type of solar panels. In other words, when sunlight hits them, more of it turns into electricity than other types. As a result of their high silicon content, they're also more expensive, but you need fewer of them. That's why they're ideal for roofs. You can tell if you have a monocrystalline solar panel by its square-ish cells