Monocrystalline vs Polycrystalline (Multicrystalline)
Commercially available solar modules are usually made of either monocrystalline or polycrystalline cells (the other main possibility is “thin film” which is currently significantly less efficient, has a much shorter useful life and is not widely used). Monocrystalline cells are made of slices of a single silicon crystal whilst polycrystalline cells come from slices of silicon formed of multiple crystals fused together.
Efficiency and loss of power when hot
Being formed from a single crystal, monocrystalline modules are typically marginally more efficient than polycrystalline and suffer a little less loss of power when the module becomes hot (typically losing 12-15% at 50°C / 115°F as opposed to 14-23% for polycrystalline panels).
Useful life
Monocrystalline modules are also expected to degrade less than polycrystalline so that a guarantee of still producing 80% of the original power after 25 years of use is possible. In fact, many monocrystalline modules are expected to continue working well for over 50 years.
Polycrystalline modules have historically been significantly cheaper than moncrystalline modules although advances in production means that the costs are now largely similar for both.
As monocrystalline modules remain typically slightly more efficient, lose less performance in high temperatures and are projected to have a longer useful life, they are generally the first choice for most purposes.