Flat plate collectors are generally more efficient than ETC in full sunshine conditions. However, the energy output of flat plate collectors is reduced slightly more than ETCs in cloudy or extremely cold conditions. Most ETCs are made out of annealed glass, which is susceptible to hail, failing given roughly golf ball -sized particles. ETCs made from "coke glass," which has a green tint, are stronger and less likely to lose their vacuum, but efficiency is slightly reduced due to reduced transparency. ETCs can gather energy from the sun all day long at low angles due to their tubular shape.