Energy Star is a certification program for energy-efficient products, including space heaters. Energy Star certified heaters are generally more energy efficient than non-certified models, which can save you money on your energy bills over time. The difference in energy efficiency between Energy Star certified heaters and non-certified models can vary, but in general, Energy Star certified heaters are designed to use less energy while still providing the same amount of heat.
As for the Energy Star website being down, it's not uncommon for websites to experience temporary outages. You can try visiting the website again later or find more information about Energy Star certified products by searching on the U.S. Environmental Protection Agency (EPA) website.