When sourcing these windows, you must prioritize the U-Value (thermal transmittance) and the Solar Heat Gain Coefficient (SHGC). A lower U-value indicates better insulation, while the SHGC should be chosen based on the climate; lower for hot regions to block heat, and higher for cold regions to harvest solar warmth. Additionally, ensure the Low-E coating is on the correct surface (usually surface #2 for cooling climates and #3 for heating climates) and verify the use of Argon or Krypton gas fills between panes to maximize energy efficiency.