Sixty to eighty percent of software still in use is legacy. That is the share that published research puts on old code. These systems were written when computer time was cheap and energy was invisible. They run all day and all night, often on many machines. Most have never been measured for energy. And nobody wants to touch them. So the biggest pool of avoidable energy is often the oldest code. Here is the fix you can use today. Rank your systems by how much they run, and how many copies exist. Map and test the top few before you change them. Fund the work as maintenance, with a measured energy and cost result. GreenCode's legacy work starts from this research. Size each codebase and find its bugs, using tools like cloc and SonarQube. Then refactor across the whole codebase, using tools like OpenRewrite. GreenCode helps tackle this by mapping old code, then writing its missing tests and documents. Only then does it refactor, and each change comes back for your review. That was GreenCode Tech Tip number thirty-five. Follow for the next tip, and find more on greencode.ai/tech-tips.