In crossword clues, common answers for “unit of heat” include THERM and CALORIE. Another frequent option is JOULE, depending on the clue length and crossing letters. Here are the typical possibilities by length you’ll encounter:

• 3 letters: BTU (British Thermal Unit) is a frequent fit in energy-themed clues.
• 5 letters: THERM
• 6 letters: JOULEs (often just JOULE in singular)
• 7 letters: CALORIE

If you share the exact clue (including the number of letters and any crossing letters you have), the most likely answer can be narrowed down precisely.