What would be a good hypothesis for a science fair project that's about energy in fruit?

First, energy is not a physical unit. It is a philosophical concept and an accounting technique used to analyze mechanical and chemical exchanges. In nuclear physics it is defined as a color of light, in biology it is a synonym for metabolism, and in public utilities they say energy when they mean connectivity.

Second, when you ask for advice you have to give enough information to lead to an answer. That would the general subject (fruit is ok), your grade level, your level of expertise, your ability to buy equipment and supplies, time to spend on this, when it's due, whether you have a mentor to help you through it, and so forth. It is your teacher's job to help you choose an appropriate project, and that would include the subject of investigation, often called a thesis.

A hypothesis is an attempted explanation of something you have observed. I know the words are somewhat similar, and I doubt that anybody would complain if you say hypothesis when you mean thesis, but that sort of mistake tells people that you don't know what you are talking about. So my advice to you is to study the topic until you are able to write a thesis without anybody's help, and present it with no mistakes.