How do I explain work experience in a different field on my SOP?
There are many people who do exactly this - many have worked in a field that is almost completely unrelated to the subject(s) that they are researching.  There is a major opportunity to emphasise the demonstrable tranferable skills that you developed from your work that would benefit your research.

In your situation, as with many similar circumstances, specific examples of technical skills that you used and developed as working as a software engineer that would be relevant in your research in electrical engineering.