Building a record of the outage
You prove a no-hot-water problem with documentation: written reports to your landlord, dated notes or a log of when the hot water was out, any repair records, and even short videos showing the water not heating up.
Because a lack of hot water is not always obvious in a single photo, a clear written record matters most. Save every text and email with your landlord, since these show what the landlord knew and when. Keep a simple log of the dates and times you had no hot water, and hold on to any plumber or repair invoices.
A short video of the tap running cold after several minutes, with the date visible, can help demonstrate the problem. You do not need an expert to prove it; a consistent, dated record usually tells the story. A free review can help you understand which evidence matters most in your situation.
This is general information about California tenant rights, not legal advice. Every situation is different.