$20K for a home battery backup for someone capable of doing DIY would be far larger than what I assume he has built here. AFAIK the cheaper end is around $340 (2016) per kWh at 20 kWh that would be $6,800. In 2025 at $100 per kWh it would be $2K. If it's worth it would largely depending on a persons post tax required rate of return and how long it would take.
If you took that same time, and invested it in working at Target, or Amazon etc, would you have more or less money than it would cost to buy an off-the-shelf battery? There are obviously other pros and cons.
I spent almost as much as that for a 2 Powerwalls and installation in 2019. (Granted, I got a 3rd back from various incentives that probably weren't available for DIY.)
DIY (like this project) is only "worth it" if the person doing it enjoys the work or values the lessons.