$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 think Target isn't the right comparison here - the skills required for this project are worth much more than minimum wage bagging groceries. If you assume something like $50 an hour (on the low end for a skilled electrician), you get to the $6800 number in the parent post pretty quickly.
Of the three options, DIY battery packs, premade 100aH battery packs, or white glove powerwall a minimum wage earner would likely not have the skills to DIY the battery packs nor the money to pay for the powerwall.
Battery packs are an efficient market commodity and that’s pretty hard to beat for value for money.
Once full installations become more of a commodity then DIY with premade packs becomes less worth it.
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.