Megapack is the largest battery system that Tesla sells they can integrate it with the Superchargers so help reduce the peak energy charges that Tesla would get depending on how popular the site is.
To add a little more to that see the graphic below. SCE has Time of Use rates and most Tesla Supercharger installs fall into the TOU-EV-9 rate schedule. If Tesla is able to manage their power by charging their Megapack during offpeak hours and then discharging the Megapack when cars are charging during the most expensive time frame they are able to save substantially in electricity cost.
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This image below is the Tariff and I am just highlighting that Tesla services are under 2kV (Typically 480V) and that the on-peak generation charges are substantial compared to the mid and off peak.
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