> It has an "app" user interface, requiring both a cellular connection and the service remaining in business. Plus it wants WiFi so it can receive "software updates". What could possibly go wrong?

Do not be so dismissive so fast without reading. The FAQs and the website says that they have a local interface and requires no internet for controlling it. E.g from https://pilaenergy.com/tech-specs

"While we think you’ll love the Pila App, you’re welcome to connect your own monitoring platform with our free local APIs. We embrace and actively support open standards like Home Assistant, Matter, and Thread for local data streaming from Pila Batteries — Because your data should always stay yours. Local API documentation coming soon. "

"Pila does not require internet to provide backup power, monitoring, or smart energy management features. The Pila Battery Mesh Network keeps all batteries working together, even when your home Internet goes out. The Pila App includes a Local Connection Mode for reliable battery control and monitoring without internet.

For reliable Remote Monitoring, all Pila batteries are equipped with a cellular 4G LTE radio for backup communication when home Wi-Fi fails.

However, we highly recommend keeping Pila batteries connected to Wi-Fi to receive the latest software updates and unlock new features, enhancements, and performance improvements—ensuring your system gets smarter and more capable over time."