To give more functionality around energy management, we wanted all of the batteries to be able to work in concert. We could have gone with "any old wireless" network to coordinate the batteries. One thing that's really stuck with me from building products at Span and Tesla is how important reliable connection is over years of operation. Depending on consumer wifi is a huge pain for reliability (changed passwords, inhomogeneous equipment, etc), using cellular is OK but has latency limitations and can be expensive. Existing stacks like Zigbee just had other limitations we wanted to avoid, so we realized borrowing from the world of the mesh wifi systems could offer a lot of what's needed. If we do our job right, the result will be a reliable connection over the years, that also facilitates easy addition of new battery nodes as you go, as your needs evolve.
re: funding, We've not yet made our detailed financing info public, but you can check out crunchbase for some insights on our investors to date!
By "more functionality around energy management" you mean what? For example if a battery powering a fridge detects a loss of power, it could inform a battery powering less essential things to shut down, so that the user would have the ability to swap batteries once the fridge one runs out? Is this the type of use case you have in mind?
I'm curious about the Zigbee limitations you're alluding to. Bandwidth is limited for sure, but I would expect a sub-GHz mesh network to be even more limited. The only issue I could potentially see is that a Zigbee mesh might work great when the power is all on, but might suddenly partition once power is cut and some intermediate nodes suddenly go offline. That would be a surprising thing to happen in the middle of a power outage.
Zigbee is on the same frequency as WiFi and therefore suffers from all the same issues that WiFi has interference and limited range. SolarCity used Zigbee for communication with solar inverters. It was very unreliable. Tesla moved to WiFi after acquiring SolarCity.
Zigbee is sufficiently low bandwidth that interference is typically a non-issue, and repeaters for its mesh network are easy and cheap to add for extending range and dealing with interference. This simply isn't the case with WiFi.
Did you consider using homeplug, IEEE 1901, or some other network over powerline standard? I imagine it being more reliable if they're all behind the same breaker.
Unfortunately, AC powerlines are extremely noisy. Powerline comms are used by SolarEdge to communicate with their optimizers. Because it’s a small closed loop there is far less noise than the home grid.
re: funding, We've not yet made our detailed financing info public, but you can check out crunchbase for some insights on our investors to date!