T/tree: all good points. Some other comments on the Tesla PowerWall specifically:
1. must be wall (bulkhead) mounted, presumably because of its "liquid thermal control" mechanism. So this otherwise space-efficient packaging cannot be laid flat or on its side...but I wonder how it would cope with the movement (pitch,roll, yaw) of our vessels?
2. max draw is 2kW..(with a surge capacity of 3.3kW) .so around 18A if you are on the North American 110v system or around 8.5A if you are rest-of-world on 220-240v. That's just enough to run one 12kBTU air-con unit but I think more typically, would be used to supply the non-air con demands via an inverter. A genset would still be used to run air-con (but could be fully loaded to recharge the PowerWall)
3. Operating range is -4F -- 110F (-20C -- 43C). That would make risky engine room installations for those of us in tropical, sub-tropical and even temperate (Summer) locations...and have Winter-izing implications for those in colder climes
4. The advertised price before install costs works out at $250kWh according to Tesla reviewers...that's about 25% of the current typical pricing of Li-chemistry batteries. They are clearly trying to build a mass/consumer market and with volume and low margins, so the unit prices are low. Maybe its big benefit for boaters will be to change the whole pricing game for this battery technology, as the Chinese did with solar panel costs
5. Some reviewers indicate 5,000 cycles might be conservative. If $400per kWh installed and 80% efficient, that's around 15c/kWh storage cost.
A couple of reviewing sites, from both technical & economic perspectives:
Why Tesla's Powerwall Is Just Another Toy For Rich Green People - Forbes
Tesla Powerwall: A Battery For Your Home | Gizmodo Australia