I'm looking at a powerwall specifically for powering a sailboat, with solar providing the bulk of my power needs with the remainder provided by a diesel generator while underway.

EDIT: After a quick shower, I wanted to come back to expound on this a bit.

I've also wanted to lead a bit more of an independent life on a sailboat, traveling within my home. As an engineer at heart, I've always thought about how to best manage energy to have the top quality of life on a boat with minimal sacrifice. Solar is almost always the way to go, as the sun is usually shining where ever you're at (some folks do go with a wind turbine, but they're unreliable at sailboat scales).

Tesla's PowerWall product enables this. No longer do I have to consider running a generator full time, or having a dangerous amount of lead acid batteries onboard to store enough power to meet my first world standard of living. The more energy I can safely store, the longer I can be aware from a port, not run a generator, or in short, spend money. The less money I spend, the less I have to work a job I don't enjoy or want to (disclaimer: I love technology, I'll always work in it, I just want to be able to always be picky about the jobs I take)

Dense, cheap energy storage enables everyone to better manage their energy use, and energy is what (mostly) drives the world now. Thanks Tesla!

I have good news and bad news.

Bad news: the battery chemistry used in the Tesla PowerWall (Li-ion NCA or NMC) is not safe for marine applications. Only Lithium-iron-phosphate batteries, with lower energy density, are safe for this purpose. Putting a PowerWall on your boat would not only void your insurance, it would carry a real risk of setting your boat on fire.

Good news: there are already several manufacturers that sell lithium battery packs that have been safety tested and approved for marine use. GIYF.

I had spoken to my prospective insurance company, and they had assured me a lithium ion chemistry was acceptable for onboard use, even if I choose to use it as part of the power train in a series hybrid setup (this is going in a multi-hull catamaran). I'll have to call them back and get something in writing. Thanks!

No problem. Remember when the entire Boeing 787 fleet was grounded due to li-ion fires - you really really don't want that in your boat. If possible, buy something that's been certified for marine use by an independent safety auditor like Bureau Veritas or similar.

1 - Currently (I believe) only grid operators can make profit on grid arbitrage, since in all cases I know you are either paid a lower tariff (i.e. the maximum you pay is lower than the minimum you're sold for), or nothing at all. The only arbitrage you can make is as described here, by storing to avoid buying from the grid, that is, "self-arbitrage", for the back of a better term. Perhaps you could even sell your neighbor some energy, I don't know if regulations anywhere would allow that.

That said, this is obviously the way to go in the future, but it needs a good system-wide regulatory design first (from both an economic and technical perspective) to avoid things like grid instability (from the technical side) and compromising utilities and grid maintenance financially (from the economical side).