The battery packs would be able to recharge themselves whenever power rates are lower. Generators currently do not re-fill themselves whenever they run out of fuel. An additional feature would be to run the house off of battery during the day and recharge at night- effectively giving you an overall cheaper electrical rate. Upgrading a Powerwall would be as simple as connecting another battery pack to the existing unit. If you outgrow your generator, you need to toss it and get another one.

They both fulfill different needs. I like the Powerwall concept quite a bit better than a generator.