I always thought about this myself in terms of personal sized long term, high density energy storage. Compressed hydrogen with a fuel cell is the obvious solution. Excess electric is used in a electrolysis cell and a matched compressor fills a bank of storage cylinders. More cylinders = more storage. Though likely very inefficient with a risk of fire or explosion.
Are there any other long term high density electric storage technologies that can fit in someones basement, garage, or even apartment closet?
> Compressed hydrogen with a fuel cell is the obvious solution.
To achieve volumetric energy density of hydrogen at room temperature that's on par with batteries (and that's charitably assuming you're using inefficient resistance heating with batteries) you need to store it at a pressure in the order of 100 bar.
You're better off with batteries realistically speaking.
Compressed hydrogen is no joke. It can escape most containers, actively degrades many grades of steel, has a very low ignition energy, and will explode over an enormous range of air/fuel ratios. Definitely not something to keep anywhere where you care about the roof :)