Green - An environmentalist community

If you have a water stream nearby, using excess energy to pump the water upstream (so it passes through your generator again when there is no excess) is the greenest thing.

Do you have some resources on how to do that and what performance this could provide on a small scale?

If you know mechanics in your area, repurposing old car batteries (who're not powerful enough to start a car anymore but are powerful enough for quite many other things) is the greenest thing.

I agree with the sentiment of this, reduce new products first, but keep in mind that car batteries are designed in such a way that the moment you drain them close to empty, they immediately take a giant hit in both performance and lifespan. Most car batteries can only survive being drained to zero a few times before dying completely. If I recall correctly, even moderately deep cycles like going between 100% and 50% can still reduce their lifespan drastically. They also just don't have that much capacity in general.

A much better option are deep cycle lead acid batteries if you want to go this route. They're less common and more expensive, but are still very commonly used on boats, trucks, RVs, basically any vehicle where you need a decent supply of power without having to run the engine. More importantly, they're also very common for uninterrupted power supplies, and, yes, off-grid energy storage. They behave like the "normal" batteries people are familiar with, and draining them to zero, while still not recommended, isn't that big of a deal. However, it's likely that they won't really be applicable for your example where the battery can still hold power but don't have enough cold cranking amps, because the original application didn't need much current anyway and they're most likely only replaced when they're actually dead. Maybe a marine deep cycle battery that's used both for cranking the engine and power storage can be applicable in that case, but again, they're a lot less common than car batteries.