Ceiling – Reinforce ceiling joist for ceiling storage

We're still living in complicated times when it comes to figuring what's best in terms of energy usage and storage.

If the goal is to become purely fiscally self-sufficient, I think ben's answer is really good. Sell your excess for cash. Then use said cash to exchange it for 'cheap' (relatively speaking) storable energy sources like a propane tank + generator.

Whether we like it or not, fossil fuels are still the cheapest way to store energy for extended use.

If the goal is to help the environment, then ideally you are never drawing power from the grid at all if the grid is powered by fossil fuels. On the other hand, if the grid is powered by wind or solar (or, arguably, nuclear) then it actually may be better for you to draw from the grid rather than storing it yourself.

If the grid is fossil fuel based, then you're back to finding a device to store energy with.

Options:

• chemical energy = batteries (likely most common for household usage)
• kinetic energy = flywheels (my favorite concept, but not really affordable or practical at a household size)
• kinetic energy = compressed air (already mentioned)
• kinetic energy = water pumping
• heat energy = ideal if a large part of your energy needs are for heating

and plenty more options: http://en.wikipedia.org/wiki/Energy_storage

Based purely on your 3 bullet point needs, I'd say the propane tank + generator are your best bet.

Unfortunately, as with most questions like this, you're going to have to talk to an engineer. A bunch of amateur DIYers on the internet, is not the proper resource for this type of information.

You're making a major structural change to a building, and you don't want it to cause property damage or personal injury. The "correct" and responsible thing to do, is to seek the advice of a trained professional.