Compressed Air Storage
Taking example calculation at https://en.wikipedia.org/wiki/Compressed_air_energy_storage -
a 500 gallon propane tank stores .75 kWhr of energy when compressed to 200 PSI. Pressure rating of propane tanks is 215 PSI - . Taking efficiency of an air engine - generator to be 50% - we have .4 kWhr. So a 500 gallon tank would get us 0.4 kW hr of usable energy.
See comments below:
For volume of 500 gallons (propane tank) -
E=Pressure x Volume.
=200 PSI x 1.9 cu meters =1,400,000 Pascals x 1.9 cu meters =2.7 million Joules =.75 kW hr
With 60% efficient air engines - this makes it slightly under 1/2 kWhr of energy storage.
For comparison, one motorcycle battery at $20provides a comparable amount of electrical energy storage, making a weak case for compressed air storage on cost considerations.
For comparison - if one could generate an average of 50W via wind power, that appears to be an easier route of power generation.
- EPRI head states that air will be cheaper per watt hr store than batteries. - 
- Cost Comparison of Energy Storage -