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:
- 500 gal propane take - 66 cu ft x10 = 660 cu ft. Cost is $1500 + installation, so about $2000.
- 250 cu ft is $300 for Type K Cylinder.
- Advantage here is high modularity, scalability.
- Type K tanks are about 2x less expensive than propane tanks.
- 7 cfm for .2 hp at $250 
- One type K cylinder at 250 cfm runs this for 35 minutes
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.
- However, a motorcycle bat will last you a year, whereas the tank will be unlimited lifetime.
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 -