Propane

Propane (C3H8) is a common fuel for heating and cooking in rural areas. Most urban folks know it as the fuel for their outdoor grill and an alternative fuel for vehicles, and might use it in a hand-held torch for soldering copper pipe or other small jobs. Often known as LP (Liquid Propane, Low Pressure gas, or Liquefied Petroleum gas), it is a by-product of Natural Gas (CH4) production and oil refining. Portable cooking and heat equipment often runs off of disposable 14 oz cylinders or 20 lb refillable tanks. More permanent equipment is often fed from 100 lb cylinders or 300-700 gallon tanks.

Utility

If you watch the news and documentaries as much as I do, you'll notice that the common white 20 lb refillable tanks are found all over the world, and are often a primary fuel source in third world countries that don't have a developed infrastructure. Even in areas involved in war, someone will smuggle in fuel, and propane is easier to transport than gasoline. Since it has no shelf-life limitations, is in easy-to-handle containers, is fairly safe to use, and is moderately inexpensive, it is a good choice for use when natural gas pipelines are unavailable or inoperative. Having a few spare tanks for your grill is a good idea if you expect short-term power outages -- you can use the grill as an oven, and most of the newer ones have a burner on the side for warming up side dishes. 

You can also run a grill set up for a 20 lb cylinder from a 16 oz cylinder if you have the right fittings.

Safety
Propane is a gas at normal temperatures, but can be easily compressed into liquid form easily. It boils at -44° F, so it must be handled with care to prevent frost-bite. Unlike natural gas (methane), propane is heavier than air and it will settle into low areas of a structure if there is a leak. Since water heaters and furnaces tend to have their igniters near the floor, this can cause explosions capable of lifting a house -- if not destroying it completely.

Ventilation and checking for leaks is the key to preventing fires and explosions. Do I really need to explain that propane is flammable and you shouldn't smoke around it? No open flame or fire within 50 feet of open propane source, please. We don't need to hear about you on the nightly news.

Refilling Disposables
Did you know that you can refill disposable 14-16 oz cylinders? There are fittings available to let you refill them from a 20 lb cylinder as often as you need. (Read the instructions and follow them.  And wear good gloves, preferably insulated rubber gloves; getting bare skin sprayed with liquid propane at -44° means instant frostbite.)

A few years ago, the industry switched to a new style of valve that has a built-in overfill protection device (OPD) to prevent filling cylinders over 80% full. This was done to ensure that there is a vapor space in the cylinder, since liquid propane is not good for regulators and burners. When they changed the valves, they changed the connectors as well to make it easy for the person refilling the cylinder to tell if it was a new tank. The new valves use “Acme” threads, which are more widely spaced than normal pipe threads and have a flat top surface. OPD valves also have to be connected to a fitting before they will allow gas to flow, so just opening the valve may not tell you if there is anything left in the tank.

Refilling From a Tank

With a few fittings and some good hose, you can refill the 20 lb cylinders from a larger tank. You'll need a female Acme fitting to connect to the supply tank discharge valve and either a POL (fine threads inside the valve body) or Acme (coarse threads on outside of valve body) style fitting on the empty tank end. Here's what Acme threads look like:

http://www.ncagr.gov/standard/LP/LPgasConcerns/cylindervalves.htm

Do you see the little screw right above the threads that go into the tank itself? That is the fixed level gauge, a vent that will spit liquid once the tank is 80% (max safe fill level) full. That is how you fill tanks if you don't have a scale to do it by weight.

A simple method of refilling a 20 lb cylinder from a large tank:

1. Put on your gloves. 
2. Connect hose to tank discharge valve, usually Acme threads. 
3. Connect hose to cylinder valve. 
4. Open cylinder valve and slightly open cylinder vent (fixed level gauge). 
5. Slowly open tank discharge valve. 
6. Step back and wait. 
7. Once liquid starts coming out of cylinder vent, close valves and vent and disconnect hose.

If I can find better photos or prices, I may have to edit this post. I need to check a few places and take some pictures when I get some free time. Be careful, and respect the things that can kill you.