I didn't see the beginning of this thread, but... 
One of the things I lost when my house burned was a homemade LPG... 
 
Took an old gas engine; 2-3hp is best, 5 is too hard... which was 
hooked up to an old DELCO truck generator (NOT alternator) with an
8" pulley on the engine with the standard 2-3 inch V-belt pulley on 
the generator. 
 
I disconnected the gas tank and gasline, and drilled a ¬" hole in 
the air filter through which I fed a copper tubing LPG line right 
into the throat of the carb, past the choke, which is disabled. 
 
I put a 10 ohm ceramic 10 watt resistor between the A)rmature post 
and the F)ield post.  With some old generators you would put the 
resistor between the F)ld and Ground since it was already tapped 
inside the case with the Armature windings. You can test the setup 
before with a pair of jumper cables.  The resistor is needed to 
keep the field windings from frying in continuous use. IF you got 
it set up right, the generator will "motor" at a good clip. You 
may havta reduce the field resistor to around 3 ohms on some units. 
 
Bolt both the motor and the generator to a 2X10; apply 12 VDC to  
the A) or (+) and metal frame (grnd) and with the 3:1 v-belt ratio, 
the generator will turn the motor over fast enough to start it on 
LPG, which you must turn on with a gas (heater type) valve. crank  
it out till it puts out around 15 amps and let her run a while. A 
lot of old generators will do 20 amps all day w/o overheating.  I 
know you see alternators listed at 80 or 100 amps, but what they 
do not tell you is that they will only do the rated output for a 
few minutes.. ;)  and- their bearings will blow if you try 25 amps 
for a few hours. 
 
As you crack the gas valve, you'll be able to provide more than a 
cold motor needs, (choke condition) and it should start right up. 
it is a bear to start on LPG with a pull rope cause it floods out 
if you don't crank fast enough.  But with the generator doing the 
cranking, you can back off on the gas valve till it pops off. A 
5hp motor is too heavy a load for most generators to crank, but if 
you can yank the rope, most generators can keep it cranking to start. 
 
The LPG burns so clean, the motor oil stays light for months, but 
change it once or twice a year anyway; some moisture condenses in 
the crank and forms acids that eat at aluminum parts. 
 
Even if you can get an alternator hookup to start, you may see, as 
I have, that if the belt starts slapping it will tear up the bearings 
in short order, whereas the lubrilite bushings used in generators can 
take that kind of abuse till you get a round tuit. 
 
I never got around to installing the 12vdc LPG control valve I got 
for 6$, which wouldda let me turn it off remotely.  Recently I've  
seen some solid state 20 amp relays that were designed for computer 
power supplies which should work as well as a voltage regulator to 
turn off the power should you run outta gas, and which, unlike the 
mechanical voltage regulators, don't have points to burn out. 
 
Two hundred bucks will get you 240 watts of backup power; compare 
that to the cost of PV and batteries to take you through the rare 
times when you get 7 consecutive days of cloudy...
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