As of this moment, I don't have the "demand" regulator.
That one is required to make the generator run. I have run straight from a BBQ bottle and regulator for short tests - it WILL fire up and produce electricity, but isn't constant, and kills quickly, unless you can manually adjust the flow of fuel to the generator.
I have done this experimentally, and run a 70-watt light bulb successfully for up to a minute. Kinda hard to drive a car and throttle a propane bottle by hand at the same time though!
Once I get the fancy regulator in and hooked up, then I can experiment with how big of a primary regulator I need, and if a 20lb BBQ bottle will be up to the task or not.