BTU/h requirements are for any inverters that are around 4500W and/or 9000W?

90,000 BTU/h for 4500W and twice that for 9000W

Also, is it a good assumption that the same size inverter would have about the same (+/- 20%) efficiency regardless of brand?

20% is the common efficiency used for calculations. Some fuel injected versions have a slightly better efficiency, but it is not that much.

Be sure and consider the temperature vs vaporization rate for the propane.

 

Oops, forgot to say that those BTU/h numbers are at 100% load.

 

I want my planned propane system to have enough capacity for a generator. One with 4500W surge capacity will take care of my basic back-up needs, but I want to consider the piping cost for one with 9000W generator for future options.

Most the tables base the minimum piping size on the required BTU/hour. I have checked the specs and manuals of several inverter generators, but have not seen one that states the required BTU/h input at full power. One posting I read stated that this type of information is required to be shown on the generator plate with model number, serial number, etc. Does anyone know what the BTU/h requirements are for any inverters that are around 4500W and/or 9000W? Also, is it a good assumption that the same size inverter would have about the same (+/- 20%) efficiency regardless of brand? Thanks for any assistance.

the best place to start is how many CC is the engine?
second how many cyls?
and last what is the make and model on the generator?

 

When you say "propane system" I am taking that to mean an installed permanent tank.

Regardless - portable tanks or permanent - your tank pressure isn't going to be your operational pressure.

This is important to keep in mind because as the propane drops in your tank you might loose flow at your operational pressure on the bottom end - say, below about 20% of your tank capacity.

A generator engine is going to draw more fuel than any other load I can imagine you would have on your system. The bigger the engine the more exacerbated the potential issue.

And again - a propane system will have a high tank pressure, then you will drop down to your operational pressure. What operational pressure you have for your home vs the generator might be 2 different numbers. A usual value for demand regulators on engines is 1/2 PSI, or 14in H2O. You will need to drop the tank pressure to this. And again - if the volume of fuel in the tank (what ever the tank is - 100,000 gallon industrial or a 5lb portable, and anywhere in between) is too low - it doesn't matter what the "piping size" is - there won't be enough fuel squeezing out of the tank to meet a high demand and you'll starve the engine.

The line between your step-down pressure regulator (from tank pressure to the operational pressure of your gen/demand reg) is the line that will matter more than anywhere else in the circuit. I would keep the line as short as possible.

For the size gens you're working with - a 3/4" line would be a good place to start. If you plan to upgrade and get a whole house gen later (15-25kw) then you might want to go to a 1".

We have a 15kw gen as an option and it will run the whole house (including central AC starting and running) on a 3/4" gas line - natural gas, not propane. That isn't to say 1" would be better, but for the draw of the engine and the load on the gen in our case it works. That is at 1/2 PSI service pressure from the meter. So for 4500w-9000w a 1", in my opinion, is overkill.

 

This link won’t help vaporization rates and pipe sizes, but can help determine basic BTU determination.

may have to download your own copy to be able to edit the cells.

 

I made my own chart. It is 20KBTU/Hr per each KW of gen output.

 

FWIW, Generac provides a fuel consumption table. The gas company might be able to extrapolate the required info from it.

 

Generac provides a fuel consumption table.

Yes, some manufacturers do provide the consumption rates for their various generators. If they don't, then assume 20KBTU/Hr per each KW of gen output for system design and you will have enough fuel in virtually all scenarios.

Propane varies in its BTU content because it does not have a consistent makeup across the country, let alone across the world. The usual numbers you will see are between 91,500 BTU/gallon and 92,000 BTU/gallon.

For the above Generac chart, it extrapolates into this:

8KW gen at 100% load uses 1.42 gallons/hour of propane.
92,000 BTU/gallon X 1.42 gallons/Hour = 130,640 BTU/Hour

20KW gen at 100% load uses 3.85 gallons/hour of propane.
92,000 BTU/gallon X 3.85 gallons/Hour = 354,200 BTU/Hour