Propane Consumption Rate
First thing to keep in mind is that the estimate given by the manufacturer for the gasoline run time can be used for propane also because it is just that; an estimate or guess. Propane only has about 10% less Btu content than gasoline gallontogallon, so the estimate used by Yamaha for gasoline consumption will be very close for propane if not exact. The biggest mistake most "experts" make in opinionating that a propane or natural gas generator will lose power is based solely on these figures and not on any data they personally have. When figuring power output of any fuel, the size of the engine has to be a significant factor of the total equation. For instance, comparing a vehicle with a V8 engine to a generator with an 11hp generator attached can not be calculated the same. The characteristics of the fuels and how they mix with air, how far the piston is in relation the gasoline float bowl effects the power derived from the fuel among many other things. To presume that just because the btu content is less, it is less powerful, is erroneous.
But for those that want even more information we now offer some basic factors, round numbers and some good estimates to see how run time values of propane cylinders can be worked out:
BASIC FACTORS
It requires 2 horsepower to produce 1000 watts of energy per hour under load 

Under load, each horsepower consumes 10,000 BTU per hour 

Propane contains 92,000 BTU per gallon 

Propane weighs 4.2 pounds per gallon 

Cylinders are rated by their weight capacity of propane 

Bulk tanks are rated by gallon 
Cylinder Capacities in Gallons and BTU's
Size  Gallon Capacity  Total BTU Capacity 
20# 
4.8  441,600 
30#  7.1  653,200 
40#  9.5  874,000 
60#  14.3  1,315,600 
100#  23.8  2,189,600 
200#  47.2  4,342,400 
420#  99.1  9,117,200 
Using these factors we can arrive at run times based on average load for any generator.
For instance:
How long would a 5000 Watt Generator with a 10 HP engine at 50% load run on a 20# propane cylinder?
10hp at 50% load would be using 5 horse power to generate 2500 watts of energy.
5hp x 10,000 btu would consume 50,000 btu per hour.
Using a 20# cylinder that produces 441,600 total btu, the engine consuming 50,000 btu per hour would run for about 8.8 hours.
BTU consumption chart based on generator/engine size and load
Generator Wattage  Engine Horsepower  Full Load  75% Load  50% Load 
1850  3.5  35,000  26,250  17,500 
4000  8  80,000  60,000  40,000 
5000  10  100,000  75,000  50,000 
7500  15.5  155,000  116,250  77,500 
8000  16  160,000  120,000  80,000 
10,000  20  200,000  150,000  100,000 
12,000  24  240,000  180,000  120,000 
Many people want to know what size cylinders they need based on their engine size. Here are some real conservative estimates of the vaporization rate of various size cylinders based on the outside temperature.
Vaporization Rates of Cylinders
Output in BTU's per hour  Vertical Cylinder 25% full  Minimum Cylinder Pressure 10 PSI
Cylinder Size
Outside Temperature  20  30  40  100  200  420 
+60F  24,000  32,000  40,100  79,700  125,900  185,500 
+50F  21,200  28,300  35,500  70,600  111,500  164,300 
+40F  18,450  24,700  31,000  61,500  97,200  143,100 
+30F  15,700  21,000  26,400  52,400  82,800  122,000 
+20F  13,000  17,300  21,800  43,300  68,400  100,700 
+10F  10,250  13,700  17,200  34,200  54,000  79,500 
0  7,500  10,000  12,600  25,000  39,500  58,300 
10F  4,780  6,400  8,000  16,000  25,300  37,100 
20F  2,050  2,700  3,400  6,800  10,700  15,900 
For the physical properties of each cylinder, click on the "Cylinder Size" above.
What does all this mean?
Well, if you went exactly by the chart, you would need a 420# cylinder to run a 14hp engine if it was 25% full and 40 degrees outside and keep a minimum of 10 psi in the cylinder. This is a worse case scenario. For instance, when a 20# cylinder is full it can run a 16hp engine for quite some time in 40 degree weather before there will be any freezing problem. But if you wanted to use up all the gas in a cylinder, it would have to be sized according to the chart.
Here is why. Propane is stored as a liquid under pressure and boils to produce a vapor that is drawn off at the top for the engine to use as the fuel. Because propane boils at 44° (below zero), the gas will freeze if it can not absorb enough ambient heat to compensate for the boiling process. The bigger the cylinder is compared to the amount of load, the warmer it is outside, the warmer the cylinder is kept, all are a determining factor in the likelihood of a cylinder freezing up.
If a sweat or frost line forms around the cylinder at the level of the fuel, this is a telltale sign that the cylinder over worked and is in the process of freeze up. If the gas does freeze, it will stop producing vapor and the pressure inside the cylinder will drop to as low as zero psi which will cause the engine to stop running.
Natural Gas Consumption
Natural gas is billed in THERMS.  
This represents a unit or block of 100,000 btu of fuel.  
The average price per therm is around $0.80.  
A generator engine running at 3600 rpm under full load consumes on average about 10,000 btu per horsepower per hour. 
Using these figures, we can figure the estimated usage for any size engine. For instance a 10hp engine used on a 5000 watt generator running at FULL load should use no more than 100,000 btu per hour and cost approximately $0.80 to operate. 50% load (2500 watts output average) should use no more than 50,000 btu per hour and cost approximately $0.40 to operate.
To compare that to gasoline (110,000 btu per gallon) times the cost by 1.1 to arrive at $0.88 per gallon.
So if you are paying over $0.88 per gallon for gasoline, you can save by using natural gas.