ENGINE POWER. The basic formula for the power of a steam engine is:

Indicated Horse Power =
                                                                     P x L x A x N
                                                                          33,000

Where P= mean pressure in cylinder, in pounds per square inch
L= length of stroke in feet
A= area of piston in square inches
N= number of power strokes per minute.
I.H.P = indicated horse power.

P is usually taken to be 75% of boiler pressure (UK, or 85% in USA),
but will depend upon the cut-off.
N will be 2 times rpm for the normal double acting engine.

For a compound engine:

P =             HP
                 LP + ratio of LP : HP

Where:
LP = mean pressure in low pressure cylinder
.
HP = mean pressure in high pressure cylinder

For a triple expansion engine:

                IP HP
P=   LP + R1 R2


R1 = ratio of LP: IP cylinders
R2 = ratio of IP : HP cylinders

LP = mean pressure in low pressure cylinder
IP = mean pressure in Intermediate cylinder
HP = mean pressure in high pressure cylinder

STEAM CONSUMPTION.

The steam consumption can be calculated from an Indicator diagram.

One "rule of thumb" is to allow 30 to 40 lbs. of steam per horsepower per hour.
In most cases this is over the actual requirement.

Calculating the total volume of the cylinder (s) times the rpm, multiplied by the cut-off, will give a guide to the volume of steam which could be used. Again, this will give a high figure as it doesn't allow for the expansion of the steam.
However, if this and the prior "estimate" are close to each other, it is an indication that the resultant figures may not be far out.

It is possible to calculate steam consumption more scientifically than this. However, it requires some complex maths, and isn't worth the effort. Load Bill Hall's software from the Modeleng site (see the LINKS page) and let that do the hard work for you!

FUEL CONSUMPTION.
A Guide is to allow 10 pounds of wood per hour for each horsepower used..

One horsepower-hour is about 2545 BTUs.

Assuming that an engine has an efficiency of 10%, a one horsepower engine would use 25,450 BTUs / hour.

That would be about 3.6 pounds of wood / hour, or about 1.8 pounds of coal / hour.