Method of Measurement
The height of the column of water contained in the rain barrel will determine how much pressure you will get. The weight of water will create .43 PSI (lbs per sq ft) per foot of height above the point of “draw” (i.e. the location of the spigot, where water will be removed).
Example – Pressure at Spigot
Suppose you have a hose attached to a rain barrel on a stand so that the surface of the water is 10 feet off the ground. And the rain barrel has its spigot 6 feet off the ground. So you have four feet between the top of the water column and the point of draw (i.e. the spigot). If you able to measure the water pressure at the spigot, you find you have 1.72 PSI (4 ft x .43 PSI) of water pressure at the spigot...plenty to fill buckets, push water
through a garden house, power a drip irrigation system, etc.
Example - Pressure at End of Hose
Now let’s expand on this example a bit and add a 50 foot garden hose to the spigot and run it to your garden. Let’s say the point of discharge on the end of the hose is now 12 feet below the surface of the water. The math says that you will have about 5.16 PSI of pressure at the end of the hose.
If you raise the end of the hose in the air, the pressure drops... lower the end of the hose and the pressure will rise. The length of the hose is immaterial in this example (except for some minor friction complexities).
What’s Normal Water Pressure?
As a point of comparison, normal household water pressure ranges from 20 PSI (mostly RV’s) to about 80 or so PSI. You would need at least 80 feet of fall to run a garden sprinkler or hose sprayer... but the gravity option works well for drip irrigation, hand watering, etc. However, with the addition of a water pump, you can increase the PSI so that you can power your lawn sprinkler or garden hose.
Height vs. Volume
Think of it this way... the more water HEIGHT you stack atop the point of release, the more pressure you will get. The VOLUME of water above the point of release is immaterial. So, the higher you mount your rain barrel, the more water pressure you will generate.