The instantaneous power available from a microhydro system is based on two main factors:

- The quantity of water (per unit time) moving in a river or stream, and that is available to be diverted through the turbine, is called
*flow*. It is expressed as a rate such as cubic feet per second (cfs) or gallons per minute (gpm). - The pressure that drives that flow is caused by the vertical height between the intake and the turbine. The
*head*is basically the weight of that water column and can be expressed as pressure (psi or bar), but is more often discussed in terms of vertical*feet*of head since that relates directly to the topography of the site—2.3 vertical feet of water will create 1 psi.

Together, head and flow are the driving forces that spin the turbine at the bottom of the system. A hydro system designer will use these two measurements to determine the pieces and parts necessary to optimize a system. Intake and turbine locations will be chosen to maximize head, while minimizing pipe and wire runs and other site-specific challenges. Pipe will be sized to balance reducing friction loss with keeping costs in check. The number of nozzles, runner type and size, and alternator size will all be carefully balanced to work with available flows without depleting the source (and ideally, without negatively impacting the local ecology). And the efficiencies (inefficiencies) of each component in the process will be calculated for an accurate estimate of the power available at the site.

But there is a simple formula to guesstimate a site’s general hydro potential without going though all of the formulas and variables of turbine choice and pipe sizing:

**Head (vertical ft.) × Flow (gpm) ÷ Derate factor = Power (W)**

The derate factor is commonly between 9 and 13. This range has been determined over the years from the measured real-world performance of professionally installed hydro systems. A low derate factor like 9 would be appropriate for cases with good head and flow, and relatively short pipe runs and other inefficiencies. Higher derates, like 13, would be for cases where either head or flow (or both) is challengingly low, or other obvious inefficiencies will occur. This factor also takes into account the canceling out of units to arrive at watts; do not try this formula using cfs, psi, or other units of measurement.

In some of the myths in this article, this formula is used to illustrate example scenarios. Because these myths are so fallacious, we can use a nice optimistic 10 as a derate factor. (In fact, with the tiny systems described, the derate might actually be twice that.) Besides making the math easy, you’ll find that even giving a myth this benefit of the doubt won’t make it stand up to the real tests of physics, financial viability, or both.

You should feel encouraged to use this formula as a starting point in assessing your site’s hydro potential. It may help you decide if you should contact a professional system designer, or drop the idea. Besides, the variables you used in the formula will be the first questions that a pro will ask, (so make them accurate). Knowing your site’s *measured* head and flow in advance will save everyone time and money. (See “Intro to Hydropower Part 2: Measuring Head & Flow” in *HP104*.)