ASK THE EXPERTS: Microhydro Flow

Measuring flow, one of the two key components of determining available hydro power.

My family is considering installing a microhydro-electric system to power our remote cabin. We plan to use a 3-inch penstock (approximately 900 feet long) with 85 feet of head. Can a theoretical flow rate be calculated from these figures? The upper end will be submerged in a pond/creek and the water returned to the same creek.

We would prefer a batteryless AC system. This is a joint project with a neighbor. His part of the deal is installing the penstock; ours is selecting a hydro turbine. I keep telling him that the penstock has to come first so we can measure the pressure and flow. He wants me to “just buy a turbine” and says we’ll make it work. Can these figures be estimated given the information above, or should we install the line first and measure directly?

Rob Day • via email

Measuring head and flow are critical design jobs that should be done before any equipment is specified or purchased. Deciding on the penstock diameter without solid head and flow measurements could lead to wasted money or lost power. While both these measurements are more easily done with an installed pipeline—a pressure gauge for head, and a bucket and stopwatch for flow—you really need to know them in advance to choose the right pipe and size. Buy a pipe that’s too small and you could lose most or all of your power to friction loss; buy one that’s too large and you will waste money.

Once you have determined head, flow, and pipe length, consult friction-loss tables for the pipe type you choose. Next, you need to decide if the pressure (head) loss is acceptable. People typically accept a head loss between 15% and 25%. I personally favor lower losses, since the pipe cost is a one-time purchase, and the head loss is forever.

So instead of starting by specifying a pipe diameter, get some data and examine possible scenarios: “With 3-inch pipe, we’ll spend $X,XXX and lose YY% of the potential from friction loss. With 4-inch pipe, we’ll spend $X,XXX but only lose XX% of the potential.”

See our many microhydro articles for more on this and other aspects of system design. For instructions on how to measure your hydro resource, see

If you’re providing electricity for a conventional house or two, I’m doubtful, with that head and pipe size, that you’d get the power necessary for a stand-alone AC system. Your system would need to produce more than 2 kW of power to provide for the loads of a typical house. Stand-alone AC systems are limited in peak power, which equates to how many loads you can run simultaneously. Again, do the math first, and decide what system configuration is most appropriate.

Ian Woofenden • Home Power senior editor

Comments (0)