Micro-hydro potential calculator

If your land has water falling across it, you may be holding the best off-grid power source there is. Micro-hydro is unglamorous and it is magnificent: a few hundred watts that never set, never cloud over, and peak in the wet, dark months when solar is weakest. This calculator turns the two numbers that define any hydro site, head and flow, into honest watts.

Head and flow, the only two numbers that matter

Head is the vertical drop from where you take water in to where the turbine sits, measured in feet of elevation, not pipe length. Flow is how much water moves, in gallons per minute. Multiply them and divide by 5.3 and you have the theoretical watts; a real system delivers 35 to 60 percent of that after pipe friction, nozzle, turbine, and generator losses, which is what the efficiency selector applies.

The two trade evenly, 100 feet of head at 10 gpm equals 10 feet at 100 gpm on paper, but high-head sites win in practice: smaller pipe, smaller turbine, less water diverted, less silt trouble. A steep little seasonal brook with 80 feet of drop often beats a big lazy creek. Measure flow with a bucket and a stopwatch at the driest time of year, because the dry-season number is the one your batteries will live with.

Why small numbers are bigger than they look

People sneer at 150 watts until they multiply by 24. That trickle is 3.6 kWh a day, 108 kWh a month, delivered through the night, through the storm, through January. Matching that with solar in a northern winter takes a startlingly large array, which is what the "equivalent solar" row is showing you. Hydro also flatters the battery bank: with continuous charge, the bank cycles shallow and can often be half the size solar alone would demand, something you can check against the battery runtime calculator.

The practical and legal fine print

The build is mostly plumbing: a screened intake set in a small pool, penstock pipe sized generously (undersized pipe eats head as friction), the turbine in a box at the bottom, and wire back to the batteries, sized with the wire gauge calculator because low-voltage runs from a distant turbine get expensive fast. Plan for leaves in fall, ice in winter, and silt after storms; a good intake design handles all three without daily babysitting.

Water law is real: many states regulate diverting even a stream that crosses your own land, and fish-bearing streams carry extra rules. Most micro-hydro takes only a fraction of flow and returns it a few hundred feet downstream, which permits fine in many places, but check before you trench. The Water archive and Land & Real Estate archive cover water rights and evaluating land for exactly this kind of hidden asset, worth reading before you buy the next parcel, too.

Getting a useful answer out of this

Micro-hydro potential returns continuous output, per day, per month, and equivalent solar array. Every one of those is a planning figure rather than a specification: it tells you the size of the problem and roughly what it will take to solve, which is what you need before you can shortlist equipment or ask a supplier a sensible question.

The single most useful habit with any calculator on this site is to run it more than once. Change one input at a time and watch which output moves. That tells you where your design is sensitive, and design sensitivity is far more valuable than a single answer, because it identifies the assumption worth spending real effort to pin down. If a modest change in one input swings the result substantially, that input deserves a measurement rather than an estimate.

It is also worth running the pessimistic version. Off-grid systems are sized by their worst case rather than their average, and a design that only works on the numbers you hoped for is a design that works for part of the year. Put in the colder temperature, the longer run, the larger household, the dimmer month, and see whether the answer is still one you can live with.

What a calculator cannot know

Any tool like this works from the inputs you give it and from published averages for everything else. It does not know your particular site, the way your household actually behaves, the corner your building was cut in, or the ten-year-old equipment already installed that does not match its datasheet. Treat the output as a well-informed starting bracket, not as a specification to order against.

Three things in particular sit outside what any of these tools can see. The first is local rules, which decide what you may build regardless of what the arithmetic says: the laws pages cover those by state, county, and town. The second is the condition and quality of what you install, which is why two identical designs perform differently. The third is how the system is wired and protected, which decides whether it is safe rather than whether it is adequate, and which is the subject of lesson six.

Where a number here comes from a constant or a rule of thumb, the method notes above say so and give the source. That is deliberate: a result you cannot check is a result you should not act on, and the arithmetic here is simple enough to verify by hand on paper if you want to.

Where to take the answer next

A single figure rarely settles anything on its own, because off-grid systems constrain each other. The load list decides the array, the array decides the controller, the battery decides what the inverter can deliver, and the climate at your site decides all of it. The system planner takes one set of inputs and produces a whole coherent plan rather than an isolated number, which is usually the better next step once you have a figure you trust.

For location-specific inputs, the site report gives any address its real 30-year sun, climate, elevation, and distance figures, including the worst-month sun hours that size an off-grid system properly. For the equipment that the result points at, the spec tables list what manufacturers publish, normalized so that units of different sizes can be compared, and each product page works its own numbers through. For what things cost this month, the price index.

And for the reasoning rather than the arithmetic, the free course runs through the decisions in the order they actually arrive, from deciding whether the life suits you through to the first year on the land. Most of the mistakes that calculators cannot prevent are ordering mistakes, made before anyone opened a calculator at all.