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Micro-Hydro Power Calculator — Estimate Output from Head & Flow

For sites with year-round flowing water: estimate how much power a micro-hydro setup could realistically generate from two on-site measurements — flow rate and head.

Estimated power output3.43 kW

How to measure your inputs

  • Flow rate: the “bucket test” — time how long a known-volume container takes to fill from the stream, then divide volume by seconds (e.g. a 20L bucket filling in 4 seconds = 5 L/s). For larger flows, a float-and-cross-section estimate works better.
  • Head: the vertical drop the water falls, from intake to turbine — measured with a level and tape over the distance, or from the static pressure in a filled pipe.

Methodology

The standard hydropower equation: P (kW) = ρ × g × Q × H × η ÷ 1000, where ρ is water density (1000 kg/m³), g is gravitational acceleration (9.81 m/s²), Q is flow rate (m³/s), H is head (m), and η is overall system efficiency — textbook small-hydro engineering, with turbine-type efficiency presets based on typical real-world figures (Pelton/Turgo for high-head/low-flow sites, Kaplan for low-head/ high-flow).

What actually makes a good micro-hydro site

Head vs. flow — which matters more

Power scales linearly with both head and flow, so neither is inherently "more important" — but they lead to very different system types. High-head, low-flow sites (a steep stream with a big drop) suit impulse turbines like Pelton or Turgo. Low-head, high-flow sites (a wide, gentle river) suit reaction turbines like Kaplan. Getting the turbine type wrong for your site’s head/flow profile is one of the most common reasons a micro-hydro install underperforms its theoretical output.

Why year-round flow is the real constraint

Unlike solar, which varies predictably by time of day and season, a stream’s flow can vary by an order of magnitude between wet and dry season — or dry up entirely. A realistic system estimate should use your stream’s worst-case reliable flow (often measured over a full year, not a single good day), not peak flow, since that’s what determines year-round power availability.

From a kW estimate to a real system

This calculator gives theoretical output from your two site measurements — the actual delivered power will be somewhat lower after real turbine efficiency, transmission losses, and intake losses (debris screens, penstock friction) are accounted for, which is why the efficiency preset here is described as "overall system efficiency," not just turbine efficiency alone.

Frequently asked questions

How much head do I need for micro-hydro to be worthwhile?

There’s no strict minimum — even a few metres of head can produce useful power with high enough flow (a Kaplan-type low-head turbine). What matters is the combination: low head needs proportionally higher flow to produce meaningful kW, and vice versa.

Can I generate useful power with very little flow?

Yes, if you have significant head — a small but fast-falling stream (high head, modest flow) can still produce a useful kW figure with a Pelton or Turgo turbine, which are specifically designed for that head/flow profile.

Is micro-hydro cheaper than solar for an off-grid site?

It depends heavily on the site — a good micro-hydro site with reliable year-round flow can be more cost-effective than solar since it produces power continuously (not just daylight hours) without battery storage needing to cover the full night. A poor site (intermittent or low flow) may not justify the civil works cost compared to solar + battery.

Do I need a permit for a micro-hydro installation?

Often yes — many jurisdictions regulate water diversion and in-stream works even for small systems, since they can affect downstream flow and aquatic habitat. Check local water-rights and environmental regulations before construction, not after; this is exactly the kind of site-specific check Swales’ Professional Services micro-hydro feasibility consultation covers.