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How Micro-Hydro Power Works: Head, Flow and Power

Updated 21 September 20267 min readWind & Hydropower

Micro-hydro is the most predictable renewable there is: a site that produces a kilowatt at noon is usually producing it at midnight too. Everything rests on two measurements — the vertical drop the water falls through, and how much of it passes per second — and on the uncomfortable fact that neither is constant, and that the drop you can use is always less than the drop you can see.

Key takeaways

  • Power is the product of head and flow, so a steep trickle and a gentle river can deliver the same output.
  • Gross head is what you survey; net head is what remains after friction in the pipe, and pipe diameter is the main lever on the difference.
  • Halving the pipe diameter multiplies friction loss enormously, because the loss scales with roughly the fifth power of diameter.
  • Flow varies across the year far more than head does, so the site is characterised by a flow duration curve rather than a single measurement.
  • Micro-hydro's advantage is not efficiency but capacity factor: it runs through the night and through the winter.
On this page
  1. Two numbers, and everything follows
  2. From head and flow to power
  3. Gross head is not net head
  4. What the site does over a year
  5. Where the water enters, and what comes with it

Two numbers, and everything follows

A hydro site is described by exactly two quantities. Head is the vertical distance the water falls between the intake and the turbine, measured in metres or feet — not the length of the pipe, and not the slope, but the vertical drop alone. Flow is the volume passing per unit time, in litres per second or gallons per minute.

Head and flow on a micro-hydro siteA section through a hillside scheme. An intake with a screen sits in the stream at the top. A penstock pipe runs down the slope to a powerhouse containing a turbine and generator, and a tailrace returns the water to the stream below. The vertical distance between the intake water level and the turbine is marked as gross head and is clearly shorter than the sloping length of the pipe, which is marked separately. Flow through the pipe is marked as volume per unit time. A note states that head is the vertical drop and never the pipe length.intake and screenpenstockturbine andgeneratortailracegrossheadvertical onlypipe length — longer, not the headFlowvolume per second through the pipelitres per second, or gallons per minuteHead is the vertical drop between the intake water level and the turbine — never the length of the pipe, never the slope.
Head is the vertical drop, never the pipe length. Flow is what passes per second. Every other decision on the site follows from these two.

The reason both matter equally is that power is their product. A steep mountain stream with very little water and a slow river with a gentle fall can deliver identical output, and the engineering that suits each is completely different.

Everything downstream of those two numbers — turbine type, pipe diameter, generator, even whether the scheme is worth building — is determined by them, which is why site assessment is almost the whole of micro-hydro design.

From head and flow to power

The hydraulic power available is the weight of water falling per second multiplied by the distance it falls:

P = ρ · g · Q · H · η

where ρ is the density of water (1000 kg/m³), g is gravitational acceleration (9.81 m/s²), Q is flow in cubic metres per second, H is net head in metres, and η is the overall efficiency of everything between the water and the wires.

Head and flow turned into power, step by stepA worked calculation in stages. A flow of ten litres per second, or about 160 US gallons per minute, is written as 0.01 cubic metres per second. A net head of twenty metres is noted as sixty-six feet. Multiplying water density, gravitational acceleration, flow and head gives about 1960 watts of hydraulic power. Applying an overall efficiency of 0.55 gives about 1080 watts delivered. Multiplying by twenty-four hours gives roughly twenty-six kilowatt-hours per day. A note marks every figure as an illustrative example rather than a typical site.P = ρ · g · Q · H · ηρ = 1000 kg/m³g = 9.81 m/s²Q = flow, m³/sH = net head, mη = overall efficiencyFlow10 L/s≈ 160 US gal/min = 0.01 m³/sNet head20 m= 66 ft, after pipe frictionHydraulic power≈ 1960 W1000 × 9.81 × 0.01 × 20Overall efficiencyη ≈ 0.55pipe, turbine, generator, wiringDelivered≈ 1.1 kWcontinuously — day and night≈ 26 kWh per day1.1 kW × 24 hIllustrative arithmetic, not a typical siteThe numbers are chosen to make the calculation legible. Real head, real flow and real efficiency are properties ofa particular scheme and have to be measured, not assumed.The efficiency term covers everything between the water and the wires, which is why it is lower than any single component's.
Illustrative arithmetic. The efficiency term is where a scheme is won or lost — and it covers the pipe, the turbine, the generator and the electronics together.

Take a modest site: 10 L/s (about 160 US gal/min) falling through 20 m (66 ft) of net head. That is 0.01 m³/s, so the hydraulic power is 1000 × 9.81 × 0.01 × 20 ≈ 1960 W. At an overall efficiency of 0.55 — a reasonable figure for a small scheme once the pipe, turbine, generator and electronics are accounted for — the delivered power is about 1.1 kW.

Which sounds unremarkable until you multiply by time. Running continuously, that is roughly 26 kWh per day, every day, through the night and through the winter. A solar array of comparable daily output would need to be many times the nameplate rating, because it works for a fraction of the hours. Micro-hydro's advantage was never efficiency. It is that it does not stop — which is why it needs none of the storage that solar surplus demands.

Gross head is not net head

The drop you survey is not the drop the turbine sees. Water moving through a pipe loses energy to friction against the pipe wall, and that loss is subtracted from the head before any power is produced.

Friction turns gross head into net headA penstock drawn from intake to turbine with a hydraulic grade line above it. Gross head is marked between the intake water level and the turbine. The grade line falls steadily along the pipe because of friction, and the vertical gap between it and the gross head at the turbine is labelled friction loss; what remains is labelled net head. Two pipes are compared: a narrow one whose grade line falls steeply and leaves little net head, and a wider one whose grade line falls gently and leaves much more.The drop you survey is not the drop the turbine seesintake water levelturbine levelturbinenarrow pipe: energy falls steeplywider pipe: energy falls gentlygrossheadfrictionlossnet head(narrow pipe)Diameter is the leverFriction loss rises steeply as a pipe narrows — roughly with the fifth power of diameter for a given flow — soone size smaller is not a small economy. Length matters in direct proportion; roughness matters less than both.Head lost to friction is lost every second the scheme runs, for its whole life. Most schemes oversize the penstock deliberately.
Friction eats head along the whole length of the pipe. What reaches the turbine is what is left — and pipe diameter is the main thing deciding how much that is.

The lever that matters is diameter. Friction loss rises steeply as a pipe narrows, roughly with the fifth power of diameter for a given flow, so a pipe one size smaller is not a small economy but a large one in the wrong direction. Length matters too, in direct proportion, and so does the pipe's internal roughness — but neither is as brutal as diameter.

The practical consequence is a rule most schemes settle on: size the penstock so that friction takes a modest fraction of the gross head, and accept a larger pipe than seems necessary. Head lost to friction is lost for the life of the scheme, every second it runs.

What the site does over a year

Head is essentially fixed. Flow is not, and a single measurement on a single day tells you very little about the site.

A flow duration curve, and where the design flow sits on itA chart with percentage of the year on the horizontal axis and stream flow on the vertical axis. The curve starts very high for a small percentage of the year during floods, falls steeply, then flattens into a long tail of low flows through dry periods. A horizontal line marks a chosen design flow. Where the curve lies above that line, the surplus must pass the intake unused; where it lies below, the scheme runs at reduced output. The point where the curve crosses the line marks the percentage of the year the scheme runs at full output. A separate line near the bottom marks the flow that must be left in the stream.Flow is what varies. This is the honest description of a site.flow% of the year this flow is equalled or exceeded →design flowfull output up to herefloods: surplus passes the intake unusedbelow design flow: reduced outputflow that stays in the stream — not negotiableChoose highand the scheme is built for floods that lastdays, then stands underused for months.Big machine, small capacity factor.Choose lowand the scheme runs almost all year butgives away flow that was there for months.Illustrative curve — every stream has its own.
Illustrative. Designing for peak flow builds a scheme that stands idle most of the year; designing too low gives away output that was available for months.

A flow duration curve plots flow against the percentage of the year it is equalled or exceeded. It is the honest description of a site, and it makes the central design decision visible: choose a design flow, and the scheme runs at full output whenever the stream exceeds it and at reduced output whenever it does not.

Choosing high builds for floods that last days and leaves the machinery underused for months. Choosing low gives away power that was available for most of the year. Most schemes settle well down the curve, and some fit a turbine that handles a range of flows rather than one — a choice that belongs with turbine selection rather than with the survey.

One constraint sits above all of this and is not negotiable: some flow has to remain in the stream. The channel below the intake is a habitat, and a scheme that takes everything is not a scheme anyone should build.

Where the water enters, and what comes with it

The intake is the part that decides how much attention the scheme demands, and it gets less thought than it deserves.

What the intake has to deal with, and what deals with it
ProblemWhat it doesUsual response
Leaves and debrisBlocks the screen, starves the pipeCoarse screen, self-clearing geometry, accessible for raking
Silt and sandAbrades the runner and wears nozzlesSettling basin before the penstock, with a flushing outlet
Air entrainmentAir in the penstock disrupts the flowSubmerge the intake sufficiently; avoid vortices
IceBlocks the screen at the worst time of yearSubmerged intake below the freezing layer
FloodsDamage or destroy the intake structureBuild for the flood, not for the normal stream
Low flowOutput falls, air can be drawn inDesign flow chosen from the duration curve

Design responses, not a specification. The right arrangement depends on the stream: a clear upland burn and a silty lowland channel present completely different problems.

Screen, settle, then drawA detail of a micro-hydro intake. Stream water passes a coarse angled screen that sheds leaves downstream rather than collecting them. Behind it a settling basin widens so the water slows and sand drops out, with a flushing gate at its base. The penstock draws from between the water surface and the settled silt: deep enough not to draw in air, high enough not to draw in sand. A residual flow passes the intake and continues down the original stream channel.The intake decides how much attention the scheme demandssettling basin — water slows, sand drops outsettled silt and sandstreamcoarse angled screensheds leaves ratherthan collecting themresidual flow continues down the original channel — a scheme that takes everything is not one to buildto thepenstockdeep enoughnot to draw airhigh enoughnot to draw sandflushing gateMicro-hydro's maintenance is mechanical and seasonal: the screen, the basin and the tailrace, after every storm.Ice is the winter version of the same problem: a submerged intake sits below the freezing layer and keeps working.Arrangements differ by stream: a clear upland burn and a silty lowland channel need very different intakes.
Screen, settle, then draw. The penstock takes water from between the air above and the silt below — and a share of the stream always continues past.

The recurring theme is that micro-hydro's maintenance is mechanical and seasonal rather than electrical. The generating equipment is undramatic; the screen, the settling basin and the tailrace are where the work is, and they need attention after every storm.

That is the honest summary of the technology. The physics is a single multiplication, the losses are dominated by a pipe diameter chosen once, and the running of it is mostly about keeping a screen clear — the same trade between extractable energy and survivable engineering that governs wind's aerodynamic ceiling and every marine energy device that has to live in moving water.

Frequently asked questions

How much head do I need?

There is no minimum, because head and flow trade against each other in the power equation. A high-head site can work with very little water; a low-head site needs a great deal of it. What matters is the product of the two, and whether the resulting figure justifies the pipe, the intake and the maintenance that any scheme demands.

How do I measure head without surveying equipment?

A length of hose and a pressure gauge is the usual method: run the hose down the intended pipe route, fill it, and read the static pressure at the bottom. Pressure converts directly to head. A spirit level and a straight edge, stepped down the slope, works too and needs nothing but patience.

How do I measure flow?

In a small stream, divert it into a container and time how long it takes to fill — flow is volume divided by time, and a few repeats settle the reading. For larger flows, a temporary weir with a measured notch, or the float method, where a float's travel time over a measured length is combined with the channel's cross-section and a correction for the slower water near the bed and banks.

Does micro-hydro work in winter?

Usually better than in summer, since rainfall and runoff are typically higher and the flow is what limits most schemes. The risks in winter are mechanical rather than hydraulic: ice at the intake, debris after storms, and access to the works when the weather is worst.

Why is the overall efficiency so much lower than the turbine's?

Because the turbine is one stage among several. Friction in the penstock, the turbine itself, the generator, any drive between them and the conversion electronics each take a share, and they multiply. A scheme reaching half of the hydraulic power available at the intake is doing respectably.

Sources

Named organisations whose published material underpins this article. Where no link is given, the source is named rather than linked.

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Articles are drafted from primary engineering and physics references with AI-assisted tools, then reviewed and fact-checked line by line by a human editor before publication. We publish explanations, not recommendations: no products, no pricing, no country-specific rules, and no invented author personas.

Last reviewed 21 September 2026. How we research and review

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