What head of water does an Archimedes screw need?

What no source in the UK guidance sets is a single fixed minimum head for a screw, because the screw is specified per site. The two things that decide whether a scheme works are head and flow together: "a suitable site needs a water source with a combination of" the two¹. A smaller drop can still generate useful electricity if enough water is moving through it, and that trade-off is what any site assessment has to measure.

Low head is the point: what the screw is for

The defining feature of an Archimedes screw scheme is that it works where the drop is small. The Centre for Alternative Technology's guidance on micro-hydro draws the line clearly at the top end of conventional turbines: "You usually need a drop of over 10 metres for a higher head turbine to be viable"². Below that, the options narrow, and the screw is the machine that fills the gap, which is why it appears at weirs and low fall river sites rather than on steep hill streams.

For scale, the UK's consumer and planning guidance classifies hydro by capacity rather than by head. Systems with a capacity of less than 100 kilowatts, the size typically used for small-scale generation, are sometimes referred to as micro-hydro, and very small systems with a capacity of less than five kilowatts are also known as pico-hydro³. A domestic screw scheme will usually sit somewhere in that range, with the head and the flow of the watercourse setting where.

The case for screw turbines at low-head locations

High-head turbines are described in the CAT guidance as attractive where the drop exists: types "such as Pelton or Turgo turbines, are comparatively cheap, easy to install and work well in a fluctuating water flow"². But their whole design depends on water arriving at speed down a steep drop. On a lowland river with a metre or two of fall, that water simply never arrives, and those machines cannot be made to work.

That is the gap the Archimedes screw fills. Its slow rotating action lifts and lowers large volumes of water through a shallow incline, so it trades speed for volume: less head, more flow. The practical consequence for a household is that a site dismissed as "not steep enough" for hydro may still be viable, provided the watercourse carries enough flow. The trade-off runs both ways, though: at very low head the output per litre of water is small, so a scheme that would be generous on a hill stream can be modest on a flat river.

How head and flow rate combine to determine power

Head on its own tells you nothing about output. The Centre for Alternative Technology gives a rule of thumb for estimating what a site can produce: "To estimate the energy that could be available, multiply the flow rate (in litres per second) by the head (in metres), and by 10, then halve the result to account for losses"². The answer is in watts.

Worked through, that means the two figures substitute for each other only in arithmetic, not in reality. Ten litres per second falling 10 metres and 100 litres per second falling 1 metre give the same estimate, but the sites, the equipment and the civil works are entirely different. This is why a screw scheme on a low fall is always a high-flow scheme: the water volume has to make up for what the drop lacks.

A real example of the scale a river site can reach is the community-owned 50kW Archimedes screw hydroelectric turbine on the River Esk at Ruswarp, near Whitby, North Yorkshire⁴. That is a river-scale installation rather than a domestic one, but it shows the principle in practice: a shallow fall made productive by the volume of water moving through it.

Checking your head and flow rate before you invest

Both figures have to be measured at the site, not estimated from a map. Head is the height difference between the water at the top of the system and the bottom¹, and flow is how much water is moving through the system¹. Because river flow varies through the year, both measurements matter most at the times of year when the water is at its lowest, since a scheme sized on winter flow will not deliver in a dry summer.

The same discipline applies across microgeneration: the Centre for Alternative Technology's wind guidance makes the general point that it "could be worth spending a few hundred on some wind monitoring equipment" before spending thousands on a turbine⁵, and the logic transfers directly to hydro. Measuring the site first costs a fraction of the installation and is the only way to know whether the screw, the flow and the expected output match each other.

For a grid-connected scheme, the size of the generator also determines what happens next with the network operator. Scottish and Southern Electricity Networks advises that for installations of 50kW or greater, "it would be worth checking our thermal Centralised Network View map to understand constraints in your area" before applying⁶. A domestic screw will usually be well under that threshold, but the connection process still applies.

Grid connection, licensing and what owning a screw means

A screw scheme does not stand alone. Planning guidance on hydro-electricity notes that "for houses with no mains connection but with access to a micro hydro site, a good hydro system can generate a steady, more reliable electricity supply than other renewable technologies at a lower cost"⁷, which is the strongest independence case for the technology: a river that flows year-round is steadier than wind or daylight.

For connected homes, the grid remains part of the picture. Excess generation can be exported: "any unused or excess electricity can be exported to the grid and sold to the local electricity supply company"⁸. The electrical installation side is covered by building regulations, where an installer registered with the relevant competent person scheme may be authorised to self-certify the work without involving local authority building control⁹, and small-scale generators fall within the categories of notifiable electrical work¹⁰. Water turbines are also named in the legislation governing energy-saving materials¹¹.

The dependence that remains is on the watercourse itself and on the permissions that govern it: abstraction licensing, environmental requirements and the network operator's connection terms. A screw household generates its own power from its own river, but it does so within those rules, and the scheme's output still rises and falls with the water.