Size a hydro system’s guaranteed output on the driest-season flow, because power is directly proportional to flow: if the stream falls to a sixth of its wet-season flow, the watts fall to a sixth too, and any surplus water bypasses the intake. A larger turbine does not raise the dry-month output; it only harvests more of the wet months, so in the worked year below the same 10-meter site gives 350 watts at 5 L/s and 1,400 watts at 20 L/s.
Part of Micro-Hydro Turbine
A stream’s flow changes with rainfall, snowmelt and freezing, and because hydro power is proportional to flow, the year’s lowest flow sets the output the village can count on.
Power Is Proportional To Flow, Not To Its Cube
In the hydro formula P = Q × H × 7, flow Q appears once. A stream that drops from 30 L/s to 5 L/s keeps 5 ÷ 30 = one sixth, about 17%, of its power and loses 83%. That is a large change, but it is a slow one, over weeks, and it is predictable by season. Wind is a different law: power depends on the cube of speed and changes hour by hour (see Wind Resource Assessment). Do not apply the cube to a stream.
The carrier’s rule is to measure during the driest season, call that the guaranteed minimum, and design for it. The installation page repeats it: for a system that must run all year, use the driest month’s flow (Installation).
Method: Build A Flow Record
Step 1: Measure on a fixed schedule. Use the bucket-and-stopwatch or float method from Flow Measurement once a month, and after each long dry spell and each big rain. Write the date next to each figure.
Step 2: Find the lowest. One dry season is not the driest on record. Ask people who have lived beside the stream which year it nearly stopped, and look for dry stains and high-water marks on the bank. If the record is short, treat the smallest measured flow as an upper limit on the true minimum.
Step 3: Choose a design flow. The design flow is the most water the turbine will take, set by the nozzle or runner width. Water above it bypasses.
Step 4: Compare designs. Compute the energy each design flow harvests over the record, as below.
Worked Example: Twelve Months, Four Designs
These monthly flows are invented to show the arithmetic, not measured at any site. Head is 10 meters and power is Q × H × 7.
Flows in L/s, January to December: 5, 5, 6, 8, 12, 20, 30, 28, 18, 10, 7, 5.
| Design flow | Output in a dry month | Output in the wettest month | Average output | Energy per year |
|---|---|---|---|---|
| 5 L/s | 350 W | 350 W | 350 W | 3,066 kWh |
| 7 L/s | 350 W | 490 W | 449 W | 3,935 kWh |
| 20 L/s | 350 W | 1,400 W | 793 W | 6,950 kWh |
| 30 L/s | 350 W | 2,100 W | 898 W | 7,869 kWh |
Energy per year is the average output times 8,760 hours; for 20 L/s that is 793 × 8.76 = 6,950 kWh. Three things follow:
- The dry-month output is the same in every row. 5 L/s through the turbine gives 5 × 10 × 7 = 350 watts whatever the nozzle size. The turbine size does not change the guarantee.
- The wet months pay for a bigger turbine. Going from 5 to 20 L/s lifts the year’s energy from 3,066 to 6,950 kWh, 6,950 ÷ 3,066 = 2.3 times as much, but the extra nozzle, runner and penstock cost more.
- The gain flattens. From 20 to 30 L/s the energy rises only from 6,950 to 7,869 kWh, because only two months have flows above 20.
A dry-month flow of 5 L/s through a design for 20 L/s is 25% of the design flow, which is the lower edge the Crossflow Turbine page quotes for good efficiency at part flow. A Pelton with several nozzles can shut some off in dry months and keep its jets fast (Pelton Wheel).
The Head Moves Too
Flood water raises the tailwater, which cuts the head the turbine sees, as the installation page notes for a tailrace that backs up. If a flood lifts the tailwater by 1 meter on a 10-meter site, head falls to 9 meters, about 10% less power. On a 2-meter low-head site the same rise is 50%. Low-head sites lose most to floods.
Plan The Loads Around The Seasons
Put the base loads, lights and phone charging, on the dry-month 350 watts. Move workshop machines, grain milling and heating to the wet months, when surplus is highest, and send the rest to a dump load that heats water (Load Control). Frozen winter streams are a separate season: see Debris & Ice Management.
Floods arrive faster than the gauge shows
A stream that is a trickle in the dry season can rise within minutes after rain upstream. Measure flow from the bank, never wade a rising stream, and put the intake gate where it can be shut from dry ground.
Common Mistakes
- Measuring once, after rain — the number will be several times the true minimum.
- Treating the average as the guarantee — the average here is 793 watts at 20 L/s but the village gets 350 for months.
- Sizing the turbine for the biggest flood flow — the unused capacity sits idle for most of the year.
What’s Next
- Learn next: Turbine Types to match a machine to head and part-flow behaviour
- Combine with: Tailrace & Environmental Flow for how much of the dry flow you may take