Year end results for 2025 / 26 water year.

Yesterday, September 30th 2026, was the last day of the 12 months which count as my "water year". 

By convention, water years do not follow the 12 months of a calendar year. Rather they start and end at the time of year when water is scarcest. 

In Wales where I live, the streams and rivers are always most depleted in early autumn, - from August onwards, - and only begin to have decent flows again when the leaves drop from the trees and rain starts to fall in earnest. It can be very variable as to when the rains start, but by selecting October 1st as the start date for the new water year, it is usually the case that the start and end of each water year falls in the driest time of the year.

I always look forward to September 30th because I can bring together the data from the previous 12 months to see how my Powerspout has performed, and especially how it has performed in relation to previous years. It is not a huge year-end exercise to compile the data because the data has been accruing each month from the inverter which acts as the DC to AC interface for the power the turbine is generating.

Here then, are the results for the past year. In each graph, the bold black line depicts the data for the past year.

1. Daily Power output (Watts)

2.Cumulative Energy (kWh)

3. Power duration curve (number of days which had power generated at different levels)

4. Correlation of Rainfall to Energy generated.

Comments.

The year was unusual for there being a few days, just 3, when the turbine did not run. In the 13 years of its operation, there have only been 3 years when this has happened. Strangely, those 3 days did not fall in 2026 when the UK had a very severe drought, but in late October 2025. In that year, there was low total rainfall (900 mm), the winter rains did not start until well into November, and as a result by the end of October, for just 3 days there was not enough water to run the turbine.

The total energy generated in the year (4047 kWh according to the inverter data but 4109 on the rainfall chart which uses the more accurate meter on which Feed in Tariff payments are calculated) was slap in the middle of the range that is typical for my site. As can be seen from the Cumulative Energy plot, there is a 'central band' into which the total for each year usually falls. In my data this band ranges from 3599 to 4534 kWh. There are then upper and lower bands when yield has been either exceptionally good (5132 to 5254 kWh) or exceptionally poor (2773 and 3155 kWh). This pattern of yield from hydros has been seen widely and probably results from irregular climatic events, such as El Niño. You can read about it in an earlier blog here, where I wrote about how it relates to calculating the Annual Asset Earning Potential for a hydro that has not yet been built.

Weather forecasters are predicting an El Niño effect for the UK over the winter months 2026/7 in which greater rainfall and cold temperatures are expected. I do hope they are right ! As the rainfall chart shows, where two years in succession each have high levels of rainfall, it produces in the second year generation which comes into the up-side band.