How can mixing m/s, mph and km/h distort a wind-energy assessment?

The short answer

Mixing wind-speed units can produce a large energy error because the turbine curve expects a particular speed quantity. A file imported in km/h and interpreted as m/s does not contain slightly stronger wind; it contains a unit error. Unit labels and conversions must remain explicit throughout the calculation.

Verify the source and the receiving field

Record the original unit from the data provider or instrument configuration, then check the unit required by the analysis tool and power curve. Do not infer units from the magnitude of a few values. A calm site and a unit-converted dataset can both look superficially plausible.

Retain the original file and create a derived column for conversion. One metre per second equals 3.6 kilometres per hour; use a documented conversion rather than a rounded mental estimate inside repeated spreadsheet operations.

Convert before binning and presentation rounding

Apply one controlled conversion before assigning speed bins or evaluating the curve. If values have already been binned, establish the original bin edges and units before reusing the table. Relabelling the column does not convert the underlying values.

Check whether decimal punctuation or text parsing changed the imported numbers. A unit-correct file can still be wrong if the import treats a decimal separator as a thousands separator. Keep these checks separate so the actual correction can be traced.

Use a small verification sample

Select several source rows, including calm, moderate and stronger conditions, and independently verify their converted values. Confirm that the power-curve speed axis uses the same basis and that the conversion is applied only once.

Record the unit rule in the assessment notes and preserve it when later files are added. If earlier energy results used the wrong unit, recalculate the affected outputs and explain the revision. Do not describe the resulting difference as improved turbine performance or a change in the site's wind resource.