Small wind / Buyer questions
Energy and economics
Does a P50 wind-energy estimate mean the project will deliver that energy every year?
The short answer
A P50 estimate is not a promise that the project will deliver that energy every year. It describes an exceedance level under a stated probability model and set of assumptions. The buyer must know the metric, time horizon, uncertainty sources and operating boundary before using the number in a project decision.
Ask what the probability describes
Identify whether the estimate concerns a single year's energy, a longer-term average or another quantity. Confirm whether the result is gross generation, net delivered energy or energy available after specified losses. Similar P50 labels can refer to different boundaries.
Also ask which uncertainty sources are included. A percentile calculated only from historical weather years does not automatically include measurement errors, model uncertainty, future operating losses or every project risk.
Keep probability separate from a contractual obligation
A probabilistic assessment does not create an equipment guarantee. Any promised output, test condition or remedy would need an explicit agreement and a defined measurement method. Do not infer such terms from a graph labelled P50.
For planning, examine an appropriate range of outcomes and the project's ability to tolerate them. A proposal that meets its financial target only at one central estimate should show how a weaker resource or different loss outcome affects the decision.
Use the number with its evidence statement
Request the source resource, curve identity, loss basis, probability method and assessment date. Preserve those inputs when comparing proposals, and avoid mixing percentiles calculated under different conventions.
The useful deliverable is a transparent probability statement with limitations, not a precise annual kWh promise detached from its model. This lets the buyer discuss investment risk while keeping actual product performance claims and project commitments tied to the documents that explicitly establish them.
Sources
- System Advisor Model documentation: wind-energy uncertainty inputs and probability results
- NREL wind-resource assessment report: measurement analysis and long-term correlation
Examples are illustrative. Confirm product-specific performance, compatibility and supply scope in the applicable documents.