Here is a paper 
describing how they measured the power curve of one of my early-design turbines in Nicaragua. They clarify the impact of various parameters such as generator heating, battery voltage, controller settings, furling system and anemometer
position.
The above chart is an example of the sort of data you can sometimes see and the paper explains how to clean it up.
Buy a Tristar charge controller (UK only)
From: Mike Allen
Subject: Re: A Wind Turbine Recipe Book
Date: 12 April 2013 10:53:18 BST
To: jytte
I recieved the book yesterday and when it arrived I thought ‘this isn’t worth £12’.
But how wrong I was once I’d opened the envelope and settled down to read the contents with a cup of coffee. 3 hours later the coffee was stone cold and I was still reading the book.
Loads of info to be getting on with.
I’ll email Hugh when my terbine is up and running with some pics.
Thanks a lot.
Michael.