qgis - How to measure areas of features in OpenLayers plugin background maps correctly?

I can't figure out how to make areal calculations (eg. in km2). I have tried the 'field calculator', I have tried the 'measure area' function on the "attributes" toolbar, and i have tried fTools in the "Vector"-menu.

How I do:

1. Open Qgis


2. Add Google Hybrid Layer (OpenLayers Plugin)


3. Find tenniscourt (should be 200-260m2)


4. Measure
   
   
   
   • attribute toolbar->measure area - result: 65.060 m² hmm?
   
   
   • layer->new shapefile layer->polygon, save, "add feature" and drawing polygon. Then field calculator $area - result: 118.48828125
   
   
   • repeat with new shapefile - result: 4.05503328693158e-08
   
   
   • vector->geometric tools->export/add geometric columns = another random result
   
   
   
   
   

I can't figure out what I am doing wrong! For me it seems like there is some error in the program? I have tried saving the shapefile and opening it again in a "blank" project - same result - and new result every time?