My starting data :

 - a collection of points which are buildings
 - a collection of points which are electrical transformation plant
 - a network of vectors which is the public electric network from an operator
 - buildings are not connected to the electric network

I use QGIS to find which are the buildings which are located less than 250 meters
from an electrical transformation plant by following the network. I need this information more 100.000 buildings.

*To do it by hand :*

- trace a line between the building and the nearest electrical network segment,
- compute the distance by the network between this entry point and the nearest tranformation plant.
  I have found the RoadGraph plugin (http://gis-lab.info/qa/road-graph-eng.html) which works for that
- add the two distances and check if < 250m.

Now, my question is: how to automatize it for the 99.999 other buildings ?
I know Python a lot but I am a beginner with QGIS. I did not find a way to automatically interact 
with the RoadGraph plugin and source code seems not to be available anymore.

Maybe I am doing it wrong ? If you have a better option, any help would be appreciated !

[![what it looks like][1]][1]


  [1]: https://i.sstatic.net/u4x2u.png