Easy PHP, Radioactivity module: Read out a node's current energy?
My guess is that it is something like

float($entity-&gt;field_energy['und'][0]['value']);


Also, you can do 

print_r ($entity-&gt;field_energy);


to see what is contained in object.

also, it may be contained in node object so maybe it is

float($node-&gt;field_energy['und'][0]['value']);


My blind guess but it may help you :)