Ethercard and sending data ?

[pylon]

The webClient example does provide the necessary information for your task. "/foo/" is the fixed part of the URI, while "bar" is the variable part. So in your case it could be:

    int power = 200; // fill that with your value
    char buffer[30];
    snprintf(buffer, 30, "{power:%d}", power);
    ether.browseUrl(PSTR("/emoncms/input/post.json?apikey=913f55770xxxxxxxx&json="), buffer, server, my_callback);