The documentation for the REST API for the wifi power controller is a bit sparse. I found the YT video for the SCR 485 which shows ESPHome commands being issued via a browser.

I tried "http://192.168.0.46/number/max_power" and got the reply "no find file: number/max_power"

What should I be trying ?

please try

http://{localIP}/api/maxpower?x={value}

and

http://{localIP}/api/setpower?x={value}

I will summarize all of APIs of WPC3700 and list it in our blog later

I'm now trying to integrate into openHAB. 

I can use a JSONPath transformation to extract the value from the call to /api/setpower which returns a json response of {"setpower":2500} if the current output is 2500.

I will next look at setting the output from openHAB, probably will use a dimmer style UI element.