USA equivalent of European Lot 20 portable heater efficiency standard?
I'm not aware of any laws in regards to energy consumption of space heaters and Google couldn't quickly find any. I'm not sure about the EU but energy laws in the US are determined by federal, state and local governments so you'll need to make sure you do research for wherever your client is. The US does have a Energy Star program which rates products energy use so consumers know how much energy it uses and they can compare products. This is a voluntary program but I couldn't tell you the last time I bought a appliance and it didn't have a Energy Star label on it.

https://en.wikipedia.org/wiki/Energy_Star