This.

The theory beside measuring power with a multimeter you way even have done it in high school usinq a 12V power supply and a small resistor/motor. Fun exercise and you may even loop on the whole high school physics curriculum with electricity --> power --> force --> work --> Energy and efficiency calculation.

However, unlike the 12 V you work on at school the domestic 230V can kill you (same for US 110V). I don’t see a safe way to use a regular multimeter to measure a current in these voltage range. If you know what you do, you’ll build a dedicaced/enclosed set up. If you don’t buy a watt meter at the hardwarc store, it’ll be safer and cheaper than a DIY one