I bought a new poly solar cells, and the test result of the conversion efficiency is 16.8%, is it lower than usual?
It depends on what kind os poly solar cell you are using, if it is 260 watt photovoltaic solar panel, 16.8% is a bit lower.
The PV solar cells should be around 18%, any solar cells under that rate is not very efficient.
I don't think it's lower than normal, 16.8% is pretty common in most of the power plants in my area.
If the solar cells have been used for over 5 years or more, the conversion efficiency may become lower.
Since you mentioned that your poly solar cells is a newly bought one, I strongly recommend you to ask your supplier.