Nonbusiness Energy Property Credit / Residential Energy Efficient Property Credit

hi 

Wanted to check the following as it relates to Energy credit for 2021 TY.

If I have received a Solar electric system tax credit in 2016 amounting >$500 will I still be eligible for getting complete Nonbusiness energy property credit ( Tankless water heater installed in 2021)  ?

While filling the details through Turbo Tax , it indicates I can get up-to 10% of the installation of Tankless water heater,  however it is maxed out at $150 , though allowed max is $500 ( Tankless water heater +installation costed around $5400). Not sure if this is due to 2016 Credit I have rxvd for installing Solar electric system or due to my 2021 tax liability.

Thanks in advance for any pointers on this.