kernel_task

My iMac is almost unusable as a result of Kernel_tasks. Activity monitor shows 1600% cpu usage, whilst the graph shows the system is taking up 70%. How can if fix this? I have reinstalled a new system and reinstalled all my data. I am running a Catalina on a 27inch iMac with 4.2GHz Quad core intel core i& with 64GB of DDR4. Help!!