By lowering multiplier of course. There's another less sure way. In windows Control panel > Power options if you set maximum processor state to 99% that should limit CPU max frequency to it's base frequency. If you drop % even more, there should also be appropriate max frequency drop.
All of that is crippling CPU performance of course. I can't see a viable reason to do that instead of just buying 65W CPU.
I would start with ryzen master and make a profile with the lowest clocks, reducing vcore is just as important as lowering clocks and ryzen master should do that automatically.
Of course on a power outage you could also shut down one of the ccxs and do some additional core parking so that the CPU has fewer cores to power.
Keep in mind though that most of the power is being drawn by everything else and the CPU is only drawing a lot of power if it is doing something heavy so shutting down your apps will already push it down to idle comsumtion.
In that case go with Power Options solution. Make 2 Power plans, one with Max CPU at let's say 50% and one with 100%. In case of power loss and running on battery + inverter, just switch to 50% power plan and that will keep CPU up to somewhere like 2200 -2500MHz lowering power usage to minimum.