At least for peltier cooling, as delta temperature across the peltier increases, it takes progressively more power to counteract conduction inside the peltier.

At least for peltier cooling, as delta temperature across the peltier increases, it takes progressively more power to counteract conduction inside the peltier. This requires progressively more power and results in joule heating. Works great at low deltaT though. https://www.meerstetter.ch/customer-center/compendium/71-peltier-element-efficiency https://www.meerstetter.ch/customer-center/compendium/71-pel... has some nice plots and explanation of how performance varies with deltaT. A constant current driver can also be important - PWM results in increased joule heating. For 50% maximum constant current joule heating is (V/2)^2/R but when duty-cycling with 50% PWM, it's V^2/(2*R). Moving the scaling factor out from under the ^2 makes the savings more pronounced at lower deltaT as well. But constant current is more complex, especially with larger peltiers. I cants really speak for Carnot cycle coolers, but they can often be effectively duty-cycled and can be placed remotely so the compressor waste heat can be vented into the room. Performance still degrades with dT though.