We have two main models here, one with a sizable compressor enough to heat a whole home (7-20 kW heat) and a water tank on the side which doesn’t need resistive backup, or smaller, hybrid models that have a few hundred W compressor and maybe 1-3 kW heat output. The latter are almost all backed up by a gas boiler in a hybrid setup, usually uses zero gas until you run it dry - then the 25ish kW gas powered furnace can provide enough power to quickly fill a bath or send tons of hot water to a rain shower.