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Efficient with money means you get more of that world and environment as savings per dollar.
Would you choose paying €4000 to replace 600 kWh of heat pump boiler hot water with solar boiler, or 4000 kWh of basically a whole home’s energy usage including hot water?
The solar boiler doesn’t even heat well during all seasons, mine still uses gas to back it up at least half the days.
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.