Compressor cutoff temperature
The compressor cutoff is the ambient temperature below which a heat pump water heater stops using its heat pump (typically around 37–45 °F) and falls back to electric resistance heating — a key spec for garage and basement installs in cold climates.
A HPWH in a space that drops below its low cutoff loses its efficiency advantage exactly when heating is most expensive. Certified records list both low and high cutoffs; a lower bottom number means more heat-pump hours in a cold garage.
Split-system models with outdoor CO₂ compressors are built for this problem — several operate below 0 °F — while indoor units in heated basements rarely hit their cutoff at all.