Preserve the operating facts
Save the heater and generator models, nameplate photographs, exact overload message, selected heater mode, and when the trip occurs. Note whether it happens at startup, after heating begins, or after a mode change. Do not remove covers or repeat the fault to obtain a video.
Honda's wattage guide distinguishes starting demand from running demand. Its generic appliance estimates are not a rating for your heat-pump water heater. Starting and running power guidance.
Ask qualified providers to establish the heater's applicable starting and running requirements, any element operation, and the generator's actual continuous and surge ratings at the required voltage. A heat-pump-mode label alone does not establish its behavior under every condition.
Review the connection
A licensed electrician should assess the supply, protection, approved connection or transfer equipment, and actual measurements under an appropriate test. The water-heater technician should confirm model requirements and any fault or mode behavior.
Never backfeed a household outlet. CPSC generator electrical warning.
Do not bypass overload protection, alter element wiring, or try adapters to make the supply fit. Ask the providers to distinguish a capacity mismatch from a connection fault, equipment fault, or another documented cause.
Keep generator use safe
CPSC says to run portable generators outdoors only, at least 20 feet from the home and other buildings, with exhaust directed away. Never operate one in a home, garage, basement, crawl space, shed, or on a porch. Keep it away from home openings and maintain working battery-backed CO alarms. CPSC portable-generator boundaries.
Follow the generator's full instructions, including weather and electrical precautions. Arrange another location for essential bathing if the approved combination cannot provide hot water during the outage.
Your next step is a coordinated electrician and heater-service review of the exact models and connection. Resume the proposed setup only after its cause and suitability have been verified.