Is there an "unmet hours" output variable equivalent for DHW demand?

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I cannot think of a time-based output, but the output variable called "Water Use Connections Hot Water Temperature" should be compared to the intended hot water supply temperature setpoint.

If your WaterUse:Connections is the only thing on the demand side of your plant loop, or you can safely assume the rest is satisfied, then you can use the Plant Loop Unmet demand rates:

HVAC,Average,Plant Supply Side Unmet Demand Rate [W]
HVAC,Average,Plant Supply Side Not Distributed Demand Rate [W]

See documentation here