heat pump water heater for ADU?

[ericm]

My new house in Oregon is going to have a heat pump water heater. They're a lot more efficient than regular electric water heaters. We also have a separate garage with a small shop area that's going to be used to store and work on bicycles and an upstairs apartment. We need a water heater for the apartment of course. The GC is suggesting that it doesn't need a heat pump water heater and a regular electric one will do.

My current plan (which could change once we live there) is to leave the water heater for the apartment on all the time. We probably won't have a lot of guests but will sometimes use the bathroom. But mostly the hot water's going to sit in the water heater. So my question is: does an electric hot water heater use much power once the water is hot?

Before anyone says it there's no natural gas and I'm not going to install propane. Everything's electric.

 

[gmcgeo]

This would really depend on how much it would run, typically you would see 4000 watts per day.

 

[jack stand]

Tank insulation quality & room/ house temps is going to determine it's maintained minimum temperature while your away.
What's "ADU" ?

 

[The Metric System]

So my question is: does an electric hot water heater use much power once the water is hot?

No, the idle load from a water heater (electric or combustion) is very low compared to the load from heating water. The overwhelming majority of energy lost in a typical water heater goes out the discharge pipe in the form of hot water rather than through the insulated wall.

Check out the efficiency ratings (in terms of energy consumption per unit hot water) for varying sizes of heater within the same product line and you'll see there is very little difference between the smaller and larger units. In some cases the larger heater will actually work out to be more efficient overall due to the lower surface area to volume ratio as the cylinder grows.

For the application you describe I would not install a heat pump water heater. I would consider turning down the thermostat on that heater, and/or putting it on a timer that would cut power during the times of day that it is unlikely to be used.

 

[ericm]

Tank insulation quality & room/ house temps is going to determine it's maintained minimum temperature while your away.
What's "ADU" ?

Accessory Dwelling Unit. A separate living space to the main house.

 

[larry_g]

My kids put an electric on demand tankless water heater in their house. It might be a solution for you if it is being used infrequently..

lg
no neat sig line

 

[theoldwizard1]

My kids put an electric on demand tankless water heater in their house. It might be a solution for you if it is being used infrequently..

lg
no neat sig line

Or get a 120V one for the garage and set it on a timer.