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How to calculate power consumption for heat pump water heater

samueldnewman | Posted in Mechanicals on

Hi all,

I am looking at building an off-grid house here in Vermont, and am trying to get a handle on how big my PV system might need to be.  I’m having trouble really understanding how much power a heat pump water heater might use.  The smallest one I can find ( 50 gallon) says that in a typical household it will consume 915 kwh per year.  Ok, so I can just divide that by 365, but there must be a way to get a more accurate estimate, right?  For example, if I know the UEF of the water heater, is there some math I can do to calculate watts per gallon per degree F?

I know the easy solution is to use a propane water heater, but I’m trying to figure out if I can avoid fossil fuels altogether.

Thanks!
Sam

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Replies

  1. Expert Member
    NICK KEENAN | | #1

    The yellow sticker is based on a standard amount of hot water usage, I'm sure you can find out what that is.

    The biggest question is how much hot water your household uses. Since you're living in Vermont your incoming water is probably colder than what the yellow sticker people figure on so that will up your usage somewhat as well.

  2. BFW577 | | #2

    I monitored my GE Geosprings electrical consumption for a few months. The compressor draws 550 watts running and it would use around 2-4 kwhs a day depending on the season and basement temperature. There are just 2 of us do obviously a big family would have a higher number.

    1. samueldnewman | | #3

      Thank you, BFW577, that is extremely helpful! We are just two as well, so that wouldn't be too different, and the water heater would be in the first floor conditioned space, so I'm thinking we'd probably use an amount comparable to you.

      Thanks again!

  3. _JT | | #4

    I have a 65 gallon rheem with a family of 5 in a typically 60 degree basement. Power usage is 2-4.5 kwh/day in heat pump only mode. That's a bit less than the sticker price but I think the sticker assumes mixed electric heat pump mode.

  4. Expert Member
    Rick Evans | | #5

    Hey Samuel,

    We have a Rheem 50 Gallon HPHW for just my wife and I. We average about 1.5 KWH per day here in the mountains of NH. We have a massive drain water heat recovery device too that helps with the efficiency.

  5. brad_rh | | #6

    geospring 50 gal in a cold shop (~45-50F), household of 2, 1.72 kwh/day for the last 40 days. We don't take showers everyday.

  6. vap0rtranz | | #7

    Bradford White Aerotherm -- the re-branded GeoSpring when GE stopped making it -- is basically same as Brad's comment above. 50gal @ <1.5kWh per day.

    We're also 2 here and don't shower daily :)

    Will you be heating / cooling with PV / electricity too?

    I sympathize with the sizing guesstimate quandary that you're in. We went through that with our PV contractor too. I'd worry more about space heating. For all their efficiency, heat pumps still suck up a lot of electricity in winter. That is true for our home's minisplits. And unlike a HPHW, there's a lot more inputs, sizing, tweaking, improvements, etc. to do for minimizing heating with minisplits / heat pumps. A HPHW is basically a dumb appliance that just works :)

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