[RE-wrenches] Electric Water kWh/ person

Thu Mar 15 13:00:24 EDT 2018

This discussion has been a great help. The house 
is built and the plumbing is in, but could be 
modified. Wood stoves are not a likely solution. 
I think we need a different water heating strategy.

Thank you all!

Drake

At 09:21 AM 3/15/2018, you wrote:
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>This is what I came up with:
>
>The calculation is Q = mcp delta T
>
>Water Temp out =Â  120
>
>Water Temp in = ground source Temperature @ 50F?
>
>delta T = 70F
>
>m =Â  mass flow rate.Â  ASHRAE is the source for 
>this. 20gals per person per day = 20 gallons x 
>8.34 lbs/gallon =Â  167lbs/day/person
>
>cp =Â  1 BTU/lb-degree F
>
>Q = 167 x 1 x 70 = 11,676 BTU/day/person
>
>Divide this by the efficiency of the water 
>heating system (I'll assume = .86)Â  =11,676/.86 = 13, 577 BTU/person/day
>
>convert to kWh if necessarym 13,577/3412 = ~4 kWh/person/day, 120 kWh/month
>
>I checked this against one month and it seems reasonable.
>
>Website assuming ~3.5 family size comes up with 
>@ 400kWh per month. They approached it another way with assuming run times.
>
><https://www.google.com/search?safe=active&source=hp&ei=Z3KqWvXiNsGb5wKJ2Kf4DA&q=electric+water+heater+use+annual+consumption&oq=electric+water+heater+use+annually&gs_l=psy-ab.1.2.33i22i29i30k1l3.279.9222.0.12475.35.19.0.7.7.0.451.2360.0j6j4j0j1.11.0....0...1c.1.64.psy-ab..18.16.1952.0..0j0i67k1j0i131k1j0i22i30k1.0.CiU8wYZigpI>https://www.google.com/search?safe=active&source=hp&ei=Z3KqWvXiNsGb5wKJ2Kf4DA&q=electric+water+heater+use+annual+consumption&oq=electric+water+heater+use+annually&gs_l=psy-ab.1.2.33i22i29i30k1l3.279.9222.0.12475.35.19.0.7.7.0.451.2360.0j6j4j0j1.11.0....0...1c.1.64.psy-ab..18.16.1952.0..0j0i67k1j0i131k1j0i22i30k1.0.CiU8wYZigpI
>
>
>Chris
>
>
>
>
>On 3/15/2018 8:09 AM, Mike Kocsmiersky wrote:
>>Barton makes a good point with regards to the 
>>incoming ground water temperature and the 
>>amount of heat required to bring it up to 
>>shower temp. Â My recollection is that ASHRAE 
>>states 20gal/person/day for the first 2 people, 
>>then 15gpd or 12gpd thereafter.Â  However those 
>>rates vary wildly with consumers, from those 
>>that never bathe to the high schooler that 
>>needs 3 showers a day.Â  In the northeast water 
>>heating can be up to 19% of the total household 
>>energy load for the year.Â  Also, I would 
>>recommend the air source heat pump water 
>>heaters if you have significant humidity or an 
>>on demand unit.Â  Water tanks are typically 
>>under insulated and can lose 2kWh/day in standby losses.
>>Â
>>Mike Kocsmiersky
>>Principal
>>Spirit Solar Inc.
>>(413) 734-1456
>>Â
>>Â
>>Â
>>From: Barton Churchill 
>>[<mailto:barton at solarips.com>mailto:barton at solarips.com]
>>Sent: Wednesday, March 14, 2018 7:31 PM
>>To: RE-wrenches
>>Subject: Re: [RE-wrenches] Electric Water kWh/ person
>>Â
>>You first need to know your ground temperature. 
>>Then figure a delta to 100 or so degrees. Most 
>>but not all showerheads flow 2.5gpm and showers 
>>are typically 10 minutes. You can then find a 
>>calculator online to calculate how many Btus 
>>are required to raise water temp for your delta 
>>per gallon. Convert to kWh, add some contingency and you should be good.Â
>>
>>On Wednesday, March 14, 2018, 
>><<mailto:drake.chamberlin at redwoodalliance.org>drake.chamberlin at redwoodalliance.org> 
>>wrote:
>>Hello Wrenches,
>>
>>Is there a good way to estimate the power that 
>>will be consumed for electric water per person 
>>per day? We are estimating the amount of power 
>>that will be used by an all electric duplex, 
>>and the water heater amount seems high. The 
>>current model we have claims 3493 kWh/ year per 
>>26 gallon tank will be consumed. With the two 
>>units, that would add up to 6986 kWh / year for 
>>hot water, pushing the system size to humongous for a 1300 ft sq building.
>>
>>Does that seem high? There will be a maximum of 2 people per unit.
>>
>>The owner is considering going to 115 V units 
>>with a 15 gallon tanks. I think the smaller 
>>tank would help, but it would seem that the 
>>amount of hot water consumed would be the main issue.
>>
>>How can we get a good estimate for a per person 
>>energy usage for electric hot water? Any other suggestions are welcome.
>>
>>Thank you,
>>
>>Drake
>>
>>
>>
>>--
>>--Â
>>Barton Churchill
>>406.587.5295 Â
>>2430 North 7th Bozeman, MT 59715
>><http://www.solarips.com/>www.solarips.com
>>Image removed by sender.
>>
>>Â
>>
>>
>>
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>--
>
>Â Â Â Â Â Â Â Â  Christopher Warfel, PE
>Â Â Â Â Â Â  ENTECH Engineering, Inc.
>PO Box 871, Block Island, RI 02807
>Â Â Â Â Â Â Â Â Â Â Â Â Â Â  (401)466-8978
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