[RE-wrenches] Electric Water kWh/ person

cwarfel cwarfel at entech-engineering.com
Thu Mar 15 13:35:06 EDT 2018 

That would be a good analysis to share. I can see how it could be 
possible with lower module costs, but I haven't seen any studies on this 
comparison. Chris


On 3/15/2018 1:17 PM, Darryl Thayer wrote:
> When I did my study of solar water heating I found that PV water 
> heating gave more hot water at a lower cost than solar thermal.  the 
> Heatpump water heater was the big winner.
>
> On Thu, Mar 15, 2018 at 12:00 PM, Drake 
> <drake.chamberlin at redwoodalliance.org 
> <mailto:drake.chamberlin at redwoodalliance.org>> wrote:
>
>     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:
>>     Content-Type: multipart/alternative;
>>      boundary="------------EE4E9806B711DEF636076840"
>>     Content-Language: en-US
>>
>>     This is what I came up with:
>>
>>     The calculation is Q = mcp delta T
>>
>>     Water Temp out =  120
>>
>>     Water Temp in = ground source Temperature @ 50F?
>>
>>     delta T = 70F
>>
>>     m =  mass flow rate. ASHRAE is the source for this. 20gals per
>>     person per day = 20 gallons x 8.34 lbs/gallon =  167lbs/day/person
>>
>>     cp =  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)  =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.  However those rates vary wildly with consumers,
>>>     from those that never bathe to the high schooler that needs 3
>>>     showers a day.  In the northeast water heating can be up to 19%
>>>     of the total household energy load for the year.  Also, I would
>>>     recommend the air source heat pump water heaters if you have
>>>     significant humidity or an on demand unit.  Water tanks are
>>>     typically under insulated and can lose 2kWh/day in standby losses.
>>>     Â
>>>     Mike Kocsmiersky
>>>     Principal
>>>     Spirit Solar Inc.
>>>     (413) 734-1456 <tel:%28413%29%20734-1456>
>>>     Â
>>>     Â
>>>     Â
>>>     *From:* Barton Churchill [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,
>>>     <drake.chamberlin at redwoodalliance.org
>>>     <mailto: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 <tel:%28406%29%20587-5295> Â
>>>     2430 North 7th Bozeman, MT 59715
>>>     www.solarips.com <http://www.solarips.com/>
>>>     Image removed by sender.
>>>     Â
>>>
>>>
>>>
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>>
>>     -- 
>>
>>              Christopher Warfel, PE
>>            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 <tel:%28401%29%20466-8978>
>>     Â
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-- 

          Christopher Warfel, PE
        ENTECH Engineering, Inc.
PO Box 871, Block Island, RI 02807
                (401)466-8978

EEI logo <http://entech-engineering.com>
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