[RE-wrenches] Electric Water kWh/ person

Thu Mar 15 09:21:52 EDT 2018

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 monthand 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

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.  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.
>
> Mike Kocsmiersky
>
> Principal
>
> Spirit Solar Inc.
>
> (413) 734-1456
>
> *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
>
> 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.
>
> 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
>
>
>
> -- 
>
> -- 
>
> Barton Churchill
> 406.587.5295
> 2430 North 7th Bozeman, MT 59715
> www.solarips.com <http://www.solarips.com/>
>
> Image removed by sender.
>
>
>
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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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