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:bar...@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.chamber...@redwoodalliance.org
<mailto:drake.chamber...@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/>
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