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40gal hot water w/ top on ‘grid’ bottom, solar?

12VoltInstalls

life passes by too quickly to not live in freedom
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Going to be doing something for my sister/bil with an LV6548 variant and 10kWh battery, 6600W solar input.

“Averaged” daily kWh (13mo) is 19kWh with water heating separately metered 9.6kWh. I want to leave the top element on grid at at 240VAC disconnecting the bottom element from grid supplying it through lower thermostat instead with 120VAC from the AIO.

I’ve read a number of threads like this so I know it’s possible and done. I intend to switch the bottom element through a relay triggered by a photocell so that the lower element only runs when the sun is out which brings up a concern.

The lower element “should be” 1100W at 120VAC; basically 10A. Dumb question that I don’t have the experience to answer: is that ‘shock load’ (although no surge) going to present any issue for the 6548? There’s no surge on a resistance load, of course, and I guess that would have similar ‘violence’ as a hair dryer or shop vac, so maybe I’m thinking about things for no reason. I just haven’t done the hot water thing before.

The only reason for switching is that all they can do right now is two 5kWh batteries for 10kWh and their 9.6kWh average hot water consumption would by wildhat guessing suck 5kWh out of the batteries from evening consumption.

Am I doing what i do well (overthinking), or is my reality check: reality.

?Thoughts?

Option “B” would be to buy the smallest cheap battery-less AIO that will do it on 6 of their 375W panels and just let it cycle on its own. I didn’t see any threads of anyone doing that. But at that point just buying another 5kWh battery would be virtually the same expense.
 
I would suggest you replace the lower thermostat with an "upper" one which has an additional temperature disconnect. I am unsure if you are talking DC or AC on this element.
 
This pretty much what I do except I run lower heater direct from 48V DC (thermostat bypassed). 40 gal tank could store 6.2kWh of heat (77F - 140F) when PV is generating so you avoid having to cycle the batteries.
 
upper" one which has an additional temperature disconnect
So I don’t know everything - though I’m cursed by the situation that in my friend group I’m the guy that has all the mech knowledge…

I didn’t know the upper tstat had two ‘switches’ in it. I will have to explore this.

Unless that’s not what you meant?

“I KNEW I DECIDED TO ASK GOR A REASON”
 
Upper thermostats have an extra two switch contacts in the over temperature switch in case the thermostat fails. This breaks both sides of the line AC coming in in case the heater element shorts to earth. Go to a big box store and compare them on the rack.
 
I would suggest you replace the lower thermostat with an "upper" one which has an additional temperature disconnect. I am unsure if you are talking DC or AC on this element.
I was planning on 120ACV from the LV6548 on the lower element only, but through the lower t-stat leaving the upper tstat to function as ‘normal’ but sourced from grid 240ACV- and perhaps running the top tstat at a slightly lower temp than the bottom tstat.

I watched this useful video which was useful.

However, unless I’m an idiot- an entirely possible scenario LOL- I’m not understanding the advantage of a second disconnect.

EDIT:
Upper thermostats have an extra two switch contacts in the over temperature switch in case the thermostat fails. This breaks both sides of the line AC coming in in case the heater element shorts to earth.
Ah- that posted will I was typing.
That makes sense; thank you.


The goal is to bill-shave for the hot water portion of the electric bill with ‘surplus’ solar. Determining what ‘surplus’ is isn’t decided as there really isn’t any pnp solutions for carrying out the logic of if/then other than supervtech’s post in another thread (which would be too much for the users of this- a photocell and a properly rated light switch or disconnect is probably the best functional solution for them)
 
Why not take about 120V DC directly from the panels to the existing 240V lower heat element. To a heat element there is no difference between AC & DC. It would be best to use a "high voltage DC contactor" (Littelfuse DCN or equal) as the existing water heater thermostats/thermal switches cannot reliably handle DC power loads, even 10 amps at 120VDC has a problem w/arcing & welding contacts.
 
If you want to store more power in the water heater you can install a thermostatic mixing valve and let the water get up to 85C or so. With the mixing valve you can fit 9.6kWh in a regular sized tank no problem. This is what I do, I made a calculator website that explains my setup.


Basically you store half your energy in the tank going form tap-cold to 40C, and the second half from 40C to 85C+. But you can only really use that "top" half of the energy because the wife and kids will complain if the shower even slightly cool. (thermal stratification helps somewhat) So you install a mixing valve and now you have a lot more headroom before the kids start whining.
 

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