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Persuade me - Hybrid Electric Water Heater.

tomuo

Member
Mar 15, 2021
49
17
Los Angeles, CA
I've run some numbers, but I'm still on the fence whether to replace my 12 yr old (and thus inefficient) gas water heater with a hybrid electric.

Calculating raw per kilowatt hour cost equivalent (lots of assumptions here)
current gas heater - 11c/kWh equiv.
new gas heater with power damper - 9c/kWh equiv.

hybrid electric heater, running during cheapest power - 5c / kWh equiv.

so it wins hands down in terms of running cost, and especially with the cost of natural gas only expected to go up in the next few years.

looking at some posts here and elsewhere, looks to be anywhere between 75 and 150kWh per month usage, or maybe 2.5 - 3 kWh per day.

The negatives (for my situation).
water heater is in the garage, I have a 70amp fed sub panel there which would take the load easily, but is not powerwall backed up.
In the event of a prolonged outage (SCE), I'd have no hot water and no way to feed it directly (as its 240V, not 120V), with a Honda little inverter gen.

This extra load would probably put me over my solar generation on a yearly basis.

Would need to automate the heater somewhat to make sure it runs during cheapest time (if the kids take a late night shower, let it sit at lower temp until the morning, etc).

But I have identified that I have a 100 watt server (cameras & media) running 24/7 that I can replace with a little NUC that should get my usage down again.

So my worry here is the loss of hot water during an outage, but if you consider a long outage might be from Earthquake damage, natural gas would probably be turned off during that time too?)
 

nwdiver

Well-Known Member
Feb 17, 2013
8,474
11,682
United States
I've run some numbers, but I'm still on the fence whether to replace my 12 yr old (and thus inefficient) gas water heater with a hybrid electric.

Calculating raw per kilowatt hour cost equivalent (lots of assumptions here)
current gas heater - 11c/kWh equiv.
new gas heater with power damper - 9c/kWh equiv.

hybrid electric heater, running during cheapest power - 5c / kWh equiv.

so it wins hands down in terms of running cost, and especially with the cost of natural gas only expected to go up in the next few years.

looking at some posts here and elsewhere, looks to be anywhere between 75 and 150kWh per month usage, or maybe 2.5 - 3 kWh per day.

The negatives (for my situation).
water heater is in the garage, I have a 70amp fed sub panel there which would take the load easily, but is not powerwall backed up.
In the event of a prolonged outage (SCE), I'd have no hot water and no way to feed it directly (as its 240V, not 120V), with a Honda little inverter gen.

This extra load would probably put me over my solar generation on a yearly basis.

Would need to automate the heater somewhat to make sure it runs during cheapest time (if the kids take a late night shower, let it sit at lower temp until the morning, etc).

But I have identified that I have a 100 watt server (cameras & media) running 24/7 that I can replace with a little NUC that should get my usage down again.

So my worry here is the loss of hot water during an outage, but if you consider a long outage might be from Earthquake damage, natural gas would probably be turned off during that time too?)

Do you have a power wall just not backing the panel the HPWH would be on? The compressor only uses ~500w. I run my POS GE Geospring off-grid with a 4.4kW inverter so a power wall would easily handle a newer Rheem. It uses about as much electricity as a refrigerator in HP mode.
 

SabrToothSqrl

Active Member
Dec 5, 2014
3,844
3,246
PA
So, my house came with a 50 gallon propane water heater. once I got my 1st Model S back in 2015 i went nuts on efficiency (mostly for my wallet). With cheap propane, I think the propane water heater says it's $300/yr to run. (way more on overpriced propane I had). HPWH was like $120/yr in hybrid mode.

But my propane was only a few years old and worked great. what to do, what to do....

Well water --> filters --> softener --> HPWH in Heat pump only mode at 140 --> Propane water heater at 120.

HPWH does 95% of the work, and now I have 100+ gallons of hot water on tap. Since it's 100 gallons, even set to 120, we don't run out, even with laundry + jacuzzi + large shower.

Propane tank is basically cheap storage of lots of water warmed by the HPWH, which can run heat pump only mode, since the propane one will pickup the slack if needed.
 
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tomuo

Member
Mar 15, 2021
49
17
Los Angeles, CA
Do you have a power wall just not on the circuit the HPWH would be on? The compressor only uses ~500w. I run my POS GE Geospring off-grid with a 4.4kW inverter so a power wall would easily handle a newer Rheem. It uses about as much electricity as a refrigerator in HP mode.
This is a partial backup situation; the breakers in the gateway are completely full, and its easily 60-70foot around the house to the garage even if there was space. I had the garage sub panel feed upgraded to that 70 amp with the PW installation, but from the main panel which would be isolated during a grid outage.
 

tomuo

Member
Mar 15, 2021
49
17
Los Angeles, CA
So, my house came with a 50 gallon propane water heater. once I got my 1st Model S back in 2015 i went nuts on efficiency (mostly for my wallet). With cheap propane, I think the propane water heater says it's $300/yr to run. (way more on overpriced propane I had). HPWH was like $120/yr in hybrid mode.

But my propane was only a few years old and worked great. what to do, what to do....

Well water --> filters --> softener --> HPWH in Heat pump only mode at 140 --> Propane water heater at 120.

HPWH does 95% of the work, and now I have 100+ gallons of hot water on tap. Since it's 100 gallons, even set to 120, we don't run out, even with laundry + jacuzzi + large shower.

Propane tank is basically cheap storage of lots of water warmed by the HPWH, which can run heat pump only mode, since the propane one will pickup the slack if needed.
putting the new hybrid in series with a smaller nat gas heater for backup would work, but there's really only space for one unit there unless I give up half my workbench and storage on that wall. It's more expense, but maybe worth it.
 

jrweiss98020

Tessa's Tesla
Jan 9, 2020
502
374
Edmonds, WA
I've run some numbers, but I'm still on the fence whether to replace my 12 yr old (and thus inefficient) gas water heater with a hybrid electric.

Calculating raw per kilowatt hour cost equivalent (lots of assumptions here)
current gas heater - 11c/kWh equiv.
new gas heater with power damper - 9c/kWh equiv.

hybrid electric heater, running during cheapest power - 5c / kWh equiv.

so it wins hands down in terms of running cost, and especially with the cost of natural gas only expected to go up in the next few years.
What about a tankless gas heater? Mine uses about half the gas of the old one...
 

nwdiver

Well-Known Member
Feb 17, 2013
8,474
11,682
United States
What about a tankless gas heater? Mine uses about half the gas of the old one...

That's pretty unlikely unless your old water heater was REALLY... REALLY terrible or you only used it once a week. A HPWH uses ~70% less energy than tankless. Tankless generally uses ~10% less energy than tanked.
 

Dave EV

Active Member
Jun 23, 2009
1,923
1,629
San Diego
I've run some numbers, but I'm still on the fence whether to replace my 12 yr old (and thus inefficient) gas water heater with a hybrid electric.
Just do it. The current Rheem HPWHs are great. Putting a new gas water heater in means another 8-15 years of another gas appliance, and we need to get off fossil fuels as fast as possible.

Calculating raw per kilowatt hour cost equivalent (lots of assumptions here)
current gas heater - 11c/kWh equiv.
new gas heater with power damper - 9c/kWh equiv.

hybrid electric heater, running during cheapest power - 5c / kWh equiv.

so it wins hands down in terms of running cost, and especially with the cost of natural gas only expected to go up in the next few years.
In my situation, I figure the cost is basically equivalent. But my main goal is to electrify everything as that means my appliances will only get cleaner over time.

looking at some posts here and elsewhere, looks to be anywhere between 75 and 150kWh per month usage, or maybe 2.5 - 3 kWh per day.
My total electricity usage (family of 4) for my HPWH for the last year is about 700 kWh or about 60 kWh/month according to the app. Usage goes down the summer to early fall, then goes up in the winter. As low as 40 kWh in Jul-Sep and as high as 90 kWh in Dec/Jan. I assume that this is due to a combination of incoming water temperature and garage air temperatures, mostly. I have not verified how accurate the app is.

I used to run solely in HP mode, but recovery time could be slow, so I changed to Energy Saver mode (except during peak TOU rates) which does turn on the resistance elements occasionally. But it seemed to not make much of a difference in total energy usage, though you can see when it happens in the app as instead of a couple hundred Wh used in an hour, you will see 2-3 kWh used in an hour.

The negatives (for my situation).
water heater is in the garage, I have a 70amp fed sub panel there which would take the load easily, but is not powerwall backed up.
In the event of a prolonged outage (SCE), I'd have no hot water and no way to feed it directly (as its 240V, not 120V), with a Honda little inverter gen.
As nwdiver states, in HP mode, it only pulls 600-700W at the most, so it could run off a small generater at 240V if needed. But honestly, in a prolonged grid-outage scenario is hot water your primary concern? If it's that important, then I would look at a 240V generator or even better, a Powerwall as that will also let you shift solar power to peak TOU rates.

This extra load would probably put me over my solar generation on a yearly basis.

Would need to automate the heater somewhat to make sure it runs during cheapest time (if the kids take a late night shower, let it sit at lower temp until the morning, etc).
I have found the ability to control when the HPWH runs somewhat lacking and often finicky when using the app. For some reason, when crossing scheduled periods the heater often kicks on for a short period, despite no change in temperature set point. So if your peak period starts at 4PM, for example - set it up to start at 3:30PM.

You can also play around with temperature set points and heating modes to store energy in off-peak periods and limit energy usage during peak periods. But your ability load shift with temperature is significantly limited unless you also have a thermostatic mixing valve to limit temperatures to 120F, for example. For this reason, I would highly recommend installing one when you install the new water heater - then you can preheat water during off-peak rates as high as 140F (supposedly Rheem is working on newer heaters that go even hotter) but maintain a safe 120F water output temperature.

But I have identified that I have a 100 watt server (cameras & media) running 24/7 that I can replace with a little NUC that should get my usage down again.
If you could eliminate that, that should just about offset your HPWH usage. Not sure how much energy NUCs pull these days. I have a Raspberry PI for running TeslaMate, and it only pulls a couple watts, but it probably isn't compatible with your workload. You might be able to get the NUC down to 10-15W? ROI for this would be a few years depending on your electricity rates. These types of vampire drains really add up, though!

So my worry here is the loss of hot water during an outage, but if you consider a long outage might be from Earthquake damage, natural gas would probably be turned off during that time too?)
It depends. It is generally only recommended that you turn off your gas to your house if you suspect a leak.
 
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Jenny73

Member
May 31, 2021
5
2
California
I know several people that also have a Rheem. They're all doing great... as opposed to the GE GeoSpring which I believe was rebranded under AO Smith and appears to have a ~110% failure rate after ~20 months...
Well this is my first experience with any tankless model. Before I had storage tank and literally they were a headache:(😞
 

Jenny73

Member
May 31, 2021
5
2
California
As I told, I recently Installed Rheem, I hired recommended certified plumber. I haven't experienced any temperature fluctuation or any other issue yet. I think this is this tankless model is the best investment I have made so far. It heats water within seconds which is cool.
 

nwdiver

Well-Known Member
Feb 17, 2013
8,474
11,682
United States
As I told, I recently Installed Rheem, I hired recommended certified plumber. I haven't experienced any temperature fluctuation or any other issue yet. I think this is this tankless model is the best investment I have made so far. It heats water within seconds which is cool.

Do you have a HPWH or a tankless water heater? Tankless is 'great'.... it just uses ~3x more energy than a HPWH.
 
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jrweiss98020

Tessa's Tesla
Jan 9, 2020
502
374
Edmonds, WA
However, that energy may well cost less... I just looked at the Rheem site. Tankless gas heaters have a UEF of ~.93, while the hybrid electric are ~3.5. So the energy use of the gas heater is 3.73x the electric one when it is using the heat pump. However, electricity is 3.2x the cost of gas. So if the hybrid ever uses the resistance heater, it will likely cost more to run. Then there's the efficiency factor of producing the electricity, especially if you're in an area that has coal-fired powerplants...
 

ItsNotAboutTheMoney

Well-Known Member
Jul 12, 2012
11,054
8,865
Maine
However, that energy may well cost less... I just looked at the Rheem site. Tankless gas heaters have a UEF of ~.93, while the hybrid electric are ~3.5. So the energy use of the gas heater is 3.73x the electric one when it is using the heat pump. However, electricity is 3.2x the cost of gas. So if the hybrid ever uses the resistance heater, it will likely cost more to run. Then there's the efficiency factor of producing the electricity, especially if you're in an area that has coal-fired powerplants...
OP is in LA, so unlikely to be using much coal.
But even then, at a UEF of 3.5 it'd be a clear efficiency advantage. Average US coal plant efficiency is 2019 was 32.3%.
(For comparison, average NG efficiency was 44.1%)
 
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nwdiver

Well-Known Member
Feb 17, 2013
8,474
11,682
United States
However, that energy may well cost less... I just looked at the Rheem site. Tankless gas heaters have a UEF of ~.93, while the hybrid electric are ~3.5. So the energy use of the gas heater is 3.73x the electric one when it is using the heat pump. However, electricity is 3.2x the cost of gas. So if the hybrid ever uses the resistance heater, it will likely cost more to run. Then there's the efficiency factor of producing the electricity, especially if you're in an area that has coal-fired powerplants...

Not that it needed further reinforcement... but the lethal heat wave that hit the PNW really highlights the urgent need to kick our pathetic addiction to fools fuel ASAP. The process is not complicated. Just two steps that can be worked on simultaneously. Electrify EVERYTHING and power EVERYTHING from renewables. A HPWH helps in two ways. It uses ~70% less energy than a fools fuel powered water heater AND it's 'discretionary load' that can be programmed to heat when there's a surplus of renewables, basically acting as a thermal battery.

Tankless water heaters are terrible....

 

tomuo

Member
Mar 15, 2021
49
17
Los Angeles, CA
I'm convinced.
The only downside is if there is a multi day grid outage, I would need to find a way to temporarily get 240V from the powerwall gateway to the garage,
or add another PW to switch to whole house backup sometime in the future.
 

iPlug

Member
Sep 14, 2019
626
908
Rocklin, CA
I'm convinced.
The only downside is if there is a multi day grid outage, I would need to find a way to temporarily get 240V from the powerwall gateway to the garage,
or add another PW to switch to whole house backup sometime in the future.

Our emergency case plan for such a situation:

We have a couple propane tanks and a propane BBQ (we rarely use) with stove top style burner. Idea would be to boil water and mix down with tap or pool ambient temp water in a large Home Depot bucket then transport to the shower area where we would then use a USB/battery rechargeable camping shower head/pump...
 

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