New Panel vs Heat Pump Water Heater

[frankge]

So I apologize as this is before I do the math which I will get to and post...

I was considering adding a additional 295w panel to my existing install of 11.310Kwh system with a 10k SE inverter. I don't get much clipping and have headroom to oversize. I can add a panel myself which is about $400.00 (panel, opti, rack hardware, shipping) or purchase a heat pump water heater for $700 or $1000 and install that myself. Im interested to see if it makes sense to go with the panels and keep my existing storage 50gal electric WH. We will be two people soon (last kid and his 30 min showers almost gone!). I had one of the original GE Geosprings and it was garbage. I still feel these things have iffy longevity but my geek kung-fu is very great and I will "f" with things endlessly like a lot of you on this forum! I have a Sense installed but it has not done a good job of detecting my WH so I don't know actual usage yet. I figure dollar wise its about $300 a year in electric for hot water.

https://pvoutput.org/list.jsp?id=68094&sid=60513

[emartin00]

Where are you located?
If you are somewhere in a warm climate, I would definitely go with the HP Water heater. Free A/C with your hot water is a great benefit.
I installed an AO Smith 50 gallon HPWH in my previous house, and absolutely loved it. Even in NH, it worked great (although I didn't utilize the HP during the winter). I went from a direct coil on my oil boiler, so it saved me about 100 gallons of oil a year. Plus it was a garage under colonial house, so the basement and garage were always nice and cool during the summer.

As you mentioned, the GE units had a lot of problems. It really gave HPWH a bad rap overall, but that isn't really the case with most of the others. The AO Smith unit is very quite, and unless it was in your bedroom, you probably would never notice it.

[J.P.M.]

My household uses ~ 40 gal. of hot H2O/day and we're pretty miserly for 2 people. The water tank is electric resistance and it, as well as the hot water supply lines, have the living crap insulated out of them. Additionally, all faucets are fitted w/ low flow nozzles. The total annual energy usage for hot water usage is pretty close to 12.2 MM BTU/yr., of which ~ 8.5 MM BTU (~2,500 kWh/yr.) is used to heat the water and ~ 3.7 MM BTU/yr. (1,100 kWh/yr.) , to make up for tank standby losses and line losses.

On the PV supply side, I live in a pretty sunny climate and each installed STC kW of PV will produce ~ 1,700 kWh of electricity per year. That means I would need (2,500 + 1,100) kWh/yr./(1,700 kWh/yr. per installed STC kW) = ~ 2.1 STC kW of PV.

Unless you live in a sunny climate and your DHW use is like mine (which, for reference only for SWAGging purposes, is ~ 1/2 - 2/3 as much use or less per person as the U.S. average),

Assuming a HPWH will require ~~ 40% as much energy as resistance heating (a C.O.P. of ~ 2.5 or so), my situation would require ~ (2.1 STC kW)*0.4 = ~ 0.84 installed STC kW to displace my DHW load.

While some of our (yours and my) situation is similar, I'd guess my loads and solar potential are probably less than yours. So, If I think I'd need ~ 800-900 STCW of PV, your estimate of adding 295 W of PV may be a bit light.

Take it for what it's worth from a back of the envelope perspective.

[frankge]

Where are you located?
If you are somewhere in a warm climate, I would definitely go with the HP Water heater. Free A/C with your hot water is a great benefit.
I installed an AO Smith 50 gallon HPWH in my previous house, and absolutely loved it. Even in NH, it worked great (although I didn't utilize the HP during the winter). I went from a direct coil on my oil boiler, so it saved me about 100 gallons of oil a year. Plus it was a garage under colonial house, so the basement and garage were always nice and cool during the summer.

As you mentioned, the GE units had a lot of problems. It really gave HPWH a bad rap overall, but that isn't really the case with most of the others. The AO Smith unit is very quite, and unless it was in your bedroom, you probably would never notice it.

I'm in Tampa and it did a great job cooling the garage where I reload and my workshop is. it kept things from rusting TOO.

[frankge]

My household uses ~ 40 gal. of hot H2O/day and we're pretty miserly for 2 people. The water tank is electric resistance and it, as well as the hot water supply lines, have the living crap insulated out of them. Additionally, all faucets are fitted w/ low flow nozzles. The total annual energy usage for hot water usage is pretty close to 12.2 MM BTU/yr., of which ~ 8.5 MM BTU (~2,500 kWh/yr.) is used to heat the water and ~ 3.7 MM BTU/yr. (1,100 kWh/yr.) , to make up for tank standby losses and line losses.

On the PV supply side, I live in a pretty sunny climate and each installed STC kW of PV will produce ~ 1,700 kWh of electricity per year. That means I would need (2,500 + 1,100) kWh/yr./(1,700 kWh/yr. per installed STC kW) = ~ 2.1 STC kW of PV.

Unless you live in a sunny climate and your DHW use is like mine (which, for reference only for SWAGging purposes, is ~ 1/2 - 2/3 as much use or less per person as the U.S. average),

Assuming a HPWH will require ~~ 40% as much energy as resistance heating (a C.O.P. of ~ 2.5 or so), my situation would require ~ (2.1 STC kW)*0.4 = ~ 0.84 installed STC kW to displace my DHW load.

While some of our (yours and my) situation is similar, I'd guess my loads and solar potential are probably less than yours. So, If I think I'd need ~ 800-900 STCW of PV, your estimate of adding 295 W of PV may be a bit light.

Take it for what it's worth from a back of the envelope perspective.

Reason for a single PV is that's what I have room for without going to a place with shading or north side of the house. It would be pretty easy to hook this single into the existing setup. On a day like today, 70's and full sun I produce ~60kWh

[Ampster]

I also have a heat pump WH. I am on a TOU rate so I run it at night to 129 degrees then set back in daytime to 110. I do that because my super off peak rate is $0.14 vs $0.44 during peak. Essentially I am storing energy. Depending on your rates the heat pump water heater can payback in 3 years. The EPA numbers are different because that calculation assumes a much lower cost of energy than California. As others have said it cools the garage and also dehumidifies the garage. Your mileage may vary. NOTE; If you are going to run it that hot for safety you should also install a mixing valve to prevent scalding.

[PNPmacnab]

I have a heat pump water heater and it is easy to focus on a single aspect. They do not work well over 120F and recovery times are slow. I also heat with solar resistance and with that temps of 150F can be reached for real storage potential.

[J.P.M.]

I have a heat pump water heater and it is easy to focus on a single aspect. They do not work well over 120F and recovery times are slow. I also heat with solar resistance and with that temps of 150F can be reached for real storage potential.

By "solar resistance" do you mean PV that powers a resistance heating element or something else ?

[frankge]

Yeah so I completely get how the hpwh works having had one that is no longer, im trying to balance out if adding a panel will have more overall benefit than installing a hpwh again. My payback is completely skewed because I have PV primarily netting my electric (estimated) to zero. Mind you have only had solar since May so I don

[Ampster]

Yeah so I completely get how the hpwh works having had one that is no longer, im trying to balance out if adding a panel will have more overall benefit than installing a hpwh again. My payback is completely skewed because I have PV primarily netting my electric (estimated) to zero. Mind you have only had solar since May so I don

Yes, I figured out that you were familiar with them further down the thread. To directly answer your question, I view a heat pump replacing a resistance water heater as a conservation measure. That is always a better investment than more capacity. As I mentioned earlier a water heater is an energy storage device as well. But that applies to a resistance WH as well. I have had a GeoSpring and Rheem and I know for sure the Geospring can be programmed by IFTTT for various setback scenerios. Before i discovered the IFTTT programming i was using a simple timer to shift that load.

[J.P.M.]

Yes, I figured out that you were familiar with them further down the thread. To directly answer your question, I view a heat pump replacing a resistance water heater as a conservation measure. That is always a better investment than more capacity. As I mentioned earlier a water heater is an energy storage device as well. But that applies to a resistance WH as well. I have had a GeoSpring and Rheem and I know for sure the Geospring can be programmed by IFTTT for various setback scenerios. Before i discovered the IFTTT programming i was using a simple timer to shift that load.

A water heater is a device that inputs thermal energy into water. A water heater MAY also be a storage device, but not necessarily, as in the case of a HPWH. A water heater of the electric resistance type may be, and sometimes is used as an inline or "instant" heater. Combining storage and heating of water in one device is often convenient but writing or implying that "a water heater is an energy storage device as well" is incorrect as written. Some are, some ain't.

[Ampster]

A water heater is a device that inputs thermal energy into water. A water heater MAY also be a storage device, but not necessarily, as in the case of a HPWH. A water heater of the electric resistance type may be, and sometimes is used as an inline or "instant" heater. Combining storage and heating of water in one device is often convenient but writing or implying that "a water heater is an energy storage device as well" is incorrect as written. Some are, some ain't.

Yes, you are correct. The subject of the thread implied storage because I don't know of any instaneous heat pump water heaters. The term water heater does include instantaneous water heaters. I should have been more clear, so that unsuspecting nu bees wont be fooled. I also wont modify my original post so my mistake can serve to illustrate your point. Hopefully you and I agree about the value of energy storage to leverage the OP's solar panels. My first solar installation was a solar thermal one in 1980 and I did another in 2005. The low price of solar PV panels and emergence of heat pump water heaters has left much of the thermal hot water uses mainly in pool heating.

[Mike90250]

There are also reports of HPWH being non-repairable, expensive compared to simple resistance element water heaters, and the first generation units failing rapidly after a couple years

[Ampster]

There are also reports of HPWH being non-repairable, expensive compared to simple resistance element water heaters, and the first generation units failing rapidly after a couple years

Yes, my GeoSpring inlet connection corroded and a friend had to replace his GeoSpring under warranty. By the time mine needed to be repaired GeoSpring was sold and it was easier to make up new inlet pipe out of copper than go through hoops. The Rheems and A O Smith ones have better ratings. I would not buy a GeoSpring again.

From my perspective, the savings have been worth the few risks. In total I have four, 1 GeoSpring, 2 Rheems and an off brand retro type that is going on 8 years with no problems.

[PNPmacnab]

By "solar resistance" do you mean PV that powers a resistance heating element or something else ?

Yes, the panels are operated at power point with excess solar and this feeds into standard heater elements. Too bad these systems haven't reached the general public yet. A couple of panels can maintain tank temps from heat loss.

I bought a NYLE external HPWH. Should the tank go, just buy another tank. The compressor, fan and Gundfoss pump are also easy to replace if into that. They will fail eventually. I got a deal on this one for $385 shipped. Payback on that is a no brainer. Wife likes it dehumidifying the basement.

Like politics, all solar is local. Heating water is very complex. Exporting power tends to be worthless. might as well put that into heating water with resistance heating. So you want to turn on the water heater those two hours of max sun. They have controllers that do this, but not sure how well they actually work around your life. It can take 70W just to maintain tank temperature. An extra panel probably won't help as you likely have excess now. I heat water solely with PV in the summer without an issue. Come October I don't even have a chance due to almost continual clouds.