What would be the best electric hot water heater to put in when using PV panels? Not solar hot water panels, PV panels. Can't afford to add the solar HW now, so need suggestions for an electric water heater to use with PV.

Given my limited knowledge, my initial reaction is that using PV to energize an electric water heater is not the best use of your dollars. I think you'll get a lot more hot water out of a solar hot water panel at a lower upfront cost.

If you already have the PV panels and no where to put the power then I guess it makes sense. I assume any electric water heater will work if you are converting your PV output to the needed power.

Given that solar HW is far more efficient than any PV (40%+, as opposed to sub-20% typical efficiencies) there's no such thing as "can't afford solar HW now", if it is/was prior to installing the PV.  If it's a available roof area situation, hybrid PV/HW unit (eg. SunDrum) can be a good solution.

Assuming you're grid-attached and that you're heating with a cheaper fuel than electricity, any tank-top heat pump HW heater will outperform a resistance-type electric heater.

See: http://www.energystar.gov/index.cfm?fuseaction=heat_pump.display_products_html

Your first-hour gallons are a bit limited for fillin' the spa/soaking-tub but they're not misearbly low. During the cooling season it'll lower your homes' cooling load slightly.  During the heating system it's efficiency/cost is whatever your heating system is when replacing the heat drawn from the room, with the factor for the coefficient of performance of the heat pump. (It's clearly a winner in cooling dominated climates.)

How are you heating the place? Any chance of incorporating DHW into that system?

Do you mostly bathe, or mostly shower?  If shower, you can get a significant performance boost (like 50%+!) out of drainwater heat recovery:



They're not all created equal- Natural Resources Canada has a handy apples-to-apples testing performance comparison:

http://oee.nrcan.gc.ca/residential/personal/retrofit-homes/questions-answers.cfm#q45

Cost effectiveness estimates @ 7.5 cents/kwh:

http://www.renewability.com/uploads/documents/en/analysis_dwhr_minnesota.pdf

Takeaway message from the cost-effectiveness analysis: Get the biggest-fattest one that fits (the increased performance outweighs the increased cost.)

I wouldn't dream of heating HW with electricity (or solar) without it.  It extends the apparent capacity of an electric tank by a factor of ~2 in showering mode while cutting the hot water heating operating cost (or size of the solar hot water array) by 1/4 to 1/2.

If you go that route, pay attention to plumbing details- setting it up to feed both the tank and the cold water (to the whole house is fine, but at a minimum the cold to the shower) is a 20-25% performance improvement than just to the hot water heater.

(I have the  4" x 48" PowerPipe on my combi-heating/DHW system- it cuts the size of the boiler output required by at least 30KBTU/hr, and I have endless hot-water tankless style when taking showers.  I still need some storage to fill tubs though, but not as much as with an electric tank.)

You might consider a hybrid electric heat pump water heater. Rheem and GE each have a model. AirGenerate has an add-on for an existing heater.

It has an efficiency factor of 2+.

We plan to locate ours in the pantry (large, and open to the kitchen, dining room, living room space) where it can draw on waste heat from freezer and refrigerator in a symbiotic relationship. The cool dry air it puts off will be perfect for the pantry.

I am off grid with a small PV system of 3.6KW

I installed a SINGLE element heater, 1500 watt, 19 gallon water heater from Lowes and for my wife and I it is great!  We only turn in on when the genset is running or sunny day between 10 and 2 PM.  Takes about an hour to heat up total.  One shower only.

Plan on installing another in series and heat them alternately for 38 gallons of HW.

we just finished building a new home in south jersey.We installed geo thermal heat with a desuperheater,that feeds into a Marathon buffer tank and then to a Rheem heat pump hot water heater.We have a 5.3 pv system.We had thought about a tankless heater or solar(we had solar in our last house for 30 years and it works great)but after a lot of dicussion with the builder,who offers solar we decided to go with the above.We move into the house next week,after we,ve been in awhile,i'll post again

I do not have a spot for solar water right now. Wish I did. The Solar Voltaic will go on an outbuilding. Rheem said it does not advise putting a HPWH in a conditioned basement which is where the water heater will be. They take heat from the air and might make the basement too cold. Marathons don't leak but I have concerns about their being plastic and having questionable compounds leaching into the water. Guess I will just find the best insulated electric heater and go with it because natural gas is not available.

might be wise to invest in a water heater insulating cover, for that extra insulating effect.

Water heating efficiency is far more than the tank.  Typical distribution plumbing heat losses are 15-20" of the total energy, and uninsulated near-tank plumbing accounts for a large fraction of the system standby loss.  It's not rocket science- insulate the UN insulated stuff first!

No matter what you use for a hot water heater, it's worth putting the 5/8" wall closed cell foam insulation on the near-heater plumbing (both hot & cold, plus the down tube for the pressure/temperature safety valve), to cut the standby loss by a significant amount (well into double-digit percentages).  It's also worth putting it on the entire hot water distribution system.  Most electric tanks have pretty good insulation, and while some could use more, insulating the plumbing will have a greater effect on energy use than adding an extra blanket on the tank.

The big box stores tend to only sell the 3/8" wall stuff, but some Ace Hardware stores sell it for half-inch pipe.  Grainger also carries it for a variety of pipe diameters.  You also get it online from multiple vendors. Thermwell, Thermacel, Frost King, Insultube, etc etc all have fairly similar stuff. The 3/8" wall stuff is only ~ R2, but the 5/8" stuff is ~R4.  There is 3/4"-wall R5 stuff around too, well worth it for the near-tank plumbing in high priced electricity/solar markets.

That's an interesting idea. Makes sense to try to use the heat generated by the fridge and freezer as well as heat generated in cooking. I've worried about those things cooling off a room too much. Be sure to let us know how that works out.

The math on the back of a napkin:

Heat pump water heater:

1lb of 50F incoming water raised to 120F takes 70btus out of the room.

But with a COP of 2.0 the compressor's power use is putting half that back, for a net draw of 35BTUs/lb from the room.

Running cold water:

1lb of 50F water drawn and allowed to stagnate to a 70F room temp takes 20BTUs out of the room.

Toilet flushing (cold water stagnation to room temp):

A low-volume toilet uses 1.6gallons, which is 13.34lbs.  A 20 degree stagnation rise to room temperature over then next hour or so takes ~265BTUs out of the room.  An old skool 5 gallon flusher takes ~835BTUs out of the room. (A measurable heat load in a small bathroom.)

It's all a matter of rate of hot water use.  If you use 60 galllons of water/day, with a heat pump hot water heater it draws 14595 BTUs from the room in 24 hours, an average of only 584 BTUs/hr, which isn't a heavy heat load. But hot water use isn't evenly distributed it time, with huge peak loads when recovering from bathing (~40% of typical hot water use.)

With a 10 minute shower (2gpm of 120F hot water, 0.5gpm of 50F cold for a 106F shower) you'll be using 20 gallons of hot water in a short time.  For that tank recovery you net (20 gallons x 8.34lbs/gal x 35BTUs/lb=) 4865 BTUs drawn from the room- about the same as running a very small window air conditioner for an hour(!).  This is a very significant heating load for a small room like a 100 square foot galley kitchen or pantry (HUGE for a  30 square foot utility closet), but for an open 800-1200 square foot basement, not so much.

Let's say it's located in an area open to the kitchen with very free air exchange: If you're running 2 burners at 1/4 fire on a gas range in the same room for 10 minutes cooking breakfast,  you're putting ~850 BTUs back.  Run the toaster for a minute or two, that's another 50-100BTUs. It adds up when you're cooking, but it takes a lot of cooking to take the chill off.   If you're adding more than 1000BTUs to the room from cooking breakfast, you're probably eating too much. :-)  If there's good correlation between cooking hours and tank-recovery periods the chill won't be as big though.

The peak abatement you get out of waste heat from a refrigerator & freezer aren't all that much during the tank recovery period, since it's mostly continuous & background. An Energy Star version of either only uses ~500kwh/year per- call it 1000kwh/year for thepair. Dividing that by the 8760hours in a year, your average heat from the freezer & reefer is ~114watts, or ~390BTUs/hr- it won't make up for the chill following a shower. While multiplying that by 24hrs it's ~9360BTUs/day it's about 2/3 of the daily uptake by the hot water heater (assuming 60gallons/day use), but it's distributed fairly evenly over time, whereas the hot water tank draw is very spiky, with huge peaks during/after bathing.

Bottom line:  It's better if placed in a large open area with free air exchange to minimize spot cooling during recovery, and it's a net win during the cooling season.  If you lock it up in a tight closet with the refrigerator & freezer to isolate the chill from the rest of the house it'll be very noticeable whenever there's a large draw on the hot water, and it'll be FREEZING in there during the heating season unless the room is actively heated. If your hot water use is well below the average US household it could strike a decent daily balance with the refrigerator & freezer though.

A fascinating analysis! I'd never considered, for example, that water allowed to stagnate to room temperature, then flushed, removes heat from a building. Thank you, Dana1.

Our pantry is 5x13x10 for 650 cu ft. The GE heat pump water heater I ordered from their web site (it's not in distribution in Texas yet) wants to see 700 cu ft.

We'll have an induction cooktop, so less heat there.

Just my wife and me and two dogs taking showers (the two cats refuse) at odd, brief times at low flow. I think we'll be happy with the results. I will report, but completion is not expected until March/April, so it will be a bit of a wait.

With drainwater heat recovery you could cut the heat loss in half, but at minimal shower use the payback would take decades.

Do the dogs shower together??  ;-)

Austin TX has a 3000+ cooling degree day climate and a short heating season- you can BET you'll be happy with the results!  But the pantry can still potentially get cold at times if the natural air flow to the adjacent spaces isn't good.  Keep the doors open and I'd expect you'll be OK.

Also, in Austin your mid-late winter incoming water won't be 50F, but probably more like 60-65F at the coldest, which limits your net loss to 30BTUs or less per pound of incoming water (not the 35 BTU/lb in the 50F case) and your 10 minutes of shower recovery would be a ~4000BTU worst-case: