Pairing Water Heaters with a PV System: Page 3 of 6


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This Rheem Marathon 40-gallon electric water heater’s tank is made with cross-fiber polybutene that is impervious to rust and corrosion. Because of this, it requires no sacrificial anode rod and carries a limited lifetime warranty against leakage. It has an EF of 0.95, meaning that it has standby losses (heat transfer through the tank walls) of 5% or less per 24-hour period.
A.O. Smith’s Voltex hybrid electric heat-pump water heaters, like this HPTU-50N 50-gallon model, offer up to four different modes—efficiency, hybrid, electric, and vacation. With up to a 3.24 EF rating, they can offer significant savings compared to tank-style heaters. This model boasts an EF greater than 2.0 and carries a six-year limited warranty.
Stiebel Eltron’s Tempra tankless water heaters have special flow control/microprocessor technology to ensure a constant temperature output that matches the setpoint—no matter the demand for hot water. It carries a three-year parts and seven-year leakage warranty.
This cutaway illustration of a Stiebel Eltron Tempra 36 Plus shows its three heating elements. This unit requires three 50 A, 240 VAC breakers in the AC distribution panel.
Solar thermal flat-plate collectors (shown) and balance-of-system components make up this SunMaxx solar water heating system.
Solar thermal flat-plate collectors and balance-of-system components (shown) make up this SunMaxx solar water heating system.
With 25.82 square feet of absorber area, Stiebel Eltron’s SOLkit 2 collector and system is designed to offset conventional water heating for a two- to three-person household. The kit includes an 80-gallon storage tank with in-tank backup electric element. The system comes with a 10-year warranty on the collectors, racks, and tank and is priced at about $7,800 (not including tankless backup shown).
This A.O. Smith EJC-2 point-of-use heater with a 2.5-gallon tank uses a 1,500 W heating element at 120 VAC.
At 5.5 inches tall, 7.5 inches wide, and 2.75 inches deep, Stiebel Eltron’s Mini 2 on-demand heater can fit unobtrusively in most bathroom, kitchen, or laundry sink cabinets. It operates at standard 120 VAC and requires a flow of only 0.21 gpm to activate.
At 5.5 inches tall, 7.5 inches wide, and 2.75 inches deep, Stiebel Eltron’s Mini 2 on-demand heater can fit unobtrusively in most bathroom, kitchen, or laundry sink cabinets. It operates at standard 120 VAC and requires a flow of only 0.21 gpm to activate.
Therma-Coil’s stainless-steel heat exchanger for wood-burning applications has almost 3 feet of tubing inside the wood heater.


WHAT: Tankless on-demand (aka instantaneous) electric water heater.

HOW: When a hot water tap is turned on, the flow of water through the heater signals an electric element to turn on, heating the water.

ENERGY: Electricity.

HOW: Via a high-wattage, resistance-based heating element (typical range is 15,000 to 28,000 W).

SUITABILITY: Grid-tied RE systems.


  • Can work as a backup for a solar water heating system when space for another storage tank isn’t available.
  • More energy efficient—no standby heat losses. The U.S. Department of Energy estimates that for homes that use “41 gallons or less of hot water daily, demand water heaters can be 24% to 34% more energy efficient than conventional storage tank water heaters.” For homes that use more than this, demand heaters can be 8% to 14% more energy efficient. Depending on pipe lengths, greater energy savings (between 27% and 50%) can be achieved by installing a point-of-use, on-demand water heater at each hot water outlet.


  • Output temperature of the water depends on incoming water temperature and if simultaneous demands for hot water are being made. While there are different sizes of demand water heaters to meet different loads, they may not be able to supply adequate hot water to multiple simultaneous demands (i.e., showering while the dishwasher runs).
  • High electrical energy use—may be more expensive depending on utility rate structure (i.e., time-of-use programs, demand charges, etc.). If on a tiered rate plan, peak demand charges could ensue if your heater consumes electricity over a certain threshold during certain periods.
  • Large amperage draw versus voltage draw; may need to upgrade amperage on the home’s electrical service.
  • In locales with hard (mineral-rich) water, scaling may result, plugging the heat exchanger. Some units may have removable filters that can be cleaned regularly and descaling the unit every few months will help prolong its life. In areas with extra-hard water, a water softener may be necessary. Check the warranty for the maximum pH the water can be.
  • Increased complexity (compared to a tank-style heater) means less ability for DIY repairs or parts replacement.

PRICE: $500-$600

TIPS: Unless the unit has a microprocessor to maintain output, you may need to compensate for use with low-flow fixtures, etc., by installing a higher-wattage heater or by installing two heaters in series—the first warms the water halfway, and the second warms it to the final temperature.

Comments (7)

marvhen's picture

I thought this article might mention hybrid PVT systems that reduce electrical resistance in PV panels due to the increased heat. Anyone have info on such a system in the US? The web seems to mostly mention installations in Europe.

Michael Welch's picture
Many companies have tried in the U.S., none have succeeded over the long term. Here's why:
Complex plus expensive. If either the SHW or PV portions fail, the entire module has to be replaced.
Generally speaking, electricity and water don't co-mingle well. That contributes to the complexity and cost per above.
To get water hot enough to use, the water will often be so hot that it will HEAT the PV module, not cool it.

It seems better to use separate systems, or use PV electricity to handle both.
RMichael Curran's picture

I briefly considered putting a 120 volt tankless water heater under my kitchen sink to save water wasted waiting for the hot to arrive. Instead I keep a 5 gallon plastic bucket nearby and run water into it until it's hot. I keep another bucket in the bathroom for the same purpose, and I use both buckets for flushing the toilet. Saves flow thru my septic system, too.

I put an extra layer of insulation around my water heater and have it on a time switch, and these really cut down its energy use.

Glenn Nichols's picture

My wife and I built a new house in the Texas Hill Country. We considered solar hot water heating, but ended up using our geothermal heat pump's optional desuperheater to provide heat to the water heater (with electrical backup) and to use the space where the solar hot water panels would have been to install additional PV panels. It made more sense to us as the solar water panels would only be functional for a small fraction of each day in order to meet our hot water needs, but the solar electrical panels would provide electricity all day long, producing more electricity than the small amount needed to top off the desuperheater provided water heat. In our case this combination made the most economic sense.

lml999's picture

Help us consume more PV-generated electricity!

We have a traditional colonial house, with baseboard hot water heated by a natural gas boiler. Five zones plus an 80 gallon indirect hot water tank. The indirect hot water tank has an option for solar hot water heating (second set of coils). The house is relatively tight and well insulated. We spend about $1200/year on NG for heat, hot water and cooking, with NG bills running about $30 in the summer.

We are in New England and are currently running several window air conditioners in the summer. At some point we may install ductless minisplit for cooling (and shoulder season heating via heat pump option.)

Last year we installed a 12.75 KWh solar array and now have significant excess power...on an annual basis, it looks like we will generate thirty to forty percent more than we consume.

So...I'm thinking about how I can make hot water for heating with some of the excess electricity. I'm wondering if either a tankless water heater or electric hot water tank preheating the water for the radiator systems (on return, before the water goes through the boiler) makes sense.

I'm concerned about a couple of issues:

- Will a domestic electric hot water heater (or tankless) keep up with the demand for multiple zones?
- If the boiler does not run, will heating the house take longer as I will lose some heat due to transfer in the boiler?

If an electric hot water heater does not keep up with demand, the boiler will heat the water to the necessary temperature...but then is the investment and effort worth it?

The house was built in 1994 and the boiler is original to the house. The indirect hot water tank is new. At some point I expect we will have to replace the boiler, although it seems to be running strongly.

We expect to be in the house for a long time and value longevity and reliability over short term cost issues. Should I just put my effort into the minisplit cooling/heat pump approach? We're likely going in that direction anyway. I haven't lived with a multi zone mini split system before and wonder how heating individual rooms will work with the existing baseboard heating system.

Comments and thoughts appreciated!

Ian Woofenden's picture
A mini-split system will in one investment give you cooling and heating for your home. This is the direction I'd go in first, since space heating is your largest load, and you mentioned that you want air conditioning as well. It's a bit tricky because you or someone before you has already invested in the boiler/baseboard system. But in your shoes, I would be moving toward abandoning that and leaning on the more efficient and more-easily-solar-powered mini-split system, and reducing or eliminating your natural gas usage. Depending on the size and layout of your home, you maybe be able to do this incrementally, since one outdoor unit can power multiple indoor units. I've just installed a mini-split to use some of my solar and wind surplus, and I can already see that the wood pile is not going to shrink anywhere near as quickly, and the convenience and comfort levels are high. I can't use it all the time because I'm off-grid, but I recommend mini-splits to most of my on-grid clients. Ian Woofenden, Home Power senior editor
RMichael Curran's picture

You might want to consider a hot water diversion heater. These install in your existing water heating tank and can run from your PV battery storage system, if you have one. Lacking a battery system, you would have to look into a similar device that runs at house voltage, although I haven't seen these. The ones that run at battery voltage are relatively inexpensive, and are actuated by a relay in the battery charge controller that senses when your battery is full.

Maybe you could disconnect a couple of your PV panels and dedicate them to water heating. Butler Sun Solutions makes a product that does this.

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