Pairing Water Heaters with a PV System


Inside this Article

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.

Modern water-heating technologies and efficiency strategies can offer energy savings in any household. With water heating consuming up to 20% of a household’s total energy usage, it’s a significant load. Here are more sustainable choices that can be offset by a PV or other renewably fueled system.

Electric Resistance, Plus

WHAT: Superinsulated electric tank-style water heater.

HOW: Electric resistance elements inside the tank are energized and transfer heat to the stored water. “Extra” insulation in the walls of the tank reduces the water’s heat loss through the tank walls. Although the conversion of electricity to heat is 100% efficient, heating a large volume of water is energy-intensive.

ENERGY: Electricity.

SUITABILITY: Grid-tied RE systems, may also be PV-direct (see “Web Extras”).


  • DIY installation.
  • Easy to retrofit/replace, though superinsulated tanks have a larger footprint than typical or older models.
  • Stainless-steel and polybutene tanks last longer than glass-lined steel.
  • Relatively inexpensive.


  • Recovery time—the tank may run out of hot water during periods of high use, depending on capacity.
  • More expensive than “typical” tank-style heaters.

BOOST CONSERVATION BY: Adding a timer to turn off electricity to tank when not needed. For example, if the house is unoccupied during the day, set the timer to turn on just before inhabitation.

WHAT TO LOOK FOR: Look for a high “energy factor (EF).” Rheem’s Marathon tank and A.O. Smith’s ProLine, for example, have an EF of 0.95.


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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