Did you know you can get 100% of your household hot water from sunshine during summer?

If yours is one of the 750,000+ homes in Maine, New Hampshire, and Massachusetts that heats domestic hot water (DHW) with an oil-fired boiler, it’s hard to ignore the roooooooar in your basement as the boiler fires up to make hot water while sunshine pours down onto your roof.

This is a terribly inefficient (and therefore environmentally harmful) way to make hot water but the good news is that there are four cost-effective ways to correct the problem.

You can save more than 300 gallons of oil per year for an average family of four by switching to solar hot water.

Decision Tree for Modern Solar Water Heating Solutions

Part of our free home solar consultation is actually more than just reviewing your home for solar – our solar design specialists will also be looking at opportunities for you to save energy in water heating and space heating. Every family’s needs are a bit different, and we don’t offer any ‘one size fits all’ solutions.

We recommend different water heating systems based on factors such as:

  1. Number of people in the home & water usage patterns
  2. Age and efficiency of current water heating solution
  3. Amount of solar-viable roofspace
  4. Project budget

Solar Hot Water – The Most Efficient Way to Heat Water


Solar thermal customer Andy Chase discusses his solar evacuated tube array, which has reduced his home’s oil use by roughly 300 gallons per year!

Solar water heating actually dates back to the ancient Egyptians (who used it as a form of hydronic heating in pyramids, as did the Romans), but we’ll start in the early 2000s when solar thermal was 50% of the work we did.

By adopting European best practices, we developed a solar hot water (SHW) design that was built to survive 20+ of the harshest winters Northern New England could throw at it (and, based on customer feedback, has met and exceeded those expectations).

At that time the cost to generate solar electricity was upwards of $8/watt and oil prices were better than $3.5/gallon, making the economics of SHW extremely attractive. Flash forward 8 years, and the economics of water heating are much different. Solar electric panels have dropped in price by over 74% since 2004 and electric water heating solutions have improved quite a bit.

Does that mean that solar thermal is dead? Not at all.

solar hot water flat plate array in maine

Two or three 4×7 solar hot water collectors will meet 75%+ of a home’s annual domestic hot water needs.

Solar hot water remains the water heating solution with the lowest operating costs. Basically, the more hot water you use, the higher the ROI. So, for a retired couple who are water misers, it may not be the most cost-effective choice. However, for an active family of 4-5, or for a multi-unit building, or commercial setting, it may significantly outperform any other technology over the long-term.

Solar hot water is also significantly more efficient on a per square foot basis, meaning, less roof space is required to meet your household or business’s energy needs. For example, 84 square feet of solar thermal collectors will handily meet the needs of a 4 person home. Covering the same amount of load with solar electric panels powering an electric water heater will require around 210 square feet!

Solar Hot Water


  • Minimal ongoing energy usage
  • Meets 100% of water heating needs May-October and more than 50% the remainder of the year
  • Least roof space required to meet DHW load
  • Long-lived equipment carries 10-year warranties and system should last 20+ years
  • Best solution for relatively high consumers of hot water


  • Highest upfront cost ($10,500 – $12,000 prior to 30% credit, in NH there is a ~$1,500 state rebate)
  • Maintenance required (roughly every 5 years)

Heat Pump Water Heaters – A More Efficient Form of Electric Water Heating

heat pump water heater powered by solar

The Stiebel Eltron Accelera Heat Pump Water Heater

Using a refrigerant process similar to an air source heat pump, heat pump water heaters leverage the ambient air of your home for water heating.

Some facts about Heat Pump Water Heaters

  • Instead of MAKING heat, they simply move it from one place to another, making them much more efficient than conventional electric water heaters, or other ways to make hot water.
  • They produce hot water 200-600% more efficiently than a resistive electric tank.
  • While they move heat, they also help dehumidify the room they are in.
  • Highly insulated, meaning the energy you do use for hot water is not wasted in ‘standby losses’ as the tank sits idle.
  • Available in large tank sizes, which means that you can easily meet the needs of a 4+ person household with a heat pump water heater.
  • Many states have cash rebates for installing them.
  • One of the only water heating systems that can be solar powered!

We currently use Stiebel Eltron Accelera heat pump water heaters for a number of reasons. We are very familiar with the brand, having used Stiebel Eltron’s equipment in SHW installations for years, and appreciate their excellent quality and long warranty (10 years).

Due to its much lower electric consumption, a heat pump water heater needs only about 1/3 of the PV modules to generate the electricity for domestic hot water as compared to a resistive electric tank.

Heat pump water heater design schematic courtesy Stiebel Eltron

Heat pump water heater design schematic courtesy Stiebel Eltron

Heat Pump Water Heater w/ Solar PV


  • Very simple and straightforward installation, can usually be done same day as Solar PV install
  • Excellent balance of efficiency and cost
  • Dehumidifies basements
  • Long-lived tank with 10-year warranty
  • Superior to ‘hybrid’ water heaters sold in big box stores, avoids using backup electric element whenever possible


  • Relatively noisy compared to other options
  • Consumes some amount of conditioned air to operate
  • Has longer tank recovery times compared to other options

Resistive Electric Water Tank – Durable and Simple

electric water heater powered by solar

The stone-lined Vaughn electric water heater

For modest consumers of hot water (up to about 55 gallons/day), a high-efficiency resistive electric hot water tank is an excellent choice. We currently install Vaughn tanks, a USA-made stone-lined water tank with 3″ of insulation and a 10-year warranty.

Resistive electric tanks are the simplest form of electric heating, consisting of an electrical element which converts kilowatts (kW’s) into heat (BTUs). They have the advantage of being a simple installation and lower upfront cost than other solutions we offer.

However, the limits of this technology mean they are also less efficient, and hence more expensive to operate. For smaller households, the savings in upfront cost is worth the slight operating premium.

Powering a resistive electric tank with solar will require roughly 4 PV panels (~1kW) per person to meet the daily water heating load.

Electric Water Tank w/ Solar PV


  • Simple and straightforward installation, can usually be done same day as Solar PV install
  • Lowest upfront cost ($1,750-$2,500)
  • Stone lined tank carries 10-year warranty and requires no maintenance
  • Build quality significantly better than typical electric water heaters


  • Requires the most roofspace dedicated to water heating, when using Solar PV to offset electric consumption
  • More expensive to operate for homes with higher hot water usage
  • Stone-lined tank is fairly heavy, so good access to mechanical space is necessary

On-Demand Electric Water Tank – Compact and Efficient

Stiebel Eltron Tempra series electric on demand water heater

Stiebel Eltron Tempra series electric on demand water heater

A relatively new addition to our lineup is an on-demand electric water heating option, the Stiebel Eltron Tempra Plus.

On demand units are also called “tankless” water heaters since they heat water as it is used, rather than storing hot water in a tank. Like propane or natural gas fired equivalents, electric on-demand water heaters fire up when demand is created from a shower, sink, or other fixture. Unlike gas, electric on-demand units can be powered by solar energy.

Its very ‘tankless’ nature is both a strength and weakness for these units.

Strengths: these units are very compact, and require no venting, making them the smallest physical footprint water heater out there. Also, because they do not have a tank, they are inherently more efficient, since there is no large batch of hot water sitting in your home slowly cooling off (what we call ‘standby losses’).

Weaknesses: the units have a max of 2.3 gallons of hot water per minute, which is adequate for a single shower, but would struggle to keep up with a heavy water usage household. The Tempra Plus is unique, however, in that it will reduce flow to fixtures when needed to ensure there is always hot water while competing units will simply let cold water reach the tap.

Also, they are electric hungry. Because so much electricity needs to be available instantaneously to heat the water, a home needs to be prepared for a full 100amp draw in order to accommodate the unit. So, additional electrical work beyond just the water heater may be required.

However, the simplicity, sleekness, and efficiency of a solar-powered electric on demand heater make it an appealing contender for many homes.

On-Demand Electric Water Heater w/ Solar PV


  • Low upfront cost (~$2,000)
  • No standby losses mean more efficient operation. Great for seasonal use / second homes.
  • The compact tankless design will work in homes with no mechanical room or basement.


  • Warranty (7-year leak-free, 3-year defect-free) less generous that other water heaters
  • Will struggle to keep up with heavy hot water demand
  • Like all tankless units, may have ‘cold sandwiches’ where a small jet of cold water is introduced between cycles of hot water demand

Water Heating That’s Right for You

Whether it’s an electric option, or ‘true’ solar hot water, getting your oil boiler turned OFF this summer is a great goal. And if you’re burning propane or keeping an old electric water tank alive, there is still plenty of savings to be had by finding a better way.

Typical Yearly Water Heating Costs for a Family of 4*

Solar hot water (w/ electric backup) $190
Heat pump water heater powered by grid $264
Heat pump water heater powered by solar $165 **
Electric water heater powered by grid $792
Electric water heater powered by solar $495
On demand electric powered by grid $654
On demand electric powered by solar $408
Oil boiler (oil at $1.99/gal) $647 ***
Oil boiler (oil at $2.99/gal) $971
Propane water tank $1,056
On demand (tankless) propane $432


  • * Load calculations use 65 gallons a day of hot water for 4 person household or 16.25 gallons per person. To calculate consumption for a different number of people, or different consumption, just do the math!
  • ** Grid electricity priced at 16 cents per kilowatt-hour, solar at 10 cents per kilowatt-hour (roughly the 20-yr cost of solar PV for an average 5kw system), propane at $2.4/gallon.
  • *** Oil and propane tanks calculated at ‘average’ efficiencies. There are a few different ways of using oil and propane to heat domestic hot water (direct fired, indirect tank, tankless), this chart uses an average.
amounts of solar required to produce water heating for a family

Sq ft. solar collector needed to supply hot water for a 4-person household. From left: Solar Hot Water collectors, Solar PV powering a heat pump water heater, Solar PV powering an electric tank (or on-demand electric unit).