Energy Factor and water heating technologies

A different sort of green home

According to the US Dept of Energy's website, energy factor (EF) indicates a water heater's overall energy efficiency based on the amount of hot water produced per unit of fuel consumed over a typical day. This rating applies to a variety of water heater equipment: tank-style (storage), tankless/on-demand, and heat pump water heaters (HPWH).


Solar water heating has a very similar rating called the Solar Energy Factor (SEF).  Solar equipment is evaluated for thermal performance efficiency by an organization called the Solar Rating & Certification Corporation (SRCC).  The federal tax incentives for the installation of solar equipment are contingent on the solar water heating equipment having been evaluated by the SRCC.  The SRCC developed the SEF for the purpose of comparing solar water heating systems to a standard 50 gallon residential water heater in an apples-to-apples sort of way. Many natural gas utilities use the industry standard EF rating system to award rebates for high performing water heating equipment.  Since SEF is the solar equivalent of EF for a standard water heater, my view is that solar water heating systems that meet EF ratings for high performance equipment should qualify at a minimum for the existing utility rebate programs.


So how do different water heating technologies measure up with respect to energy efficiency?


Tank storage water heaters

Bradford White 40 gallon gas water heater

Typical residential storage water heaters range in size between 40 and 80 gallons.  This volume of water is heated up to the desired temperature (120 degrees F); the heating element or burner cycles on and off as a thermostat in the top portion of the tank detects adequate heat or not.  This is usually the least expensive water heating technology to install but is also normally the most expensive to operate. Recovery of the heat in the tank during and after use can be slower than with other water heaters which leads to the cold showers in high use situations.


EF rating for gas heated tank water heaters:

• Low efficiency -- < 0.62
• Medium efficiency -- 0.62 to 0.67
• High efficiency -- 0.67 to 0.82

EF rating for electric heated tank water heaters:

• Low efficiency -- <0.90
• High efficiency -- up to 0.95

Keep in mind that EF rating is only with respect to the energy efficiency of the appliance.  The electricity that heats the water in an electric tank model could be coming from sources with varying degrees of efficiency and environmental impact.

Tankless

Tankless water heater (top right) in conjunction with a solar water heater

Also called on-demand water heaters, these units heat water as it flows through the device.  They are capable of reducing water heating costs 10-20% due to the elimination of standby losses (constantly heating a tank of water even when no hot water is needed).  Tankless units come in electric, natural gas, and propane versions for different markets.  Not only are these a bit more energy efficient than tank style water heaters, they are also compact, wall mounted devices to save space in the mechanical room and they can provide almost unlimited amounts of hot water.


The EF rating for gas-fired tankless units ranges between 0.82 and 0.96. 


Heat Pump Water Heaters

From DOE

In a heat pump water heater, electricity is used to move heat from the air around the HPWH into the water tank versus directly heating the water with resistance coils.  Since the heat pump cycle can take a significant amount of time to heat or reheat a tank of water, electric resistance coils are included in HPWHs to augment the recovery of the water heating and thus minimize cold shower scenarios.  HPWHs can be very energy efficient and cost effective when the HPWH is operating in the heat pump mode; they revert to EF ratings of electric water heaters when the resistance coils kick in however.  Like with many things in the sustainable building world, efficient design is only part of the total picture.  How something is used in the real world effects its overall efficiency. 


EF ratings of HPWH in hybrid mode is 2.20 and fall off towards 0.93 in electric mode (average ~1.60).


Solar Water Heaters

In solar water heaters (SWH), the heat from the sun's radiation is transferred to drinking water during the day and stored in a tank of water for use throughout the day and night.  In homes where a 40 gallon tank water heater is appropriate, you'll find a 60 gallon solar storage tank. Because the amount of energy to move the heat from the solar collectors to the drinking water is minimal and the sun's energy is free, the operating costs of SWHs are the lowest of any water heating technology today. They are also the lowest carbon option when the system offsets fuel from gas appliances or electricity from coal-fired plants. SWHs have the highest upfront cost in the water heating space, but federal and state incentives exist to encourage their adoption since they provide a benefit to both the system owner and society as a whole.


Calculating EF from the SEF rating developed at the SRCC is done with the following equation: SEF x (1 - SF) = EF. SEF and SF (Solar Fraction) are calculations from the OG-300 report on each system found at www.solar-rating.org. EF ratings on SWHs range from 0.93 up to over 5.70 depending on the size of the system, location, and back up fuel option.


Summary
The less you pay upfront for a water heater, the more you pay over time.  As fuel prices of all sorts rise in the future, an investment in efficiency today will pay increasing dividends over time.

Why propane users need to investigate solar

Propane is a dominant heating fuel in rural parts of the US. Propane users typically purchase a large volume of fuel all at one time. It would not be unusual for them to have to write a check for $1250 1-2 times per year.  What this means is that like with a solar water heating system, they are pre-paying for the fuel they use to heat their home and domestic water.  Unlike with natgas and electricity customers, it's not great a leap for propane consumers to understand how a solar water heating system would benefit them in the long run since they can't spread payments out over time.  Additionally, propane customers tend to come from agricultural communities were communities tend to have a longer view of financial prudence.

To give a little flavor of the economics, I modeled a 2 panel, 70 gallon tank system pitched at 30 degrees facing due south in Des Moines, IA.  Using a solar water heater with propane back up tank, the homeowner would save about $450/year in water heating costs (using $2.50 per gallon for propane).  Switching to an electric back up tank with the same solar system, would save this homeowner about $500/year.  Either system would reduce the home's greenhouse gas emissions by about 1.3 tons of CO2.  Even with only the federal tax credit, this system would have a 10-year payback or 6.5% rate of return.  If propane prices go up to $3.50, the payback is cut to just over 7 years.  If your state has any additional incentives, you'll see paybacks as low as 5 years. 

A couple of good resources for propane related information would be Build With Propane and this propane supply outlook paper.

Save now and save later with solar. With a solar system, you fix your energy costs today and hedge against rising prices tomorrow.