Comparing costs to heat hot water

Adding up electricity, gas and propane, most (probably half to two-thirds) of the energy you use in your house is for heating, ventilation and air conditioning. One of the most significant other uses is water heating.

In most homes, the same hot-water source is used for bathing, dishwashing and laundry. To compare water heaters, I looked up the specifications for similar, popular water heaters at one of the big-box stores. The propane water heater requires about 0.267 gallons of propane per hour. The natural gas water heater requires about 0.209 therms of gas per hour. The electric water heater requires about 5.06 kilowatt-hours (kWh) per hour.

To make comparing these numbers easier, I converted them into equivalent electric energy. The gas water heater would require an amount of gas containing energy equivalent to 6.12 kWh per hour. The propane water heater would need an amount of propane containing energy equivalent to 7.16 kWh per hour.

The amount of time per day that a water heater is on depends on how many people are in a household and how the household uses water. For discussion, an average water heater in the U.S. is probably on for about 2.5 hours a day, or a little over 900 hours/year. Thus, an electric water heater would need about 4,600 kWh/year, or about $600/year of electricity.

A conventional tank-style water heater stores some heat that ultimately dissipates through the tank’s insulation without being used. A tankless water heater reduces the total energy required for water heating, because energy is only put into the system while hot water is being used. Energy use for water heating can be reduced by about a third by using a tankless heater. A tankless heater would use even less energy, probably about 3,200 kWh/year.

It will probably surprise readers to note that this is almost as much energy as is required for an electric car traveling 12,000 miles/year (about 3,400 kWh/year). It’s different for petroleum fuels. A natural gas water heater would require about 190 therms of gas per year, equivalent to about 165 gallons of gasoline. An average gasoline car needs about 472 gallons of gas a year. So, if you’re using petroleum fuels for both, you need more gasoline for your car than you do natural gas for your water heater.

But, as shown above, if you use electric energy instead, you might need more energy for your water heating. The reason for this isn’t that electric water heating is inefficient. It’s that the gasoline car is inefficient. A gasoline car doesn’t actually use most of the energy in its fuel to move the car. It loses two-thirds of the fuel’s energy through waste heat that exits the radiator and exhaust, and another 5% or so in mechanical drivetrain inefficiencies.

In La Conner, solar photovoltaic panels will produce, on average, about 1,100 kWh/year per installed kilowatt. To produce enough energy to power a conventional water heater, a homeowner would need to install about 4.2 kilowatts of solar panels. At a cost of $1.65/kilowatt (projected low end of Solarize Skagit 2024 cost, minus 30% federal tax credit), this would cost about $6,900. A new solar installation, just used for water heating, would pay for itself in a little over 11 years.