Gas versus Electric
Most houses are “dual fuel”, meaning in our case that some systems run on natural gas, and others run on electricity. Typically, gas is used for cooktops, heating water, space heating, and drying clothes. Electricity is used for space cooling, ovens, washing machines, dishwashers, and everything else in the house.
In designing our net-zero energy house, our goal is to produce enough energy on-site to cover both electric and gas usage. This means that having a zero electric bill isn’t quite enough. The house must generate enough surplus electricity to compensate for the gas we use in the house. That way our our true energy balance is zero.
Some net-zero houses, such as the inspiring Green Idea House in nearby Hermosa Beach, CA, make the net-zero calculations simple by using all electricity and no gas. This way, the electric bill is the scorecard by which net-zero energy is measured.
However, gas is more efficient in some cases, and demands consideration when minimizing the overall energy load (and total energy bills) is the goal. Why is gas sometimes more efficient?
- Electricity in Southern California, at least the last unit of electricity (on the margin) is made by burning natural gas at the power plant. When you consider the efficiency of the power plant itself and the transmission losses to the plant and to your house, up to 65% of the energy has been lost.
- In heating applications, gas seems better suited to the task… gas burns fairly efficiently and generates a great deal of heat per dollar spent. Electric heating is accomplished using resistors and compressors, and does not seem as efficient. So drying clothes, space heating, water heating, and cooking all seem suited to gas.
- Gas is relatively cheap in the U.S.
A simple look at our cooktop decision illustrates the difference. We looked at two 30” cooktops made by the same manufacturer, in the same product line. The gas version had a 15,000 btu/hr burner as its most powerful burner. The induction version had a 3,700 W burner. Let’s compare energy usage and cost of these two burners.
Gas: 15,000 btu/hr = 0.15 therms/hr @ $1.15/therm = $0.17 per hour
Electric induction: 3,700 W = 3.7 kWh/hr @ $0.31/kWh = $1.15 per hour
The electric cooktop costs an extra buck every time you use it! That could add up to $200 in a year, some big money. These are beautiful appliances, with clean lines, and we are strongly tempted with PV panels on the roof to look at electric cooking. Furthermore, we like the idea in our small kitchen to have electric cooking surfaces that can serve as extra counter space when not in use, thanks to their durable flat surface. Gas can’t do that. Our analysis showed that the numbers didn’t pencil out. Add the 65% transmission loss in electric power, and the electric cooktop looks even worse.
Let’s look at this another way, in terms of energy content instead of dollars.
Gas: 15,000 btu/hr = 4.4 kWh/hr1
Electric induction: 3,700 W/hr = 3.7 kWh/hr
Now the thinking gets muddy! The electric cooktop actually uses 16% less energy in an hour than the gas version. If we have free electricity coming from our roof, we have to consider the electric option again, since the cost is no longer what the electric and the gas utilities charge us, but $0.00 from our roof PV and $0.17 per hour from the gas utility.
Is that really the full story though? We can sell excess electricity back to Edison at the net metering rate, which let’s say is $0.15/kWh. We could sell the 3.7 kWh used to power the induction cooktop for an hour back to Edison for about $0.50. We could use that $0.50 to power a gas cooktop for $0.17 and net $0.33 in the deal. So it’s not as simple as thinking of electric power as free, is it?
The stakes aren’t that high with a cooktop, as the hours of use per year are low. It probably comes down to your personal preferences, and lack of counter space could push us toward the induction model. But when you’re looking at an energy-intensive application like water heating, and you are building to a net-zero energy standard, you can’t ignore cost and energy efficiency differences from choosing gas or electric systems.
1The conversion is 3,413 btu/kWh