What Is a Zero-Energy House?
Homes that produce as much energy as they use + sustainable living = a green and happy earth.
A zero-energy home (ZEH) produces as much energy as it consumes over a year’s time. The only way to achieve this is through energy-efficient design and practices coupled with energy-producing technologies.
Reducing Energy Demands
In addition to a lifestyle committed to energy conservation, building a zero-energy home requires energy-conscious design and technology. “There can be no compromise on anything,” says Danny Parker, Principal Research Scientist for the Florida Solar Energy Center. How the house is oriented and designed are as important as the photovoltaic panels used to generate energy.
Reducing the energy demands of a home is the most important first step in creating a zero-energy home. “The key question is: ‘How do I reduce the loads in the building to be as minimal as possible?’ ” Parker says. The answer will vary regionally, as the nature of those energy demands depends largely on climate. A home in Minnesota will face significant heating demands, while one in Florida will consume most of its energy cooling the house. “You need to pull out all the regional tricks,” Parker says.
To that end, designing and implementing an energy-efficient HVAC system can reduce energy loads by as much as 80 percent. For a home in Florida, using light-colored roofing tiles and spectrally selective windows will reduce the heat load of the building, and that, in turn, reduces the demands of the cooling system. Internally, Energy Star-rated refrigerators, washers, dryers, and other appliances will also lower the energy required in the home.
Producing Energy for the Home
When a home is designed, built, or retrofitted to be as energy-efficient as possible, the second step is to employ energy-producing technologies, starting with the sun. Solar thermal technology uses the sun to heat material that is then stored thermally for later use. This technology includes hot water systems. Solar thermal systems are cost-effective solar solutions and are priced far less than their photovoltaic (PV) counterparts. “Solar thermal doesn’t get into the 4 or 5 digits in cost, like PV can,” says Tim Merrigan, senior program manager at the National Renewable Energy Laboratory.
Once a home is energy-efficient and set up with solar-thermal water and heat collection, PV technology adds the electricity-producing component that helps bring the energy tally to zero. PV technology harnesses the power of the sun to generate electricity that is collected as direct current (DC) and either sent to the power grid for credit or transmitted to an inverter within the home for use as household AC electricity.
Either way, surplus energy is typically sent back to the utility to power other homes and credited against the energy producer’s usage. Federal and state incentives can drop the price of these systems, and resources are available to builders, contractors, and homeowners to determine how big a system will be required to generate the necessary amount of energy for the home.
Monitoring Energy Usage
Maintaining a zero-energy household requires a commitment to a zero-energy lifestyle. Parker compares the ZEH to a hybrid car: A hybrid car is designed to get remarkably high mileage, but driving it irresponsibly will result in average or poor mileage. ZEH owners must monitor energy usage and make energy-wise decisions at all times. Energy monitoring devices allow homeowners to monitor real-time energy usage in the home and root out inefficient energy drainers. “You become real concerned with the Watt,” says Parker, and “how to eliminate every single one.” Monitoring devices let homeowners see actual energy usage and can inspire them to make efficiency changes like installing compact fluorescent lightbulbs or purchasing equipment with wattage in mind.
In order to keep energy usage as close to zero as possible, homeowners should calculate in kilowatt-hours (kWh) the average daily production of their system. Parker advises people to use the Florida Solar Energy Center’s method for calculating daily energy production. “You take the kW rating of the system and multiply it by 4,” says Parker. “That will give you the kWh per day that the system can be expected to produce.”
Parker suggests homeowners use 3.5 or 3 for the cloudier Pacific Northwest states and 4.5 or 5 for the sunnier Southwest states. Then match daily energy expenditure to daily energy production to keep the home at zero energy.
Forecast for Zero-Energy Homes
“The truth is, there are far fewer true zero-energy homes out there than people think,” says Parker. It is more accurate to say “near-zero-energy,” in most cases. “Homes in California are getting to 50, 60, and even 70 percent to zero-energy,” Merrigan says, adding that the Department of Energy’s goal for a “marketable” zero-energy home by 2020 could be attained even earlier as continued research brings costs down and federal and state incentives increase.
Even with cheap technology and incentive-reduced cost, however, homes built in some parts of the country will only benefit so much from the addition of solar technology, due to low kWh per day measurements. Additionally, the trend in certain areas of the home industry—home entertainment and home office, in particular—points toward the popularity of technology that increases energy usage, rather decreasing it. “Everyone wants the 50-inch flat screen plasma TV,” says Parker, “but no one thinks about the fact that it draws 400-500 Watts.”
Investors in a zero-energy home should also be wary of the payback trap. “People are always asking about payback,” says Merrigan. “They should be thinking that this is an upgrade — an investment — just like upgrading any part of the house.” Like finishing a basement, adding a bathroom, or installing a pool, investing extra in a home that approaches or achieves zero-energy is an investment that will increase over time. “Whatever value you put into the home, you will get back,” Merrigan adds.