Zero-carbon footprint and net-zero energy
The terms zero-carbon footprint and net-zero energy mean essentially the same thing. Since even a single light bulb uses electricity (which is a form of energy) a house can never use zero energy. However, since you can generate electricity from solar panels it is possible to have a house that generates all of the energy it uses over the course of a year. If you generate enough energy to power your home every single day of the year then you have gone “off grid”. If you generate as much energy as your house uses over the course of a full year, but not every day, then you have a net-zero energy house which, unless you are able to read in the dark, will require a connection to the grid via a net meter so you have power, and light to read by, at night. If the energy used by the house is all generated by zero-carbon sources (such as solar panels or a wind turbine) then the terms net-zero energy and zero-carbon footprint mean the same thing. In theory, if you used a diesel-powered generator to make all the electricity your house used (and that was all the energy your house used) then you would have a net-zero energy house, but this is not usually how the term is used. In common usage a net-zero energy house means a house which generates all its energy on site and that energy is made from zero-carbon sources like solar panels or wind turbines. Hence it is equivalent to a zero carbon house.
PassivHaus (Passivehouse), PHIUS
PassivHaus is a standard for low-energy or zero-energy houses. It was developed in Germany by the PassivHaus Institute. There is a U.S. branch of the PassivHaus Institute called the Passive House Institute US or PHIUS. PassivHaus certification requires compliance to their exacting standards for both building materials and methods of construction as well as the final result in terms of energy efficiency. Every element of the construction must be audited back to its source. The PassivHaus standard for total energy use is 60 kilowatt-hours per square meter of floor space per year. This is equal to 5.6 kWh per square foot per year. If your house has 2,500 square feet of living space then your total energy budget to meet the PassivHaus standard (including heating fuel like natural gas or heating oil, plus your electricity use) is about 14,000kWh per year. The average U.S. house uses about 11,000kWh per year in electricity alone. When you add in the extra energy used to heat the home, often five times the amount of energy used in electricity, the average U.S. house uses about 500% more energy than the PassivHaus standard.
After we installed the fab four (insulation, heat pumps, solar panels and triple-glazed windows) our house is using about 6.5 kWh per square foot per year or only about 16% more than the PassivHaus standard. This is a remarkable result given that the Passivhaus standard was created for new houses and our house is a 1970’s design with no sealing of the walls against air infiltration and with only average insulation in the walls. Zero Carbon Home and PassivHaus have similar approaches to reducing energy consumption such as better insulation, low-E triple-glazed windows, heat pumps and solar panels. But because of the prescriptive details and audit trail on building components required by PassivHaus it is almost impossible to apply it to the renovation of an existing house and it is only applicable to new construction. It is both expensive and time consuming to do the audit.
LEED (Leadership in Energy and Environmental Design) Certification
LEED Certification is a program of the U.S Green Building Council and is somewhat similar to the PassivHaus standard except it provides more flexibility on reaching overall energy efficiency performance rather than requiring the strict adherence to the prescriptive building elements of the PassivHaus standard. However, LEED does not currently offer a standard for renovating an existing house, it only applies to new construction.
Zero-Energy Ready Home (ZERH) and Home Energy Rating System (HERS)
The Department of Energy offers its Zero Energy Ready Home (ZERH) program but it is more aimed at certifying builders rather than buildings. Hence, just like the PassiveHaus and LEED programs it is focused on new construction, not how to go zero on your existing home. The ZERH program relies heavily on EnergyStar standards for appliances and windows and the HERS (Home Energy Rating System) for performance. HERS is focused on energy use relative to a benchmark house (i.e., how your home compares to a model house of the same floor area) rather than minimizing energy or spending. A HERS rating is only available on new houses, not for existing ones. A review of the HERS rating system in Home Energy magazine found that, in practice, “there was no clear relationship between the rating score of an individual home and actual energy cost.” Hmmm.
These existing energy efficiency rating programs have significant drawbacks. None of PassivHaus, LEED or ZERH/HERS quantify either how much you can cut your carbon footprint or how much money you can save, they don’t work on existing houses and are also burdensome (read: costly) to implement usually requiring you to hire experts to do an audit.
As if that wasn’t bad enough, none of these standards adjusts the energy needs for the climate in which your house is located. An average house in Massachusetts consumes far more energy than an identical house in Arizona simply because it is much colder in winter in Massachusetts than it is in Arizona. A bigger house will consume more energy than a smaller house. It seems to me that there is a need for a new standard. For lack of a better term, lets call it the Green Standard. I propose kilowatt-hours (of total energy used) per square foot of living space per heating-degree-day per year as the measure of energy efficiency (“Green Efficiency”) and kilowatt-hours net of on-site zero-carbon energy production per square foot of living space per heating-degree-day per year as the measure of “Greenness”. I welcome any critique of these numbers and any suggestions for improvement, you can reach me at firstname.lastname@example.org You can get the heating degree days for your area just by typing your zip code into the very helpful site at www.degreedays.net By default it calculates in degrees Fahrenheit and for a 65 degree Fahrenheit standard. This is a common standard and the one I used in all my calculations. You must use this standard if your numbers are to be comparable to mine.
For our home the numbers are:
. 2016 2017 2018
Green Efficiency: 3.9 1.5 1.1
Greenness score: 3.9 1.1 0.0
These are expressed in watt-hours/heating-degree-day/square foot of living space/year rather than in kilowatt-hours/heating-degree-day/square foot of living space/year as the numbers in kilowatt hours are just too small. To convert the numbers in kilowatt-hours to watt-hours you just multiply by 1000.
The 0.0 score for Greenness in 2018 is because we have reached a zero carbon footprint as all the energy used by our home (for heating and electricity combined) is offset by our solar power production for a full year. Thus the Greenness score is quite intuitive as a zero means you have “gone zero”!
These scores not only have intuitive meaning but are easy to calculate from just your heating fuel and electricity bills. You do not need to hire an expert! They also work for all buildings, new or old, and wherever your house if located in the U.S.
To calculate your Green Efficiency from your heating and electricity bills just find the most recent ones and follow the guidance in this paragraph. The bills usually have the full last year of energy use. For electricity it is already measured for you in kilowatt-hours (kWh). Heating oil is usually reported in gallons delivered rather than gallons used but if you take what is delivered over a full year it will be pretty close to actual use. If the full year’s delivery is not written on the bill just call your delivery company and ask for the last full year’s deliveries. Heating oil contains about 40kWh per gallon so just multiply the number of gallons by 40 to get the energy you used from heating oil. Natural gas has about 293 kWh per million BTU’s (British Thermal Units – an archaic measure of energy still widely used for heating but for almost nothing else) so just multiply the number of million BTUs (sometimes written MMBTU) used by 293 to get the kilowatt-hours used. If your bill measures the amount of gas in Therms (equally archaic and limited almost exclusively to natural gas), don’t worry. One Therm is 100,000 BTUs. So multiply the number of Therms by 29.3 to get the number of kilowatt-hours. Then just add up the total kilowatt-hours used for heating and electricity in your house. For example:
If you had 1000 gallons of heating oil delivered in the last year (which is fairly typical in Massachusetts) you had 1000 gallons x 40 kWh/gallon = 40,000 kWh of energy delivered from heating oil. If you used 1,000 Therms (100 million BTUs) of natural gas (also fairly typical in Massachusetts) then the energy you used in kilowatt-hours was 1,000 Therms x 29.3 kWh/Therm = 29,300 kWh. If, in addition, you used 7,000 kilowatt-hours of electricity (about average in Massachusetts) then your total energy used is 7,000 + 40,000 = 47,000 kWh in the heating oil example and 7,000 + 29,300 = 36,300 kWh in the natural gas example. If your house has 2500 square feet of living space (about average in Massachusetts) and you live in the 02030 zip code (where we live) where there were about 5600 heating degree days in the last year then your Green Efficiency is:
For the heating oil example:
47,000 kWh / 5600 HDD / 2500 square feet (multiplied by 1000 to get it into Wh rather than kWh), gives a Green Efficiency of 3.4.
For the natural gas example:
36,300 kWh / 5600 HDD / 2500 square feet (multiplied by 1000 to get it into Wh rather than kWh), gives a Green Efficiency of 2.6.
Our Green Efficiency was 3.9 before the fab four. So before the fab four we were less energy efficient than the average home in Massachusetts heated by heating oil and a lot worse than the average home heated with natural gas. We heated with heating oil before we installed heat pumps. After the fab four, our Green Efficiency was 1.1 or much better than the average house in Massachusetts whether it was heated by heating oil or natural gas.
To calculate the Greenness Score simply subtract the kilowatt-hours of solar electricity you generated for the year from the total kilowatt-hours you used. The total kilowatt-hours you generated can be read off the production meter installed with the panels, from your electricity bill (look for the credit they gave you for solar power production) or from the app on your smartphone that usually comes with the panels. For example, if you had a 10 kilowatt array that generated 10,000 kilowatt-hours in a year (fairly typical for Massachusetts) then, continuing the examples from above, your net energy use for the year was:
For the heating oil example: 47,000 kWh used less 10,000 kWh generated by zero-carbon solar panels = 37,000 kWh used net of solar production. 37,000 kWh / 5600 HDD / 2500 square feet (multiplied by 1000 to get it into Wh rather than kWh), gives a Greenness score of 2.6.
For the natural gas example: 36,300 kWh used less 10,000 kWh generated by solar panels = 26,300 kWh used net of solar production. 26,300 kWh / 5600 HDD / 2500 square feet (multiplied by 1000 to get it into Wh rather than kWh), gives a Greenness score of 1.9.
If you find this stuff interesting, as I obviously do, please calculate the scores for your home and email them to me at email@example.com I will publish the league table and award a prize for the one with the lowest score for Green Efficiency. The target to beat is 1.1 watt-hours per heating-degree-day per square foot of living space per year. Let the race begin and may the best home win!
Getting to zero on Greenness is its own reward as it means you have achieved a zero-carbon footprint on your home!