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Net Source Energy

This page documents the progress toward meeting the net-zero source energy goal up to the current date.  To be sustainable, a house must meet net-zero source energy.  It may be "green" without meeting this goal, but to be sustainable, it must meet the goal. 

The PV panels were installed and became operational on 18 May 2010.  The house was first occupied toward the end of June 2010.  Each month of operation so far, the PV panels have generated more than 100% of the electricity used by the house, and since the system is grid-tied, this excess electricity has built up a reserve of energy that can be traded off against the natural gas used for space heating, hot water heating, and cooking.  The expected trend is for energy to be stored up by the house during the warm part of the year, and those reserves used up in the winter, and that is what is observed in the figures below. The net energy use is summarized in the figures below.  In each figure, the net source energy for each month is presented, as well as the cumulative net source energy startng at zero on July 1, 2010.  The house has been fully occupied since that date except for vacations.  The first four figures shows the monthly net source energy and the net cumulative source energy expressed in kWh for the first four years of operation. 

These results show that the house currently is operating with a surplus of energy, and has exceeded the goal of generating more source energy than it consumed for the first three years.

 Net Source Energy in kWh - Yr. 1

Net Source Energy in kWh - Yr. 2

Net Source Energy in kWh - Yr. 3

Net Source Energy in kWh - Yr. 4

The four figures below show the same information in millions of Btu (MMBtu):

 Net Source Energy in Million Btu's - Yr. 1

Net Source Energy in Million Btu's - Yr. 2

Net Source Energy in Million Btu's - Yr. 3

Net Source Energy in Million Btu's - Yr. 4

So during the first year of operation, the house generated 2527 kWh (8.62 MMBtu) more source energy than it consumed, and during the second year, the house generated 1256 kWh (4.29 MMBtu) more source energy than it consumed.  During year three, the house used more energy than it generated, by 1978 kWh (6.75 MMBtu) because the winter was significantly colder than the previous two winters.  After three years,
the house has generated 1355 kWh, or 4.62 MMBtu, more source energy than it has consumed.  Thus, the house has exceeded its design goal over the first three years of operation.  The addition of a whole-house humidifier in year two has increased the energy use in the house relative to the first year.  
The details for the energy generated and the energy used in the Residential Energy Laboratory are given in Energy Use Details.

The Details Behind Source Energy

"Source energy" means evaluating energy use including the energy used to drill, refine, generate, and/or transfer the energy from the primary energy source to the house.  Thus, for electricity, this starts with the fuel that is burned to generate the electricity, a process that typically requires about 2.5 times as much fuel energy as the electrical energy produced. There are then significant resistance losses in the electric lines, with some of the electrical energy converted to wasted heat energy.  The U.S. Environmental Protection Agency (EPA) has computed that over the whole United States, it takes on average 3.340 times as much primary energy as the amount of electrical energy that is available at a house.  This factor also includes an accounting for electricity generated at nuclear plants and renewable sources.  Details are provided in the EPA report "ENERGY STAR Performance Ratings, Methodology for Incorporating Source Energy Use, August, 2009."  A link to this document is as follows: source energy details.  The document contains the following table that summarizes the source energy requirements for a variety of fuels.  

Fuel TypeSource-Site Ratio
Electricity (Grid Purchase)3.340
Electricity (On-site Generation)1.00
Natural Gas1.047
Fuel Oil (1,2,4,5,6, Diesel, Kerosene)1.01
Propane and Liquid Propane1.01
Hot Water1.35
Chilled Water1.05

In the Residential Energy Laboratory, the power sources are natural gas for space heating, domestic hot water heating (supplemental to solar thermal), and cooking, with electricity used for everything else.  Electricity generated on site by photovoltaic (PV) panels is used to displace grid purchased electricity, so the source-site ratio of 3.340 is used.  For natural gas, the source-site ratio of 1.047 is used.  There is no wood-burning stove or other energy sources except for the solar thermal hot water system.