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100% Solar Home - Energy Analysis

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House 13-04-30 at solar noon


After you have a conceptual plan for your home (the basic shape, size, orientation, layout, windows and insulation levels), there are any number of next steps you can consider that won’t cost too much time or money, before you have actually identified the building site. For example: you can get rough quotes from experienced builders based on trim level and square feet; you could start discussing the floor plan and kitchen layout; or you could work up your initial energy plan. For me there was no choice, I needed to know if what I wanted to build could be 100% Solar or not. To answer this question, there are two other questions that need to be answered:

  1. How much energy will it take to power your home?
  2. How much energy will your solar resources provide?

By working through an energy analysis you will learn if your initial plan will meet your energy goals or not. If your current design does not meet your energy goals, then some major rethinking may be needed, such as reducing the size of the house, improving the wall assemblies and insulation levels, upgrading the windows, and maybe altering the shape of the house to allow more passive solar gains, etc. All these are pretty major, so the sooner you can check that the house is in your energy ball park the better.

Note: This step is typically not done today because energy use is not much of a concern. It is assumed that the heating load will be calculated by the HVAC contractor and the furnace and AC will be sized correctly (usually a bit big, but that is another issue). The point is the house’s energy requirements are not considered, after the initial decision on whether to build a “code compliant,” Energy Star, or “something better” house is made. Unless! the house plan exceeds the financial budget and then a reduction in the “nice to have” insulation is one of the first things to be reconsidered! It won’t be until the house has been lived in a few years, that a sense for just how much energy it takes to operate the house will be determined. This won’t always be the case, but sadly for most homeowners and builders it still is today!

Conceptually, to answer the primary energy questions, we simply need to identify the energy demands, add them up; identify the energy sources, add them up; and calculate the difference. However, this is very burdensome to do manually or even with a spreadsheet. So we use energy modeling software and enter: the surfaces (walls, windows, doors, ceiling, basement, etc.); the appliances and lighting specified; the heating and cooling systems with temperature set points; and the solar resources. Press a button and presto - the electric and firewood for heating and the electric for all other functions are presented. Unfortunately, as you will see, I learned this is not yet quite the case!


There are several, probably many, energy modeling programs “out there.” BUT, some are very expensive (Polysun), too difficult to use with poor documentation (Energy 10), proprietary (Passive House), or have limited functionality for my purpose (RETScreen, TREAT and RemRate), for example. By the way, don’t assume Polysun or Passive House software, even if you can get access to them, will model every detail of your house, as they too were developed for a finite set of objectives and may have missed an energy feature or two you would like to include!

I had used TREAT extensively for over five years modeling existing homes and validating those models against actual energy used. Over this time I have developed a high level of confidence that the heating energy the model requires will be close to the energy actually required by the house. Plus, it does handle zone heating with varying set points, which is critical to my design. However it does not allow modeling of some solar features, such as heat storage in a thermal mass or insulation between interior spaces and thus the restricted heat movement. Nor does it have the specific wall or ceiling components that I planned to build, and it does not handle hourly simulation at all. But, for my purposes, I am confident that total energy required to heat the house will not to exceed what TREAT estimates.

There are two major components of our home energy requirements: energy for heat; and energy (electric) for “everything else.” I break it down this way because I have been analyzing our electric usage for “everything else” for years and feel pretty confident we can live very well for less than 5,000 kWh per year. I based this assumption on using about this amount in 2010, and my plan to move the same family into a smaller space that has fewer lights, a smaller water heater (and a greater solar fraction), expected limited use of a very efficient AC unit, more energy efficient cooking, TV and water pumping. Eventually, I will know how much less than 5,000 it will actually take, but that will be a minor detail compared to how much energy it will take to heat the house.

So, my primary focus was on: 1) how much energy is required to heat the house; and 2) how much energy can be generated from PV and active solar (the passive solar will be taken into account with TREAT).

I modeled the house with TREAT and obtained an estimate it would take 21.5 million BTUs for heat. As I said earlier I’m confident it will take less because the model surfaces are not equal to my actual surfaces and the model does not fully address the internal temperature variations and the thermal masses involved, both of which should lessen the amount of BTUs actually required to maintain our desired set point temperatures.

There are two types of solar energy that will contribute to the house’s energy: electric and thermal. The electric for heating was easy. Olya Prevo calculated that the 6.2 kW array they were installing would generate about 7500 kWh and indeed the year before we moved in it generated about 8000 kWh. This means that we will have approximately 2500 kWh per year plus whatever we can save in actual use of the electric used for “everything else,” for heating and a future electric car

My first attempt to get an estimate of how much thermal energy the solar thermal collectors could capture during our heating season was to contact Heliodyne (a major manufacturer of such panels) and ask them for an estimate for their panels. They took my information but never responded.

Then in 2009, I paid for a little coaching and consulting from Ben Nusz, one of the two authors of “Solar Water Heating.” It was from their book that I got the beginning of my ideas on how to heat with the sun. During one consulting session, Ben used RETScreen and estimated that ten flat plate collectors could harvest about 19 million BTUs during the heating season.

At that time, I didn’t understand all the variables involved in estimating solar thermal BTU harvesting, but for my purposes, Ben’s estimate of 19 mmBTUs, plus the fact that I plan to burn about a face cord of firewood (stored solar energy = 5 million BTUs with a 75% efficient wood stove) to make thermal ends meet was good enough for me to feel comfortable I could provide the estimated 21 mmBTUs that would be required to heat our house, with minimal electricity (for circulators).

Two Updates

First, I came to understand from Pete Skinner that there are a number of dynamic factors involved in any calculation of how much heat you can harvest from the sun. Two of the biggest factors are the load you place on the collectors and the resultant returning water temperature (a small load heats up quickly and returning water temperatures increase too much to continue efficiently collecting more heat). Pete estimated that with just five efficient panels I’ll be able to collect about 20 million BTUs due to the large solar battery (thermal mass) we are trying to keep charged and the expected lower returning water temperatures.

Second, during our first winter (moved in 12/20/12) I was able to measure how much energy we using to heat the house. During February, March and April we used just 7 mmBTUs for heat, which extrapolates to about 15 mmBTUs for the past heating season, and about 17 mmBTUs for a more normal heating season.

So, with less heat needed by the house and more heat available from the sun, we should be comfortable and use less electric for hot water (1500 kWh included in the “everything else”), and ultimately have more electricity available for a future electric car.  

Thus, now in 2013, we am ready for Pete to design a drain back system that will cost effectively generate our heat and hot water.


Dan Gibson is Reporter and Chief Coordinator of Our Energy Independence Community and Chief Energy Auditor and Solar Designer for www.HomeEnergyAdvisors.com. He previously spent 5 years performing home energy audits in NYSERDA’s Home Performance program. He is also currently “finishing” their 100% Solar Home – Yes, three years in the making and it's not finished yet. He can be reached at DanG@OEIC.us or at (518) 899-2400.

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