Design - Photovoltaic Array

Not "If" but "When"?

If we install photovoltaic cells, our electric company will allow us to use reverse metering to export electricity to the grid when we produce more than we consume. Unfortunately, unless the rules change, we will be credited at a lower figure when exporting (meter running backwards) than what we will pay for electricity when importing (meter running forwards).   Our goal is to break even with the power company without over-spending on photovoltaics but we won't know until we live in the house for awhile what it will take to do so.  Then we can size the photovoltaic array to our needs or perhaps decide that the return on investment for an array would be problematic.

Other Advantages of Waiting

In addition to monitoring our needs before investing in solar, there are other advantages to waiting.  Solar prices keep coming down and the panels almost certainly will be cheaper in the near future.  Utility company rebates and government rebates and tax credits come and go but I think the probability of this kind of help may actually increase over time pending which political party dominates at the federal and state levels.

Disadvantages of Waiting

Installing the panels initially would afford the opportunity of having them double as an

overhang for the second level clerestory windows, as shown on the accompanying

Click on drawing for expanded view

drawing, and thereby eliminate the cost of a conventional overhang.  The conduits and wiring from the array to the service panel would become part of construction instead of an afterthought.  However, we are more inclined to delay construction of the overhang for a couple of years anyhow in order to use the heat from the summer sun to jump-start the AGS system. Moreover, in order to function as a overhang for our latitude, the tilt of the PV panels would likely not be optimal for generating electricity.

Compromise Plan

We will probably decide to wait at least a year before investing in PV panels.

Instead of using the panels as an overhang for the second story clerestory windows, they will probably take the form of a free-standing array on the backfill behind the house that will be an easy electric hook-up and will be scarcely noticeable from the street. Free-standing, as opposed to attached, could include an upgrade to a mobile sun-tracking system to maximize solar gain.

Estimating Needs

The average rate-payer in the St Louis region uses 10,000 to 13,000 kilowatt hours per year, depending on whose figures one uses.  The 13,000 figure is about the average for the country as a whole.  The modern rental house in which we lived when first moving to Collinsville would be a better barometer of our electric usage than the 100-year-old farmhouse that we have since bought and occupy next door to the construction site. The rental unit had HVAC, electric range and electric dryer as the main demands on electricity.  We consumed 6,400 KwH/yr which is not much more than half the area-wide average, due in part to our sustainable-centric lifestyle and the reasonable size of the house.

Based on our experience in the rental unit, we are guestimating our electric needs in an energy neutral home to be 1,600 KwH/yr despite it being a larger house.  The load will be diminished, compared to the rental unit, by the intentional design of the house. Gas will be the choice for cooking and heating water (energy efficient tankless heater).  There will be no conventional electricity-scarfing air conditioning.  Abundant natural light in all spaces will minimize the use of lights during the daytime.  Most of the lighting will be task lighting.  General lighting will be controlled by manual override, dimmable motion activated switches so as to match use with occupancy. Energy-saving LED will prevail over CFL. Phantom loads will be minimized via strategic switching, not just for electronic equipment, but for mundane appliances like the toaster, microwave and clothes washer.  

Return on Investment
If an average consumer at 10,000 - 13,000 KwH/yr were to buy PV with enough capacity to make some dent in his/her energy consumption (forget about breaking even with the utility) the initial cost would be dependent upon how big a dent s/he wanted to make.  But the return on investment from energy savings would be protracted because her/his home was probably not designed for serious energy conservation.  My take is that adding PV to the average production home with 2 x 4 walls, or even 2 x 6 walls, and questionable attention to air-sealing, or to older homes like ours with no wall insulation, homes with solid masonry construction or homes with leaky windows and doors is like rearranging the deck furniture on the Titanic.

As counter-intuitive as it may be, PV for our super-insulated, passive solar project may not be a good investment.  Our consumption will be so low that the payback on a PV system that breaks even with the utility may not make sense.  On the other hand, it might.  The decision to invest in enough solar to be energy neutral might have less to do with cost-savings now as it does with insulating us from escalating energy prices in the future. If we add PV, it will be with the conviction that, with the current and future pressure on fossil fuel energy, the cost of electricity will only go up.

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Update:  Summer of 2019.

It has become apparent that generous government incentives for purchasing PV are in danger of diminishing or disappearing.  So we are planning to invest in an array before the end of 2019.

 

Update:  Late summer of 2024.

By now, we have lived in the house approaching 3 years and recently posted on its thermal performance over an 18 months period.