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Hal Levin

The hidden payback - "wise use." (Forgive the long post, please.)

We all know that reducing the loads is the first step toward low energy use/low carbon emissions. You don't need PVs on the roof to learn where your electrons go, but if you have an incentive like I did, you might learn a lot and reduce your loads and your use as well as moving it to the hours when the emissions are lower. More on that below. First my story...

For those who don't know me, I live on the outskirts of Santa Cruz, California, with a blessedly mild climate, in a not very efficient house that I did not design or build. I installed PVs six years ago with 1/3 of the total PV system cost ($21K) subsidized, mostly by State of California's program, partly by the federal tax break.

I have an attached office with a full-time employee and a small, detached, energy efficient rental unit (I designed and built) all running on my electric meter. The house has way too much glass (although I have removed most of the north-facing glass). The hot water radiant heating system installed in the slab failed before I bought the house. Now heat is provided in some part by the sun and otherwise by a wood stove and individual electric heat sources. (More on that later.)

I have just done an analysis of 6 years' daily data on my electric energy use (in my all-electric house) and the performance of the PVs on my roof. I read the meter daily and sometimes multiple times a day, especially after an activity that I think might be a considerable energy hog. My hook-up to the grid is through a time-of-use meter. It gives me a reading of total net energy use and net peak period energy use. The utility, PG&E, charges roughly 3X as much during on-peak summer (M-F, 12-6) as they do the rest of the week. The differential in the winter is barely noticeable. The huge difference in the rates led me to do some very careful meter watching. My analysis showed that during the last three years I used 25% less total energy as I did during the first three years I had the PVs on the roof and the time-of-use meter.

This was the result of the learning I did by the frequent meter reading with annotations of my uses. The "bottom line" here is that careful documentation and analysis was the source of knowledge that has produced much lower loads.

-- garden watering turned out to be a huge energy hog as I have a well 250' deep. I used to water my substantial vegetable beds, berry vines, and fruit trees in my garden and, it turns out, over-watered them. I could easily use 15 to 20 KWH during an afternoon of garden watering. I have cut way back on watering without reducing the productivity. If you never ate fruit from unirrigated trees, you are in for a treat - the flavor becomes so much more concentrated in the smaller fruit from an unirrigated tree. Of course we have no rain in most of California for most of the growing season, so irrigation is the norm. Or, perhaps, over-irrigation, as it turns out was my habitual practice until I discovered the energy cost.

My solar hot water heater provides all the hot water for 1/2 the year when the back-up heater is off, and a variable fraction the rest of the year when the back-up heater is on. But as installed, the system was virtually useless. After morning showers and other hot water use, the back-up heater would come on and heat the water, preventing the sun from being able to do its job. I installed a time clock that turns the back-up heater on very early in the morning to add only what is need for showers. Otherwise, the back-up heater is off. A $35 time clock that makes a $2000 solar hot water heater useful rather than a nearly total waste of money and resources.

Replacing my 25-year old refrigerator saved about 5 KWH/day according to my reading of the meter.

All my clothes washing is done in off-peak hours, and the expensive, energy-efficient clothes dryer has almost never been turned on due to the use of the solar clothes dryers - outdoors in dry weather or on lines in the "sun room" which also serves at the recycling room.

The 2500 watts of PVs on the roof are at a lower than "optimal" slope because they are on a lower roof south of the clerestory windows through which an important part of the solar gain occurs. I did not want to block the sun's entry through the clerestory windows. At less than a 10 degree slope, one would think their performance might suffer. But, in fact, their output is right at their theoretical maximum in the summer. And the lower slope of the panels works to my advantage during the early and late daylight hours in the summer's longer days of insolation -- approximately twice the hours than during the winter. If the slope angle had been steeper, the early and late gain would have been reduced in the summer when the sun's path is so much farther to the north than in the winter. The low slope allows the first and last quarters of the sunny hours to produce a substantial fraction of the peak-rated output.

The "bottom line" here is that careful documentation and analysis was the source of knowledge that has produced much lower loads.

On the heat question, since I am all electric, I have learned that spot heating with radiant sources is most effective. With air temps in the low 60s, thermal comfort is good with a radiant source. The parabolic reflectors provide a far more efficient way of doing this than the free-standing, liquid-filled types I used to use. By lowering the "design temperature," a huge saving is realized. Cut the loads first, then provide what you must with the most efficient delivery mechanism -- what you need, where you need it, and when you need it.

Finally, a word about carbon calculations from buildings. Energy use does not correlate perfectly with GHG emissions. Emissions from on-site combustion is a function of fuel used and equipment efficiency. Electrons used at your house do not correlate one-to-one with GHG emissions either. Time of electricity use is critical since the inventory of power plants on the grid feeding your house is constantly changing, and it is different in every part of the country. You can use EPA's quick and dirty GHG emissions carbon calculator for emissions from your annual average electric use available at EPA's web site, http://www.epa.gov/cleanenergy/energy-and-you/how-clean.html. But an accurate analysis requires breaking the time of use down into discrete time periods that reflect the inventory at work. Peak electric energy and off-peak electric energy can vary by as much as a factor of two in some parts of the country. In others, where the inventory is fairly homogeneous, the difference can be negligible. Base load is usually the cheapest source available to your utility -- often coal and hydro, where available. If there are nukes in your grid region or sub-region, they are likely on almost all the time -- nukes average about 90% on-time in the U.S. But if your region has both coal an natural gas power plants, then time of use matters since coal plants emit about twice the GHGs per KWH as nat gas. And hydro, solar, geothermal, and nukes are all about the same, somewhere around less than 10% of nat gas. You can read more about these issues in a paper on my web site at http://www.buildingecology.com/articles/calculating-greenhouse-gas-emissions-from-buildings/.

You can see a pretty good time-of-use GHG emission calculation tool for California at http://www.ethree.com/public_projects/ghg.html and click on GHG Tool for Buildings in California

Unfortunately, as far as I know, no such tool exists for the rest of the country.


The current generation of home power meters available, such as the Powerhouse Dynamics eMonitor, allow you to see energy usage over the day, which is the information you harvested manually as you informed yourself about your usage by reading your meter.

Your watering energy is a good catch, though the amount surprises me. My old well pump in Meriden was about 1 kW and pumped 6 gallons/minute. So 1 kWh for 360 gallons. 15 kWh would be 5400 gallons. A guideline for watering is about 1 inch/week, so a gallon waters 1.6 ft2/week. So if you were watering 8,000 ft2 efficiently that could amount to the 15 kWh. Your solution of course is the best - don't water!

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