A Net Zero Energy Day

When we first starting thinking about building a house, our plan was not to build the most efficient house in the world.  But as we got started with the design phase, we decided that we would let energy efficiency be the driving force behind many of our decisions.  Of course, we also decided that we were not prepared to live uncomfortably; what's the point of designing and building a house if you're not going to be able to enjoy it a little?  But as the process continued, we were able to include so many energy efficient features that we began to think that the house might actually end up "net zero energy."  We started monitoring our usage and production pretty closely starting on Jan. 1, 2011 and on February 15, we had our first net zero energy day!

The Sun Lights, Warms, and Powers our Home (Click image for more photos.)

Net Zero Energy (NZE): In theory, determining our net energy usage is trivial since everything in the house runs on electricity.  None of our appliances use gas, oil, or propane.  The only complicating factor is the masonry heater, which burns wood for heat.  Should the wood count towards our energy usage?  It's clearly a source of energy.  However, wood is a renewable resource and when burned efficiently it creates almost no pollution.  Furthermore, while burning wood does produce carbon dioxide, it produces no more than would be produced naturally as the wood decays in a forest.

The U.S. Department of Energy (DOE) Building Technologies Program defines “A net zero-energy building (ZEB) is a residential or commercial building with greatly reduced energy needs through efficiency gains such that the balance of energy needs can be supplied with renewable technologies.” The DOE definition seems very reasonable for our purposes since it gets at the heart of what we are trying to do.  It also makes the record keeping extremely easy; since everything in the house runs on electricity, we simply need to read the electricity meter on a specific day and then check the meter one year later.  If it reads the same or less than it did one year ago, we've achieved net zero energy.

Of course, being a physicist, I don't want to simply check the electricity meter once per year.  What's the fun in that?  I want to record data and chart our progress throughout the year.  I suppose I could check the meter once a month or once a week, but it would be much more satisfying to be able to see a real-time display of our net energy usage any time at all.  But how?

Meet TED: A little research turned up a few options for consumers to hook into their electrical breaker box so that they can monitor their electricity usage in real-time.  We opted for "The Energy Detective," affectionately known as "TED."  TED uses current transformers in the breaker box to monitor electrical current delivered to the house.  This information is then transmitted through the electrical lines in the house to a "gateway" unit that connects to a computer router via an ethernet cable.  A software program can then access the gateway to display the data in a variety of formats.  The gateway also sends this information to a wireless display unit for easy viewing without using a computer.

TED Monitors Our Incoming Electrical Power (Click image for more photos.)

It's pretty cool having TED around. If you turn on a light, within seconds you can see the result on TED's display. This makes it particularly useful to see which appliances are the energy hogs in a house. Not surprising, the heating system, oven, water heater, and clothes washer/dryer are the real energy draws in our house. On the positive side, energy production from the solar panels is quite evident, particularly when the sun is shining brightly. Interestingly, we still get pretty decent solar generation even when it's cloudy, as long as it's bright. If it's a dark and gloomy day, we produce essentially nothing.

Our Net Energy Use: A true Net Zero Energy building will produce as much energy as consumed over a period of one year. We began keeping records of our energy use and production at the start of 2011. In our part of the country, heating is one of the biggest energy draws for a house. When you factor in the extra lighting required for the short days, this makes winter the most energy intensive time of the year. In addition, winter is when the sun is at its lowest point in the sky which means solar production is at its worst.

The result of all this is that we use a lot more energy than we produce right now. As an example, from Jan. 15 through Feb. 14 our total total energy use was about 1,050 kWh (kiloWatt-hours) and our production was about 300 kWh. This gives a net energy use of 750 kWh. At first glance, this sounds like a lot. It turns out that it's not. I'll discuss the details of US average energy consumption in a later post. For now, all we can say is that we produce about 25-30% of our energy needs at the worst time of the year.

But things are changing quickly as the sun rises in the sky. On February 15, 2011 we had our first "net zero energy" day! We used 23 kWh of energy and produced 26 kWh of energy for a net -3 kWh. Although this was not a typical day (the sun shone brightly all day and we used less energy than usual), it does demonstrate that as spring approaches, we should start to break even on a day-to-day basis. Of course, to be truly NZE will require that we overproduce enough during the good months to offset the underproduction we face during the bad months. At this point, it's very difficult to predict whether we will make it or not. We'll just have to wait and see.

Power Usage and Generation on a Good Day (Click image for more photos.)

The graph above shows our usage and generation for the majority of Feb. 15, 2011, our first net zero energy day. The Blue curve is our actual usage, the red is our generation, and the black is our net usage.