Zero Energy Home: Getting to True Zero

As I reported in my previous post, our solar electric system has been installed and operational. Since then, our electric meter has been replaced with one that measures both inflows and outflows, which means more numbers for me to play with (see the table at the end of the post). The weather hasn't been cold enough for long enough for me to get a good feel for how much energy will be used to heat our house this winter. Because our home has a ground source heat pump (aka "geothermal heat pump"), both heating and cooling consume electricity. That means that all of our energy needs consume electricity (no natural gas appliances). Which brings me to the subject of my post.

If you're really trying to get to true Net Zero energy, you have to have a way to generate all of the energy you use. Jokes about my diet aside, I have no way to generate natural gas, so my wife and I chose a heating and cooling system that uses only electricity. Our intent is to have our solar electric panels provide as much electricity as we consume. Of course, that's net electricity - balancing the electricity we consume at night with the excess electricity we generate during the day.

Electricity is a very versatile form of energy. We use it to create light and heat, to cool things, to move things around and to power a myriad of electronic devices. It's not surprising that demand for electricity has grown so high. Finding cleaner, more sustainable ways of generating electricity is one of our society's great challenges. Finding ways to use it more efficiently must be part of the solution.

As far as the generation part of that equation, our solar electric system consists of 24 Sunpower 215 watt modules with Enphase M190-72 micro-inverters (5 kW DC, about 4kW after conversion to AC). The installation comes with online system monitoring -- you can see current and historical performance for our system here.

Enough preaching.  On to thenumbers for this month.

The good news is that even in November, on clear days our system is generating about 26 kWh per day.  This means that during the summer months, with more hours of cloudless sunlight, we'll be generating significantly more than that.  And since our average daily consumption during the peak heat this August was about 27 kWh, we should be in good shape, even if my wife decides to buy a plug-in electric car.

Date1Indoor °FOutdoor °FDifferential °C2Excess Humidity3Dishwasher LoadsLaundry LoadskW·h

MinMaxMinMaxSolarUsedInOut

11/1/20107176658741117021713711

11/2/201068785285-101181013131010

11/3/201067705164-186270041192

11/4/201067685271-12136102812723

11/5/201065693870-187880328211118

11/6/201065683675-21375002711723

11/7/201064703776-17443112819817

11/8/201066714978-104160126151021

11/9/201067725180-8529012614921

11/10/201067765583-55641022141018

11/11/201072766581-49107021518129

11/12/201071756882231470113131010

11/13/201069754681-18039002512619

11/14/201066714363-2264500918145

11/15/201066714668-1764011013118

11/16/201066704571-13741126201117

11/17/201066724781-13933002512720

11/18/201065724177-16473002613821

11/19/201063713373-233581326281513

11/20/201066715177-126240015141011

11/21/2010707569822514610111595

11/22/20107176708528142001514910

11/23/20107377698630160021219147

11/24/2010748073872714710151599

1Calculations are approximately 5:00PM to 5:00PM 2Differential is the absolute value of the calculated difference between indoor and outdoor temperature summed over the 24-hour period. 3Excess humidity is calculated as the difference between the vapor density of water of the outside air when cooled to the inside air temperature, and 50% relative (indoor temp) humidity.