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 the  numbers 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.

Date1 Indoor °F Outdoor °F Differential °C2 Excess Humidity3 Dishwasher Loads Laundry Loads kW·h
Min Max Min Max Solar Used In Out
11/1/2010 71 76 65 87 41 117 0 2 17 13 7 11
11/2/2010 68 78 52 85 -101 18 1 0 13 13 10 10
11/3/2010 67 70 51 64 -186 27 0 0 4 11 9 2
11/4/2010 67 68 52 71 -121 36 1 0 28 12 7 23
11/5/2010 65 69 38 70 -187 88 0 3 28 21 11 18
11/6/2010 65 68 36 75 -213 75 0 0 27 11 7 23
11/7/2010 64 70 37 76 -174 43 1 1 28 19 8 17
11/8/2010 66 71 49 78 -104 16 0 1 26 15 10 21
11/9/2010 67 72 51 80 -85 29 0 1 26 14 9 21
11/10/2010 67 76 55 83 -55 64 1 0 22 14 10 18
11/11/2010 72 76 65 81 -49 107 0 2 15 18 12 9
11/12/2010 71 75 68 82 23 147 0 1 13 13 10 10
11/13/2010 69 75 46 81 -180 39 0 0 25 12 6 19
11/14/2010 66 71 43 63 -226 45 0 0 9 18 14 5
11/15/2010 66 71 46 68 -176 4 0 1 10 13 11 8
11/16/2010 66 70 45 71 -137 4 1 1 26 20 11 17
11/17/2010 66 72 47 81 -139 33 0 0 25 12 7 20
11/18/2010 65 72 41 77 -164 73 0 0 26 13 8 21
11/19/2010 63 71 33 73 -233 58 1 3 26 28 15 13
11/20/2010 66 71 51 77 -126 24 0 0 15 14 10 11
11/21/2010 70 75 69 82 25 146 1 0 11 15 9 5
11/22/2010 71 76 70 85 28 142 0 0 15 14 9 10
11/23/2010 73 77 69 86 30 160 0 2 12 19 14 7
11/24/2010 74 80 73 87 27 147 1 0 15 15 9 9

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.