Zero Energy Home: Old Numbers, New Numbers

We've had a couple of really busy weeks around the house. In addition to all of the work on the landscaping that we've been doing, our solar electric array was installed and is now providing power to the grid (and us)! Unfortunately, that means that for a little while I won't know how much electricity we are using. That's because our current (no pun intended) electric meter only measures "inflows" (electricity that we are getting from the grid) and ignores the "outflows" (electricity that we are providing to the grid).

Texas is not a "net metering" state. There is no requirement that your electric utility buy back (at any price) the excess electricity you are generating, and they get to charge you for all of the electricity that you pull from the grid.

Fortunately, one of the locally available options for our electric service includes an option to buy back up to 500 kWh per month at the retail price, and any additional amounts at 50% of retail. See Green Mountain Energy's Renewable Rewards Buy-Back Program at http://www.greenmountainenergy.com/renewable-rewards. In addition to their generous buy-back program, all of the energy they sell comes from renewable sources.

In order to take advantage of this offer, we need to get a new meter (plus some additional paper-work). The new meter will measure both inflows and outflows (more numbers! woohoo!), so we'll be able to tell how much we're generating, how much we're using, and how much is going back out onto the grid.

As if that isn't cool enough, our solar installation includes free monitoring for a year, which will allow us to monitor our electricity production panel-by-panel via a web site.

That means this is the last of the old numbers, without the solar component being added. I've cleaned up some of my math, so you'll see some negative numbers (when it's cooler outside than inside, on average). You'll also see a dramatic drop-off in consumption as the weather turned cooler and we started opening windows to let in the cool air at night.

Here's a quick peek at the new numbers: Oct 17th, 2010 - meter indicates we used 6 kWh from the grid (all during the hours of darkness). Solar power meter shows that we generated 27 kWh. Guessing that we used about 12 kWh total for the day, that means that we used 12 and generated 27, sending 15 kWh of clean energy out into the grid. You're welcome.

Finally, the last of the old numbers. Next time, all of the numbers will be brand new.

Date1 Indoor °F Outdoor °F Differential °C2 Excess Humidity3 Dishwasher Loads Laundry Loads kW·h
Min Max Min Max
8/6/2010 75 80 78 104 165 189 0 0 25
8/7/2010 75 80 78 99 163 189 0 0 25
8/8/2010 74 81 79 97 145 186 1 3 32
8/9/2010 75 80 78 98 152 186 0 0 29
8/10/2010 75 81 77 99 154 194 1 0 23
8/11/2010 75 81 79 99 149 216 0 0 26
8/12/2010 75 80 76 100 120 211 0 1 26
8/13/2010 75 80 77 98 155 197 1 0 23
8/14/2010 74 79 80 100 171 211 0 1 27
8/15/2010 74 80 79 100 173 207 1 1 30
8/16/2010 75 80 78 102 176 170 1 1 27
8/17/2010 75 79 76 102 100 219 0 0 27
8/18/2010 75 78 79 101 177 209 1 0 28
8/19/2010 74 81 76 100 89 244 0 2 24
8/20/2010 75 80 79 98 137 214 1 0 27
8/21/2010 75 80 79 100 150 200 0 1 29
8/22/2010 75 79 78 101 165 203 0 0 25
8/23/2010 75 80 76 103 170 132 1 2 34
8/24/2010 75 80 78 105 185 139 0 0 27
8/25/2010 75 81 78 94 81 200 0 2 27
8/26/2010 75 80 77 92 83 134 0 0 21
8/27/2010 75 79 69 96 89 79 1 1 27
8/28/2010 74 79 62 97 93 3 1 1 25
8/29/2010 74 80 72 97 110 90 0 0 20
8/30/2010 75 80 80 97 134 238 0 2 29
8/31/2010 75 80 79 98 135 240 1 0 23
9/1/2010 75 80 79 99 127 232 0 1 29
9/2/2010 75 81 79 98 115 221 0 0 25
9/3/2010 75 79 72 93 48 210 1 1 26
9/4/2010 74 80 71 91 43 75 0 0 18
9/5/2010 75 80 70 93 65 145 0 2 23
9/6/2010 74 83 71 92 70 166 0 2 25
9/7/2010 74 81 73 90 33 234 1 0 24
9/8/2010 74 81 74 85 15 275 0 0 17
9/9/2010 74 82 78 92 62 271 1 2 27
9/10/2010 74 80 78 93 111 237 0 0 27
9/11/2010 74 80 78 95 116 238 1 0 22
9/12/2010 74 81 76 95 98 237 0 2 28
9/13/2010 74 79 76 93 105 252 0 1 25
9/14/2010 74 80 76 93 92 247 0 0 22
9/15/2010 74 81 78 94 115 232 1 0 28
9/16/2010 74 80 75 93 93 209 1 1 26
9/17/2010 74 79 72 93 79 169 0 0 18
9/18/2010 74 78 72 89 35 226 0 0 16
9/19/2010 75 78 72 89 36 215 1 1 24
9/20/2010 74 78 67 87 -44 158 0 2 22
9/21/2010 74 78 69 87 26 198 0 1 18
9/22/2010 74 78 74 88 47 228 0 1 19
9/23/2010 75 82 75 92 42 222 1 0 21
9/24/2010 75 82 75 92 60 221 0 0 22
9/25/2010 74 80 72 91 63 221 0 0 17
9/26/2010 74 79 70 85 28 173 0 2 25
9/27/2010 74 79 57 78 -114 24 1 1 19
9/28/2010 72 79 52 85 -141 21 0 0 15
9/29/2010 68 80 58 89 -90 33 0 0 13
9/30/2010 73 77 56 88 -90 32 0 0 12
10/1/2010 73 78 65 86 -55 45 1 2 22
10/2/2010 73 78 57 85 -75 2 0 0 17
10/3/2010 69 79 59 82 -63 0 1 0 13
10/4/2010 65 77 51 76 -95 46 0 0 10
10/5/2010 65 75 48 79 -116 32 0 2 15
10/6/2010 64 74 47 82 -115 19 2 2 21
10/7/2010 65 75 48 83 -112 17 0 0 10
10/8/2010 65 76 49 85 -107 8 1 0 12
10/9/2010 66 77 52 86 -96 1 0 0 11
10/10/2010 71 77 60 87 -62 59 1 3 24
10/11/2010 73 78 67 87 45 122 0 0 11
10/12/2010 74 78 62 88 -51 82 0 0 12
10/13/2010 72 78 60 85 -74 2 1 1 16
10/14/2010 69 77 55 81 -76 48 0 0 11

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.