Zero Energy Home: More Numbers, Some Insight

I've collected a few more days of data since my last post, and I have refined my model slightly. Once of the other interesting pieces of information that my weather station collects is the indoor and outdoor humidity. It's been a humid summer in central Texas this year, and anyone who's been outside at all can tell you that humidity can have a major impact on comfort.

My wife and I are fortunate that our geothermal heat pump does an excellent job of removing excess humidity from the air in our house, but that removal isn't free. In order to get rid of that humidity, the water vapor has to be converted into water, and a phase change of water requires a bit of extra energy.

So I've added a new column to my table, which is an indicator of how much excess humidity is in the outside air. Each time an exterior door is opened some of that excess humidity (and heat) wafts into the house. While I don't have a way to measure the amount of time that a door was open during the day, it gives some indication of the amount of energy expended dealing with humidity.

That brings up a point I've been meaning to make: not all of the relevant data is readily available. In addition to the amount of time doors are open, I haven't been trying to record the amount of time microwaves (we have two), cook-top, and oven are being used. So while I have specific data for each day, the data can't be interpreted mathematically except in the aggregate.

Here's the new data, including the new humidity information:

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

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.