The “green home” in Tiburon, CA has completed its first full year of operation and data collection so it is time to share its results with you.
This chart shows the most basic part of the results and it is the focus of this post. Results from our natural gas and water usage will follow in a separate post.
The orange bars on the chart show our electrical energy usage (positive values in watt-hours so divide by 1000 for KW-hrs). The blue bars on the chart show our electrical generation from the SunPower solar PV panels (negative values are opposite as generation is opposite from usage). Generally you can see we produced substantially more electrical energy than we used! Success.
We actually had a surplus of more than 3000 KW-hrs. Over time I expect that to decline as we get a plug in hybrid car in a few years and use the surplus to power the car.
Some other very interesting trends can be seen in the chart.
First of all on the solar production side of things you can see that we make less energy in November – February (maybe 50% less than we do during the summer months). This is clearly because the days are shorter and the sun is lower to the horizon even at the noon peak. But it is also because we have many more cloudy days in the winter rainy season than we do in the summer.
Second you can also see that we use more energy in the winter than we do in the summer. This is certainly because we need more lighting during the longer night times. Also I suspect it is because we watch more television and video in the colder rainy winter than we do during the sunny longer warm days of summer.
Lastly, while you might expect that our production curve should look more like a sin curve – symmetrical – the fact that we find we have a broad and asymmetric production period that lasts from April through October probably is due to the fact that we have so little rain or cloudy weather during those months.
Before the solar the home used to use between 500 and 700 KW-hrs per month but we also changed out our gas heating for electric heat pumps and we added a hot tub and small treadmill swimming pool. So the electrical load is substantially more but we cover it all with this design and approach.
In summary, our home has almost all of its energy needs on the electrical side (not natural gas). This includes clothes washing and drying (though we do hang some clothes up to dry naturally as well), most cooking, refrigeration, home heating and cooling by heat pumps, lighting, computers, endless pool. On average we consume 1.2 KW-hrs on a 24/7/365 basis. About 25% of this is related to the monitoring and all that we do to track the home’s performance. And we produce 1.5 KW-hrs on a 24/7/365 basis. Our net surplus therefore is about 2500 KW-hrs.