On the occasion of our first utility bill after adding solar, we found a significant reduction in the cost of the electricity we consumed from the grid. As the Tesla engineers predicted the solar provided about half of our electricity use.
As a former research person, I love data and the solar application and Xcel downloads feed my avocation.
This graph illustrates the total power used in blue, the amount of solar produced in yellow, the solar used from the power wall above the zero line and the solar fed to the power wall below. The amount of grid power used above and the amount of solar power fed to the grid below. We have not altered our power usage except that with warmer temperatures the hot tub heaters are less active. But it has also been cloudy much of the time and it is amazing how much power we actually get on cloudy days.
The Solar City app provides slightly different data. This is the solar production graph for a very cloudy and rainy day.
Since we usually get about 50-60% of our electricity from solar, it is really exciting when we have a 100% solar day. This graph shows that it is a combination of the direct solar power and the battery power wall charging then taking over when the solar is not producing enough to run the house. It is likely that a day like this also fed power back to the grid.
This is another graph from the Tesla app showing the electricity usage and solar production. There was no extra for the grid this day. The spikes in blue are the peak electrical use with the hot tub running. The solar day is moderate but the green below the line is the power wall being charged for later use. The power wall in green takes over when the sun is not producing or producing enough to power the house until the batteries are reduced to 47% then the power is fully from the grid.
I created this graph from the data downloaded from Xcel in the billing report. I have a record of all energy use from 2012 when we bought the house to the present. This is the comparison between the 2017 May usage and the 2018 Kwh from the grid. Although the use is less than half, the bill was a bit more than half due to the taxes vs. the per Kwh cost. So using less energy costs a little bit more–about 11 cents per Kwh instead of 10 cents.
We signed up to receive a check from Xcel at the end of the year for the electricity we feed to the grid. They will use an average time of use cost instead of the time of use price at production. I think that will result in a better return but it is hard to say.
The Tesla app graph showed that we fed 150 Kwh into the system the first month. If we are recompensed at 5 cents per Kwh that would be about $7.50 back on the cost for electricity this month. I can’t find a public notice of the payback for electricity fed to the grid from Xcel. They only state it is an average cost. So apparently they charge between 4.6 and 7.57 cents per Kwh but they were hoping to lower the cost per Kwh by charging for grid use which was rejected by Colorado in 2016. Obviously that would not be good for solar customers since they have to have grid use to sell excess power back to the company and the electricity would be worth less. But the compromise was to introduce Time of Use rates which will be higher for high use time periods. This seems better for solar producers who will benefit from higher rates during summer days for instance.
We did not purchase solar for the payback as much as for the energy efficiency it provides and the great boost to our LEED platinum application. It appears from the first month it has been a success.