We added solar panels to our Lennar smart home in Cypress, Texas. The smart home allows me to conduct an energy analysis using three reports:
- Our smart home includes Honeywell Lyric thermostats. These thermostats generate monthly reports that let us know how much or how little the heat or air conditioning has been running.
- Our SunPower solar system also provides monthly reports on kilowatts generated and consumed.
- And last but not least, we get a monthly bill from our electricity provider, TXU Energy (connects us to the grid and supplies power during non-sunlit hours) that shows electricity purchased from the grid and returned to the grid.
The TXU bill doesn't happen on a monthly boundary, but by looking at data from the two months that span the one TXU bill, I can get a sense of what's going on.
Summary
Based on two data points (one use case from two different systems), the SunPower systems are definitely reducing electricity bills to the tens of dollars (or even a small credit) instead of what would normally be hundreds of dollars, but the systems do not generate as much power as advertised since the system specifications are created under ideal conditions; however, over the lives of the systems, they will eventually pay for themselves.
Details
Our hardware and settings:
- Honeywell
- 3 Lyric Thermostats (see blog post for details)
- Settings
- Downstairs: 76℉ during the day, 78℉ at night
- Upstairs: 78℉ both day and night
- In-law unit: 77℉ both day and night
- SunPower
- 42 360-watt panels
- Theoretical Maximum output of 15.12 kW per hour (42 times 360, see blog post for details)
- TXU
- TXU Energy Renewable Buyback (see TXU website for the information below and more)
- When you invest in solar panels for your home, you earn bill credits when you produce more clean energy than your home uses, and when you don't produce enough, you pay a simple, flat rate for 100% Texas wind or solar power from the grid.
- Your electricity meter tracks both inflow and outflow electricity. When your solar system generates electricity you don't use, it's sent back to the grid as outflow electricity.
- Your meter only measures the electricity you consume from the grid and the surplus electricity flowing back onto the grid. If a monitoring system was installed with your solar panels (such as the one supplied by SunPower), which is separate from your electricity meter, the monitoring system is what tracks all power produced by your solar panels. (This is key as kWh produced and immediately consumed straight from the solar panels does not show up on the bill.)
- If you earn $600 or more during the calendar year, TXU sends you a 1099 form, so you can pay income taxes on the earnings. This is probable for me as I saved $129.26 in May/June alone. Maybe saving is not the same as earning?
- Rates
- $0.125 per kWh consumed from the electrical grid
- $0.115 per kWh returned to the electrical grid
- There are lower rates available in Texas for consumption, but these rates reflect that the power purchased comes from solar or wind. I am willing to pay a little extra for sustainability.
- Also, this plan includes a nice buyback rate.
- TXU Energy Renewable Buyback (see TXU website for the information below and more)
My most recent TXU bill spanned 32 days from 05/15/2019 through 06/16/2019. That means that the Honeywell and SunPower reports from May and June are of interest.
So my analysis includes:
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Based on data collected by the thermostats, the Honeywell reports show the number of hours that heating or cooling was running.
Since the heating is based on gas instead of electricity, only the cooling is pertinent for this analysis. Cooling took a big increase in June as the weather got hotter. The average high temperature in April was 60℉ but it increased to 67℉ in May and 77℉ in June. Hotter weather means more electricity consumption because the air conditioner runs more. June was the hottest month ever recorded on Earth. I may have gotten solar panels just in time.
-
The SunPower reports show the amount of energy produced and consumed.
Though the data is reported monthly, it has a daily breakdown that can be viewed anytime using the web or a smartphone app. I can specify a date range (to match the energy bill) when using the app. So for the period that corresponds to my TXU bill, I see:
The net result is that I did buy a small amount of electricity from the grid for the reporting period that corresponded to the dates on my most recent TXU bill.
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With this information in mind, TXU applies their rate formulas to convert this to a credit, or as in my case, an amount due.
The TXU values of 1124 kWh produced and 1175 kWh consumed reflect the amount of energy that was sent to the grid in excess of what was being used and the excess amount of energy consumed from the grid when not enough was being produced by the solar panels. These numbers do not take into account energy that was produced by the panels and immediately consumed because for that situation, the dial on the electrical meter does not move at all. To get the total consumed for the billing period, using the SunPower reporting, I added the total amount produced plus the extra that I had to buy from TXU minus the extra that I produced that I could send back to TXU. Using this method, if I look at the data for May and June and cull the data points from May 15 through June 16 that correspond to the TXU billing cycle, I see:
Total Energy Consumed 2329.97 kWh Cost of that Energy at 12.5 cents per kWh $291.25 Actual Cost $17.62 Savings $273.63
From the Honeywell, SunPower, and TXU data, if this $273.63 savings represents a typical month, I can see that my $37,044 out of pocket costs will take 11 years and 4 months to break even instead of my initial estimate of 10 years and 6 months.
The data from the SunPower system shows the maximum output for the reporting period. Although I have a 15 kW system, my maximum is in the 13.41 to 13.44 kW range. Part of this is due to the fact that although I have 360 W panels, I have 320 W converters. In addition, the 360 W rating on the panels is under ideal conditions.
Through my blog posts on this topic, I was contacted by another Houston, Texas resident who also has a SunPower system installed by Freedom Solar Power. We compare notes to see how we are doing.
His hardware and settings:
- ecobee
- 1 ecobee4 Thermostat (see ecobee website for information)
- Settings: 75℉ during the day, 73℉ at night
- SunPower
- 25 360-watt panels
- Theoretical Maximum output of 9 kW per hour (25 times 360)
- Green Mountain
- Green Mountain Renewable Rewards
- $0.119 per kWh consumed from the electrical grid
- $0.119 per kWh returned to the electrical grid
His results include:
- His recent electricity bill was -$16.30 (a credit!) for the period of 04/30/2019 to 05/30/2019.
- From his SunPower reports, his maximum output was only 7.73 kW, not 9 kW. He also has 320 W converters. Again, the 360 W rating on the panels is under ideal conditions.
What we are both seeing is that our systems, although somewhat cost-effective, don't generate as much power as the specifications would lead one to believe and that it will take longer to recoup our investments that originally anticipated. The good news is that eventually, our systems should pay for themselves in our lifetimes as long as we eat right, exercise, and get plenty of rest.
Solar energy analyses are alive in the lab.
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