Solar System Real Savings Example
An Installation on the Pedernales Electric Grid in Austin, TX
The system savings calculations provided by the installer of our solar power system were inaccurate. The calculation is much more complex than reducing your billable kilowatt hours by the system production.
We have an 8 Kw solar system with REC 400W Alpha Black panels and Enphase IQ8+ micro-inverters installed and turned the system on June 22, 2022. Due to the IQ8+ inverters installed on our system, the maximum output is capped at 6 Kw/hour. I've been told that taking into account the total amount of time production would exceed 6 Kw/hour and the anticipated reduction in production capability of the panels, the extra cost to use inverters that don't clip the power at 6 Kw/hour would not be justified. Also, I've been told the inverters are more efficient when they run at maximum capacity.
PEC bills power as follows:
| Service Availability Charge | $22.50/month |
| Delivery Charge | $0.028405/kWh |
| Base Power Cost | $0.0445/kWh |
| TCOS Pass-Through Charge | $0.01686/kWh |
| Temporary Storm Surcharge | $0.007/kWh |
| Sustainable Power Credit | -$0.060005/kWh |
So, we are charged $22.50 plus $0.089765/kWh for power we pull from PEC less $0.060005/kWh for power we push to PEC.
Power Usage
We have found that peak usage for air-conditioning in the summer (the largest power usage) starts around 4pm. Peak power generation is between 11 am and 3:30 pm. (In August we're still running max A/C till 9 pm.) So, in the summer, you're going to be pulling power from the utility for cooling while your solar production is low or nill. Also interestingly, the PEC metering will frequently show some power consumption as well as power generation (power you push to PEC) in the same 15 minute interval during peak solar production. So here is some real data (starting in June 2022):
| Month | Produced kWh | PEC Pulled kWh | PEC Pushed kWh | $ Saved | $ Billed | % Prod Used |
|---|---|---|---|---|---|---|
| June/July | 1,095 | 788 | 618 | 71.45 | 95.86 | |
| July/Aug | 1,176 | 1,088 | 621 | 76.65 | 90.11 | 47 |
| Aug/Sep | 1,036 | 980 | 534 | 67.47 | 90.48 | 48 |
| Sep/Oct | 1,112 | 796 | 697 | 71.52 | 62.44 | 37 |
| Oct/Nov | 755 | 614 | 503 | 47.17 | 55.13 | 33 |
| Nov/Dec | 413 | 841 | 282 | 24.73 | 88.72 | 35 |
| Dec/Jan | 796 | 971 | 473 | 47.56 | 78.17 | 42 |
| Jan/Feb | 692 | 1,215 | 343 | 42.06 | 121.63 | 49 |
| Feb/Mar | 878 | 688 | 555 | 59.63 | 55.78 | 39 |
| Mar/Apr | 750 | 668 | 436 | 51.35 | 60.98 | 40 |
| Apr/May | 915 | 710 | 572 | 61.65 | 56.89 | 37 |
| May/June | 1,194 | 672 | 693 | 84.20 | 45.94 | 43 |
| Month | Produced kWh | Proposal Est. kWh | Actual Savings | Proposal Est. Savings |
|---|---|---|---|---|
| July | 1,307 | 788 | $85.49 | $106.24 |
| August | 1,044 | 1,195 | $67.32 | $107.56 |
| September | 1,126 | 1,067 | $73.46 | $96.02 |
| October | 1,016 | 997 | $64.98 | $89.72 |
| November | 540 | 876 | $33.50 | $78.82 |
| December | 576 | 772 | $32.72 | $69.45 |
| January | 773 | 835 | $47.94 | $75.19 |
| February | 775 | 871 | $50.65 | $78.41 |
| March | 830 | 982 | $55.93 | $88.34 |
| April | 831 | 1,021 | $56.79 | $94.17 |
| May | 1,039 | 1,160 | $72.07 | $100.57 |
| June | 1,296 | 1,260 | $91.30 | $108.58 |
33-49% of our solar power generated is offsetting our usage and 51-67% is pushed back into the grid and credited at 6¢. We would obviously rather be offsetting our usage (ie. not pulling power from PEC) at 9¢ per kWh. Our actual savings per kWh produced by the system works out to around 6.4¢ thus far (the rate for generated power from PEC was recently raised from 5.4¢/kWh to 6¢/kWh effective 2/14/2023). This is the primary reason the actual savings are so much lower than the calculations provided in the proposal from the solar system installer for estimated savings and payback. They really made no attempt to realistically estimate hourly usage vs hourly production and it's effect on the value of the system production. You will definitely want to take this into consideration when sizing a system. You could install a battery system so you use all your power, but the cost/payback on these systems doesn't make financial sense right now. If you have an EV in the garage during the day, you can take advantage of the extra power and make your system payback a lot shorter. I do have the ability to charge an EV during the day, but I'm waiting on an EV system with bi-directional charging (ability to provide power to the house from the EV) before I invest in an EV. Being able to use the battery in the EV to run the house in off-production hours would be a perfect solution for me. There may be some downside to this concept as current battery technology has a limited number of charging cycles and we would probably be cycling the battery more frequently than typical driving only usage.
I hope this helps you evaluate your investment in a solar system so there are fewer surprises.