Design Process for my Tesla Solar Panels

NREL's PVWatts Calculator is great - you definitely should compare this to any solar quote you get and ask questions if there is a significant mismatch.

So after researching Solar, I ended up having "interesting" discussions with Solar vendors: SunRun, SunPower, and Tesla.

SunRun and SunPower (both "big boys" in the market) have a direct (commissioned) sales model ... so definitely motivated to close the deal ... and pretty wild how much you can negotiate a price reduction from the initial quotes. Even after that, Tesla is by far the cheapest as they came in (before 30% tax credit/subsidy) at $16,500 versus $23,500 for a 6.4kW system. Another reason for the lower price is Tesla uses cheaper String/Line inverter technology versus Micro-inverters. I won't go into the pro/cons of that, but here's a compelling video that the claim that "string inverters are 'bad'" is mostly marketing and not based on reality. Regardless, I don't have shade issues on my roof, so really isn't an issue for me.

The reps from SunRun/SunPower point another advantage of them is I have a knowledgeable person to talk to who ensures things get done "right" ... which at least for me didn't seem like that advantageous as they were mostly salespeople. Where as Tesla you get assigned a Project Advisor who's non-commissioned and not so motivated ... and as I've found out, not so smart (couldn't even tell me the dimensions of the Solar Panels) ... even though this person has been their 8 years! I suspect in both cases (and in most "projects"), you are best served by being an informed customer and taking some ownership of the process ... rather than just saying "whatever you think is best."

So I ended up going with Tesla - they are a THIRD CHEAPER than the other vendors ... which one admitted they can't compete on price against them.


So now it's time to decide on the panel layout on the roof. One thing that works against me is I only have a little bit of space on my South (192°) facing roof area ... but moderate space East (102°) and lots to the West (282°). Different orientations obviously affects your energy production ... and I made an 11th hour decision to add two more panels (a mistake in hindsight) to I went from a 6.4 to 7.2kW system with 18 panels. Using the handy PVWatts calculator (default parameters with Premium Modules and Roof Mount with 23° pitch), here's the predicted annual energy output from a 7.2kW system:

kWh Output		Orientation
6,547		0° - North
9,187		90° - West
9,597		102° - MY ROOF
10,847		180° - South
10,754		192° - South
8,659		270° - East
8,219		282° - MY ROOF
8,678		2.4kW@102° and 4.8kW@282°
8,906		3.6kW@102° and 3.6kW@282°
9,117		3.2kW@102° and 0.8kW@192° and 3.2kW@282°   -   FINAL CONFIGURATION

So the (ideal) South facing roof would generate 13% & 32% more energy than my East/West roofs. And if all my panels were (somehow!) South facing, then it would generate 19% more energy than my final configuration.

Note that Tesla predicted an annual output of 8,971kWh versus 9,117 from PVWatts. I have some slight seasonal shading from the chimney's that maybe Tesla is accounting for ... and/or I may not have the exact (secondary) inputs to PVWatts ... and/or this may just be the natural interannual variability.

Battery Backup: The reason I'm not getting a battery backup system is they are expensive. To add ONE Tesla Powerwall would cost me (after 30% tax credit and some other misc.) an additional $8,112! That buys you 13.5kWh of energy. So yes, you could use a single 100 Watt light bulb for 135 hours ... but my 4-Ton, 14 SEER Air Conditioner would only run less than 3 hours. BTW, the Powerwall warranty is (70% energy retention) 10 years.


Here's my utility bill - note I messed up the (manual) summation of the 2022 monthly bills - it was 6,700, not 7,700 - D'OH!
The reduction was because our son moved out after living with us during Covid - should have used the lower value going forward for sizing.
To compound my error, I originally wanted to do a 6.4kW system which would have produced about 8,000 kWh (based on "flatline" of 7,466-7,700kWh of usage) ... but at the last minute, I decided to add two more panels. So much for all my "great" planning! ;-)
This means the system is over-sized and I product more than I use. So Xcel gets the excess while my Solar Bank grows ... but will I ever use it?!?


This spreadsheet, downloaded from the Internet, is a first WAG as analyzing the cost/benefit of Solar. That used some earlier quotes (with 6.4kW system) with that ... but you are much better off looking at the Cost & Payback tab on my spreadsheet. Note that thanks to Inflation Reduction Act (cough, cough), American taxpayers will send me 30% of the cost back early next year as a tax credit - thank you Uncle Joey! ;-)

Per the EIA, electrical rates for Colorado over the last 25 years show a 2.5% annual increase ... which is similar to what I have seen personally. If instead of "investing" in Solar, I just let it ride in the S&P500 (which is what I do), that return is 9.6% (last 20 years to 2023) or 12.2% (last 10 years) but then knock 30% off for taxes ... so I went with 7%.

The Payback Analysis is very sensitive (DUH!) to starting electricity costs (mine was 13.5 cents/kWh in 2023) and the (assumed based on historical) future annual increases of 2.5% for electricity and 7.0% (after tax) investment rate of return.

Using those numbers, the payback is at year 16. Obviously there's the expectation that when we sell the house, even after 15-25 years, there would be residual value in the Solar System such that you would realize a higher sale price.

BTW, one thing you don't see mentioned much is there is still a "connection" fee that you have to pay. Plus you the Solar Panels should be added to your insurance policy, which I calculated to be about $25/year in additional cost.

I'm of the opinion that residential solar (added to existing roofs - managed by homeowners) makes less economic sense (for society) than doing utility solar (installed ground level with tracking - managed by people who actually know/pay attention, etc.).

People often ask me "is worth it?" I reply "What is your interest/reason for putting solar on your roof?"
If it is so you can feel good about "Saving the Planet", then go for it ... but be careful saying it's to save money! ;-)



BTW, while Gas/Electricity really haven't moved that much in price, Water has gone WAAYYY up in price ... and it's going to be substantially higher this year. Yadda, Yadda, Yadda Climate Change, Drought, etc. But Lafayette water is going up much faster IMHO ... they basically are using water to generate revenue for the city. One of these days, some local paper (HAH!) is going to look into it and realize that! ;-)

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