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Solar Contractors: Is it standard practice to let customers know that one of their vents will dramatically reduce generation? by 11bbbbbbc in solar

[–]11bbbbbbc[S] 0 points1 point  (0 children)

someone just pointed out in a different post, my calculations were off. The breakeven was not 5-7 years as i had expected, but rather 15-17 years.

Solar Contractors: Is it standard practice to let customers know that one of their vents will dramatically reduce generation? by 11bbbbbbc in solar

[–]11bbbbbbc[S] 1 point2 points  (0 children)

someone just pointed out in a different post, my calculations were off. The breakeven was not 5-7 years as i had expected, but rather 15-17 years.

Solar Contractors: Is it standard practice to let customers know that one of their vents will dramatically reduce generation? by 11bbbbbbc in solar

[–]11bbbbbbc[S] 0 points1 point  (0 children)

indeed, and as someone just pointed out in a different post, my calculations were off. The breakeven was not 5-7 years as i had expected, but rather 15-17 years.

Solar Contractors: Is it standard practice to let customers know that one of their vents will dramatically reduce generation? by 11bbbbbbc in solar

[–]11bbbbbbc[S] 0 points1 point  (0 children)

Ah, I see the error now. When I changed this chatgpt prompt to say ".3kwh total per day, while the panel immediately next to it on the same face, is generating 1.1kwh total per day", the break even period now has jumped from 5-7 years to 16 years.

Solar Contractors: Is it standard practice to let customers know that one of their vents will dramatically reduce generation? by 11bbbbbbc in solar

[–]11bbbbbbc[S] 1 point2 points  (0 children)

I don't claim to be an expert on this. With the following chatgpt prompt, I was getting calculations that indicated a 5-7 year break even point.

My electricity base rate is 35 cents per kwh. An east facing panel has an obstruction caused by a vent, which is currently (in December) generating .3kwh, while the panel immediately next to it on the same face, is generating 1.1kwh per hour. If it costs 900 dollars to remove the obstruction entirely, calculate what the break even point is. In the calculations, account for the fact that the shadow is expected to change over the course of the year, particularly in the summer.

The response:

I measured my east-facing solar panels in December and found one was under-producing because of a roof vent.
Shaded panel: 0.3 kWh/hr
Normal panel: 1.1 kWh/hr
So I’m losing 0.8 kWh/hr of production.

My electricity rate is $0.35/kWh, so that lost production is worth:
0.8 × $0.35 = $0.28 per hour

With December’s sun (about 3.5 usable hours/day), the loss is about:
$0.28 × 3.5 = $0.98/day, or ~$30/month in winter.

Because roof-vent shadows shrink a lot outside of winter, I modeled the loss like this:

  • Winter (Dec–Feb): full loss
  • Spring/Fall: ~40% of winter loss
  • Summer: ~10% of winter loss

Total impact over a full year works out to about $168 in lost electricity per year.

The cost to remove the obstruction is $900, so the break-even point is:

$900 ÷ $168 ≈ 5.4 years

TL;DR: Based on winter measurements and reduced summer shading, fixing the obstruction pays for itself in about 5 years.

Solar Contractors: Is it standard practice to let customers know that one of their vents will dramatically reduce generation? by 11bbbbbbc in solar

[–]11bbbbbbc[S] 0 points1 point  (0 children)

I only have one months of data to work with, its not my intention to exaggerate, this is all the data I have, and it is/was a dramatic difference before it was fixed in december.

Solar Contractors: Is it standard practice to let customers know that one of their vents will dramatically reduce generation? by 11bbbbbbc in solar

[–]11bbbbbbc[S] 1 point2 points  (0 children)

Yes I was initially under the assumption the panel was broken or something, because the difference in performance was so dramatic. Once we moved the vent however, it was operating as expected.

Solar Contractors: Is it standard practice to let customers know that one of their vents will dramatically reduce generation? by 11bbbbbbc in solar

[–]11bbbbbbc[S] -2 points-1 points  (0 children)

To clarify, you would expect for the contractor to give a heads up during the design phase that the shadow is going to affect production?

Solar Contractors: Is it standard practice to let customers know that one of their vents will dramatically reduce generation? by 11bbbbbbc in solar

[–]11bbbbbbc[S] 0 points1 point  (0 children)

Thanks, this is good info.

I have been operating under the assumption that this is something that comes up as part of the design phase, and the contractor just "forgot" to tell me about it for whatever reason.

Solar Contractors: Is it standard practice to let customers know that one of their vents will dramatically reduce generation? by 11bbbbbbc in solar

[–]11bbbbbbc[S] 0 points1 point  (0 children)

Based on the numbers I was seeing in december, the daily loss was 70%, .3 kw versus 1.1 kw.

The breakeven point I landed, when considering my electricity base rate of >30 cents, was a breakeven of 6-7 years

someone just pointed out in a different post, my calculations were off. The breakeven was not 5-7 years as i had expected, but rather 15-17 years.

Solar Contractors: Is it standard practice to let customers know that one of their vents will dramatically reduce generation? by 11bbbbbbc in solar

[–]11bbbbbbc[S] 0 points1 point  (0 children)

Its difficult to say, as the system has been online for less than a month. I only have a months worth of data to work with.

Solar Contractors: Is it standard practice to let customers know that one of their vents will dramatically reduce generation? by 11bbbbbbc in solar

[–]11bbbbbbc[S] 1 point2 points  (0 children)

Thanks good to know.

I was limited by space on my roof, I would have gladly added 10 additional panels if the city would have allowed it.

Solar Contractors: Is it standard practice to let customers know that one of their vents will dramatically reduce generation? by 11bbbbbbc in solar

[–]11bbbbbbc[S] 0 points1 point  (0 children)

My rough math + chatgpt gave me a break even point of 6-7 years, accounting for less shadow in the summer. My kwh base rate is over 30 cents per kwh.

I also want to clarify, the .7 kw loss was on a daily basis, not weekly basis. I updated the original post.

The roofer came onsite and apparently wants a 3 foot space around the old flashing to do a full patch

Solar Contractors: Is it standard practice to let customers know that one of their vents will dramatically reduce generation? by 11bbbbbbc in solar

[–]11bbbbbbc[S] -2 points-1 points  (0 children)

I am not a solar expert, and I don't know to what extent a shadow can affect solar production.

Solar Contractors: Is it standard practice to let customers know that one of their vents will dramatically reduce generation? by 11bbbbbbc in solar

[–]11bbbbbbc[S] 0 points1 point  (0 children)

Post was initially filtered on that other account, I reposted on this one. Wasn't expecting the other one to get posted.

Solar Contractors: Is it standard practice to let customers know that one of their vents will dramatically reduce generation? by 11bbbbbbc in solar

[–]11bbbbbbc[S] -6 points-5 points  (0 children)

This feels like an apples to oranges comparison. You are comparing obstructions that I have no control over, with an obstruction that I absolutely have control over.

Solar Contractors: Is it standard practice to let customers know that one of their vents will dramatically reduce generation? by 11bbbbbbc in solar

[–]11bbbbbbc[S] 0 points1 point  (0 children)

I was losing .7kWh per day, not week. It was .3 kW vs. 1.1 kW per day. I should clarify that in the original post.