Pre-installation Production Estimates vs. Reality

[NutmegCT]

Good morning all.

I'd really appreciate hearing from home PV users who got pre-installation estimates from the contractor.

How does the reality of what you're actually getting from the panels, compare to the estimate?

Many contractors here in New England will give very positive estimates; folks I know who actually bought/leased the system seem disappointed over what their systems are generating, and how long until the system really pays for itself. From September through April here we don't get many cloudless days - but the contractors say "we already factored that in". But can't give any specifics.

Just wondered how your "estimate" compares to what you're really getting. Examples would be very helpful.

Thanks.
Tom in CT

[sensij]

The motivation for installers to over or underestimate the system production will depend on the terms of the contract. For systems that come with a production guarantee, the production estimates are often low balled to create a bar that even a poorly functioning system will meet. That has been my experience here in CA.

J.P.M. had suggested PVWatts in your intro thread, and I'd like to 2nd that as a source of estimates with which you can compare to what an installer is telling you. The direction your roof is facing (azimuth), if roof mounted, the pitch of your roof (tilt), and your location are the three main variables that are used to get from a system size to an energy estimate. Other loss factors and modifiers are interesting if you want to dig deep, but are to some extent more difficult to control. Some big inverter manufacturers (SolarEdge, Enphase, SMA) have also developed their own estimating programs, although the nice thing about PVWatts is that NREL has extensively documented it, and the calculations can be replicated by the intrepid.

The effect of shade is another factor to consider when looking at an estimate and there are a few ways it can be accounted for and / or mitigated.

Mostly, I would agree with the idea that any numbers coming from an installer should be treated as suspect, whether it is electricity generation or some kind of financial payback analysis. Performing these calculations on your own can be rewarding, and if you share enough information, you may find forum members here willing to offer their take on it. While "unbiased" is a hard bar to hit, most here would have no incentive to mislead you.

[NutmegCT]

Thanks very much for the advice.

I have used PVWatts and found lots of numbers. But the estimates I got (last year) don't come in that format. Example attached.

But my main interest is hearing from members here who can compare what the contractor told them in advance, and what they're actually getting.

Thanks.

Tom in CT
Cost Break down.jpg

[J.P.M.]

Good morning all.

I'd really appreciate hearing from home PV users who got pre-installation estimates from the contractor.

How does the reality of what you're actually getting from the panels, compare to the estimate?

Many contractors here in New England will give very positive estimates; folks I know who actually bought/leased the system seem disappointed over what their systems are generating, and how long until the system really pays for itself. From September through April here we don't get many cloudless days - but the contractors say "we already factored that in". But can't give any specifics.

Just wondered how your "estimate" compares to what you're really getting. Examples would be very helpful.

Thanks.
Tom in CT

1.) I don't know how CT vendors compare to CA vendors in the performance estimation dept., but since most vendors make more money by putting larger systems on roofs, I'd suggest it may be common to underestimate performance, mostly to sell larger systems. Most potential customers are clueless about the situation, and no peddler ever got fired for selling too big a system. In some fairness, systems that are PERCEIVED as overproducing (actually, or probably, oversized) probably don't lead most users to be upset. On the other hand, while a slightly undersized system is often (but not always) slightly more cost effective than an oversized system, a bit either way is probably not a disaster. Gross oversizing is usually some peddler taking advantage of a potential customer's solar ignorance.

2.) Around here, vendor estimates of performance ran about 10+ % low. That seems to be coming down a bit, perhaps for some convoluted reasons, as the new PVWatts appears a bit more realistic in performance estimates.

3.) To your question: Vendors estimated my system's output about 15 - 20% lower than actual 1st year performance. I expected that as I know something of how the game is run.

4.) Just an opinion, I'd guess that user ignorance about solar potential and optimism in performance estimates by vendors are positively correlated.

[sensij]

Thanks very much for the advice.

I have used PVWatts and found lots of numbers. But the estimates I got (last year) don't come in that format. Example attached.

But my main interest is hearing from members here who can compare what the contractor told them in advance, and what they're actually getting.

Thanks.

Tom in CT
[ATTACH=CONFIG]5463[/ATTACH]

To go from PVWatts to get at numbers in that format, you will need to share more about the price of electricity (or your power company and rate plan) and your monthly usage.

The range of payback times I got from my installers ranged from 5-12 years. By my own calculations for those systems, the actual payback for those systems was 10 to 20 years. CA doesn't have SREC's to help with payback, but we do have fairly expensive electricity. Only one installer provided a payback number that was close to my own calculation.

Some ways they fool with the numbers:
Bad production estimates
Unsupportable estimates of electricity cost increases
Estimates that assume your usage will increase every year.

[russ]

Demand the output in kWh/day, month and year they will guarantee. As soon as they throw all the other stuff in it confuses the issue and it is easier to screw you. What will they guarantee?

[Carl_NH]

Hi Tom,

The installer I used was referred by a friend in Maine, as he installed a system with them in 2012. They estimated he would get a bit over the nameplate rating 4400KWH for a 4.3KW system. As you will note below he is getting a bit more nearly 5000KWH/Year.

In our case they estimated due to shading (trees and temporary) I would be under during the winter months and over the rest of the year averaging about 7500KWH total per year from a 7.2KW system My array faces 210* SE at a 38* angle. Without the shading I would be over 8000 I suspect. We just installed and wont have anything real for at least 6 months.

I am attaching his actual results here.

4.3KW System - Maine
PV Watts 2013 2014
Jan 388 440 360
Feb 412 375 446
Mar 509 462 577
Apr 374 487 525
May 353 375 361
Jun 308 379 390
Jul 341 377 395
Aug 383 470 453
Sep 394 476 489
Oct 369 453 380
Nov 283 414 295
Dec 331 278
Total 4445 4986 4671

[J.P.M.]

Carl: It looks like you may be confusing annual output in kWh and system size in S.T.C. (nameplate) Watts, or kW.

Identical systems will have different output in kWh as f(orientation,location) as well as year/year variation in output due to weather.

[Carl_NH]

Carl: It looks like you may be confusing annual output in kWh and system size in S.T.C. (nameplate) Watts, or kW.

Identical systems will have different output in kWh as f(orientation,location) as well as year/year variation in output due to weather.

Yes, I usually am confusing things

Yes and I agree each system will be different due to location, angle, sun hours, and many other factors. For instance a STC rated 4KW system in San Diego, with no shading, will generate maybe 6500KWH annually, in this case in Maine, it generates much less due to snow, cloudy days, and sun angle and so on.

The question Tom asked was related to supplier optimistic output of a system, and the reason I replied is the same supplier installed the system I refer too, and ours, and was realistic and slightly under in production estimates. What we will generate and how close we are to their estimates is yet to be seen.

[J.P.M.]

Yes, I usually am confusing things

Yes and I agree each system will be different due to location, angle, sun hours, and many other factors. For instance a STC rated 4KW system in San Diego, with no shading, will generate maybe 6500KWH annually, in this case in Maine, it generates much less due to snow, cloudy days, and sun angle and so on.

The question Tom asked was related to supplier optimistic output of a system, and the reason I replied is the same supplier installed the system I refer too, and ours, and was realistic and slightly under in production estimates. What we will generate and how close we are to their estimates is yet to be seen.

I think I understand.

I inferred from your post that you were saying that system sizes are expressed in kiloWatt hours, and that a system estimated (or actually) to have a numerically higher number of kiloWatt hours of annual output, estimated or actual, above the system size in Watts is somehow an overestimated or oversized system.

I mean no disrespect, but if, or to the degree my inference is correct, your post has possibly incorrect implications leading to incorrect assumptions, conclusions and actions by others who may read it. Maybe my opinion about pointing out what I read and believe to be incorrect information is more useful as an example of how engineers (even retired ones, and those of us accused of being politically correct) get reputations as p(r)icky curmudgeons.

[Ian S]

Well, I went with a prepaid lease and so had a production range guarantee. My first year's production was pretty much smack dab in the middle of that range, maybe a bit above midpoint.

[NutmegCT]

Most of us here in Connecticut pay about $0.22 per kWh. That's 50% electricity, 50% delivery. My average bill total is about $100/month.

I certainly agree with the statements about being overwhelmed by contractors' charts, numbers, expectations, etc. I'm beginning to think I just plain can't afford this, as a cash payment for a 4kW system is out of the question, a balloon payment lease is unacceptable to me, and I can't see (so far) actually breaking even on the deal.

Sure do appreciate hearing your personal experiences. It's interesting to me that some folks get pretty much what they were expecting (output and payoff), and some have very different experiences.

Using the info in my first paragraph, how does the contractor's estimate look to you?

Thanks.
Tom in CT

[pleppik]

I've had my solar installation for about five months (went live at the end of July). The installer estimated 5.8MWh for annual production, and we were tracking ahead of expectations right up to about six weeks ago when we got hit with the worst stretch of cloudy weather since the 1960's.

As of today we've generated about 2.1MWh since going live almost exactly five months ago. That's pretty close to being on track with the estimate, but I haven't done the analysis to see if we're a little ahead or a little behind.

The challenge with this sort of question, though, it that without several years' of data, it's hard to know if good/bad performance is because of the installer's nudging the estimate, or whether it's because of the weather.

Unless someone knows of a good source of historical insolation data...Any suggestions?

[bcroe]

I've had my solar installation for about five months (went live at the end of July). The installer estimated 5.8MWh for annual production, and we were tracking ahead of expectations right up to about six weeks ago when we got hit with the worst stretch of cloudy weather since the 1960's.

As of today we've generated about 2.1MWh since going live almost exactly five months ago. That's pretty close to being on track with the estimate, but I haven't done the analysis to see if we're a little ahead or a little behind.

The challenge with this sort of question, though, it that without several years' of data, it's hard to know if good/bad performance is because of the installer's nudging the estimate, or whether it's because of the weather.

Unless someone knows of a good source of historical insolation data...Any suggestions?

Here in NW IL we feel your pain and clouds. One recent cloudy rainy day managed to set
a new low of 5 KWH. A nearby solar guy said he got zero that day. But the seasonably
higher temps were saving plenty of energy. Won't know just how that balances out till I
see the remaining reserve on the next bill. But maybe Feb will again bring a big boost.

I don't see how these extremes can be related general historical data. Bruce Roe

[SunEagle]

Here in NW IL we feel your pain and clouds. One recent cloudy rainy day managed to set
a new low of 5 KWH. A nearby solar guy said he got zero that day. But the seasonably
higher temps were saving plenty of energy. Won't know just how that balances out till I
see the remaining reserve on the next bill. But maybe Feb will again bring a big boost.

I don't see how these extremes can be related general historical data. Bruce Roe

Not to mention that the weather just about everywhere in the US is not following historical data very closely. There are more "extreme" incidents in the past few years to results in most of the historical weather data to no longer be accurate.

Hopefully you don't get the snow fall amount like you did last year.