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I agree totally at least in the Valley of the Sun. My cases may not apply to all.
I've installed two PV systems in the Valley of the Sun. The first one, in over 3 years of operation, performed 16.7 % better than vendor estimates on energy production. The second system is 20 days shy of one year in-service and is performing at 7.8% better than vendor's projections. Based on my previous system data and others' data going back to 2009 (over 5 years), the data shows the last 11 months of solar production is about 1.72% below the average for the 5 years of the data. If I make that adjustment to my current system to compare with the vendor's estimate, my equivalent production would be at 10% higher than the vendor's projections.
All vendor quotes were very similar in production numbers making adjustments for slight differences in system size. All vendors appeared to use utility rates that were based on the standard residential tariff applied to the amount of generation and would escalate the utility rates using higher rates than what I believe will occur. Also, I did not see any models or data that accounted for excess generation under net metering tariffs in the numbers especially as the vendor's proposed systems grew in size.
So, IMO the vendors are now set up to sell a system significantly larger than one economically needs (with lower estimate of production and higher estimate of savings). Most vendors will likely propose systems to generate 100% of usage with PV or have a zero utility bill. The vendors will sell a system 10-20% larger than needed while showing a proportional savings increase from the additional generation. Heck, if you get a 12%+ (after tax) first year return on this smaller system, why not spend more. Where else can one get a 12% return after tax that will only increase as utility rates rise. However, depending on the utility tariffs, the additional $ spent are likely earning far less than 12% on the additional generation and could tank if / when utilities change net metering and buy back rates.
So bottom line on "how have your real world results been vs your system projections": since I run my models with my assumptions and do not rely on the vendors numbers, my actual generation has been very close to my expected as well as my savings. I am getting what I paid for - not paying for more than I need.
I've installed two PV systems in the Valley of the Sun. The first one, in over 3 years of operation, performed 16.7 % better than vendor estimates on energy production. The second system is 20 days shy of one year in-service and is performing at 7.8% better than vendor's projections. Based on my previous system data and others' data going back to 2009 (over 5 years), the data shows the last 11 months of solar production is about 1.72% below the average for the 5 years of the data. If I make that adjustment to my current system to compare with the vendor's estimate, my equivalent production would be at 10% higher than the vendor's projections.
All vendor quotes were very similar in production numbers making adjustments for slight differences in system size. All vendors appeared to use utility rates that were based on the standard residential tariff applied to the amount of generation and would escalate the utility rates using higher rates than what I believe will occur. Also, I did not see any models or data that accounted for excess generation under net metering tariffs in the numbers especially as the vendor's proposed systems grew in size.
So, IMO the vendors are now set up to sell a system significantly larger than one economically needs (with lower estimate of production and higher estimate of savings). Most vendors will likely propose systems to generate 100% of usage with PV or have a zero utility bill. The vendors will sell a system 10-20% larger than needed while showing a proportional savings increase from the additional generation. Heck, if you get a 12%+ (after tax) first year return on this smaller system, why not spend more. Where else can one get a 12% return after tax that will only increase as utility rates rise. However, depending on the utility tariffs, the additional $ spent are likely earning far less than 12% on the additional generation and could tank if / when utilities change net metering and buy back rates.
So bottom line on "how have your real world results been vs your system projections": since I run my models with my assumptions and do not rely on the vendors numbers, my actual generation has been very close to my expected as well as my savings. I am getting what I paid for - not paying for more than I need.
So far (10/16/2013 to now), my system has been about dead nuts w/what I expected. I have an advantage in that the Weather station about 3-4 ft. north of the array including a solar sensor and data logging, allows a pretty good tab to be kept on input to the system, both instantaneous and integrated over time. Input (weather, including solar irradiance) data logged at 1 min. intervals. Inverter input/output at 5 min. intervals.
That's mostly a description of the tools I use that finalized my opinions that most vendors oversize equipment by 10-20 % or so. That and being more than a student of the subject since about 1975 or so. Also, reviewing/recommending solar installations for the HOA around here and seeing how the solar and economically unsophisticated go to the cleaners all too often leads me to the same conclusion. finally, after selling in the capital goods business for some yrs. before an engineering career, I quickly discovering that no one ever got fired for selling too much capacity. Some things are common all over. It's just business.
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