Everyone knows that solar can help the planet, and many know it can also help their pocketbook, but in either case, the real question is: “how much?”
The amount of energy a solar array produces is at the heart of answering this question.
Solar integrators estimate the energy production of a solar array using modeling tools that predict performance based on a variety of factors, such as historical weather, system design and other technical parameters. The most common tool is the PVWatts calculator provided by the National Renewable Energy Laboratory (NREL).
Though tools like PVWatts have led the way in simplifying performance forecasting, the subject remains complicated. At Enphase, we’re also driving to better understand performance. We recently took a deeper look at the issue with some of our partners, Astrum Solar, Real Goods Solar and Solar Universe.
When our research began we discovered a number of published studies showing that common performance estimation tools overestimate solar system performance. The most extensive study we found showed that the actual performance of 480 solar installations was on average 8 percent below the PVWatts estimate.
This can be a big problem, considering that energy production estimates aren’t just for the ‘eco’-benefits of solar, they’re also essential for the ‘eco’-nomics of a solar investment.
For example, PVWatts is at the heart of many solar sales proposal generators, and it is also the tool used to define the Expected Performance Based Buydown (EPBB) incentive provided by the California Solar Initiative.
Surprised by our initial findings, we became curious how Enphase systems compared to PVWatts.
Using a similar methodology to the study of 480 sites, we examined data from 143 Enphase systems, installed by our three partners in California and the Eastern U.S. The study revealed that Enphase installations on average outperform PVWatts estimates by 8 percent. Download the study here.
When considering our findings relative to prior studies, they indicate that Enphase Microinverters offer a 16 percent performance advantage over standard inverters, relative to the PVWatts estimate.
Enphase and Standard Inverter versus PVWatts
After completing the study, we spoke with Michelle Waldgeir, vice president of marketing for Astrum Solar, a contributor to the study.
Michelle explained that Astrum Solar has been successful selling Enphase based on design flexibility, monitoring and warranty, but that energy harvest had been a trickier factor to incorporate into sales presentations.
“Conceptually, customers understand that a standard inverters can leave them vulnerable to things like, ‘The Christmas Light Effect,’—when a string of modules underperforms due to one bad module—but it’s difficult to provide them with concrete information about the long-term financial impact of this problem,” she said.
“Previously, we relied on references and individual case studies to make the Enphase technology case. We often pointed customers to a test site where we had added an Enphase array to a rooftop that already had a string inverter array on it installed by another company. In this side-by-side setup, the Enphase array outperformed the standard inverter system by more than 20 percent,” Michelle continued. “We’re excited that the new study demonstrates performance gains across a broad sample of rooftops and locations. We can now confidently provide customers with real-world data that estimates the performance benefits Enphase microinverters can deliver.”
At Enphase, we wanted to enable installers to see the Enphase advantage more easily. So, we recently introduced an Enlighten PVWatts integration, and now any installer can input a system’s design information and produce estimations right within Enlighten.
This type of integration doesn’t just demonstrate the power of our microinverters, but it also shows the value of web-based monitoring and analysis software. Through Enlighten monitoring, system owners see exactly “how much” their solar system is helping the real world.
Sounds like a terrific system and a win for everyone who installs it.
Your ‘study’ does not provide sufficient information to allow credible scrutiny. In the real world, a string PV system using 250 watt panels will outperform a micro-inverter system. This is because of the constraints of the micro-inverter. The Enphase M190 will only allow 190 watts output. During the cooler Spring and Fall months in Northern Climes when it is still quite sunny, a 250 watt panel will provide full output. In the hot summer the output will decline to about 200w. Therefore, in cooler climates where there is sun and cool air, the string inverter will do much better than a micro-inverter system, and about the same during the summer and winter. Advantage to the string inverter system.
We have systems installed using maximizers with string inverter systems where a 240 watt panel produces over 290 watts during cooler sunny days. 290 watts compared to the constraint of the M190 is a very large output difference.
Your study needs to include different climates zones, different panels, different string and micro-inverters, maximizers and performed over several years by an independent body before you can make a statement that your micro-inverter is better than a string inverter system.
Lastly, I presume your study is about M190′s since your M210 has only just come out? Also the study details are not available on the link going to your web site.
I would warn anyone who reads this to tread carefully before believing any study performed by a manufacturer that shows their product to be superior. This is definitely a biased communication talking about PVWAtts estimates that makes it look like the Enphase is superior to a string system. Don’t be fooled.
Micro-inverters definitely have their place in the industry and I am by no means saying do not consider micro-inverters. They are easy to install but not so easy to service. There is no big inverter at the side of the house, but an inverter is one point of failure at the side of the house versus many on the roof. The new Enphase micro-inverter M210 has a 25 year warranty however micro-inverters are new technology, string inverters have been proven over decades. It’s all about your choice. Make sure you do research before making a final decision, because it is an expensive decision.
Hi Philip,
Thanks for the feedback, and sorry for the delay in response.
It’s an industry standard practice to oversize the DC-side of a solar system relative to the inverter capacity (typically a 1.2x DC-to-AC ratio is used). So, even with a string inverter system, you would not capture any power beyond the DC rating of the system. We have a whitepaper on this subject here:
http://enphase.com/wp-uploads/enphase.com/2011/03/Enphase_White_Paper_Module_Rightsizing.pdf
Oversizing the DC-side is a standard practice because it leads to the best economic performance ($/kWh). Here’s why: the cost of the inverter and of all the AC electrical equipment is based on the output rating of the inverter. So, if the inverter is sized for the peak DC power level of the system, your cost will increase significantly while your actual kWh production will increase only slightly.
In microinverters, the considerations are similar: you don’t want the microinverter sized for the module’s peak power rating because you will incur higher AC electrical costs without significant improvements in performance.
Hope that answers your question.
Best,
David
Enphase just keeps looking better and better. Living and installing in Mexico I’m hoping you’ll look into a 127VAC version as many of my clients do not have 240VAC service. Congratulations on your many successes.
sure does sound great,and i even attended a small tent show ,but cant get any info on prices, parts ,or sizes of systems,how to get started,who installs locally[tucson].I mean ,do you have to be an electrical contractor,or what?is do it your self out of the ? or what?
Warren,
There are many re-sellers of Enphase and many include a complete kit with modules, racking and hardware. You do need an electrician to connect to the grid, but you can do the actual installation of the modules yourself and have the electrician do the connection. Visit http://www.SimpleEnergyWorks.com for some kits and prices.
Our home self installation is consistently outperforming PVwatts estimated by at least 10%. Hurray for Enphase. watch it work here: http://enlighten.enphaseenergy.com/public/systems/VxzZ15241
I feel a little skeptical when I see a technical evaluation done by the Marketing Manager for a company. I’m not saying that the report is not accurate but I would feel more likely to accept it if it were authored by the Director of System Engineering, and even more likely if it were done by an independent third party.
Hi Marvin (and everyone else),
I can confirm that the Enphase inverters are outperforming consistently both the state rebate calculator (csi-epbb.com) — which calculates output, and PV watts projections. We have over 2,500 of these inverters installed in both residential and commercial installations, roof and ground, and the minimum overproduction we see consistently is 3% and max is 23%.
I have been in the industry for over 6 years and have worked with all kinds of inverters. I have found, in general, both calculators are conservative, and many inverters will slightly (not to the extent that Enphase does) outperform estimates.
So where do I find this Enlighten PV Watts integration on my installer page?
Hi Bob,
Go to the Settings Page for your system and click on Array Details to enter your array information. Here’s a link to more about the latest Enlighten update: http://enphase.com/enlighten_content/enlighten-release-2-6-6/.
The study overview hints that there was shading at sites. What are the comparisons when there is zero shading?
Hi Ian,
We observed a strong inverse correlation between shade and performance ratio (i.e. less shade means higher performance ratio).
This is easily explained based on our methodology: As noted in the study, the DC-AC derate factor was not adjusted from the PVWatts default value of “0.77″. This means that we are not reducing the expected performance when there is shade on the site, which is a very conservative methodological choice, especially considering that early Enphase installations were often faced with significant shade issues.
The reason we did this is to keep the methodology consistent with prior studies such as the Gostein, et al. paper.
I question the study of 480 installations showing PVWatts being 8% higher then actual. I dont know what type of installations these were, but my experinnce is that PVWatts is conservative and underestimates production by 1-10%. There was an article in Solar Pro magazine(Apr,2010) regarding production modeling and in two actual case studies, PVWatts underestimated actual production. I’m familiar with Enphase and I know they make a good product, but I am mainly involved with commercial systems were I’m concerned about MTBF reliability with 1000′s of individual modules. However, I think string inverters or optimizers have a good future.
Surely a comparison against a shaded array with micro-inverters and a central inverter is what is required not a comparison against software predicted values, correlating those predicted values to establish a comparison is marketing hype. Micro inverters have their place but don’t spin them.
As to PV Watts one has to know how does PVwatts simulations compare to our predictions and unfortunately it doesn’t show up in this test which is enlightening
PVSyst have a vested interest in the following as their software was tested. I do not.
http://www.pvsyst.com/en/publications
http://www.pvsyst.com/images/papers/photon_survey_corr_irrad_190611_en.pdf
A proper report on Micro inverter performance and shading would take the form of something like this:
http://www.pvsyst.com/images/papers/4bv_1_81_mermoud_lejeune_valencia2010.pdf
This Enphase system will continue to generate power at 16v where line inverters need a minimum of 20v. that means they are more tolerant of some shading (dust, bird poop, low light) and shading only affects the one panel, not the entire string of panels connected together on that “string”. This is a big advantage in the real world.
My friend has a 1 string system using a string inverter and he lost 1/3 of his TOTAL output during maximum sun when the shade from 1 leaf covered one part of one cell in a normal 60 cell panel. no other panels in that string were covered but the ALL had the lower voltage and 1/3 less power generated. That was the old normal.
Enphase solves this on my system!!!! They will also generate earlier in the morning and later in the day. That is extra free power. Why wouldn’t you want those benefits. I guesstimate a 5% improvement and over the 25 yr guaranteed production of my panels, that really adds up.
“Stupid-simple” installation
solid-state reliability
String inverter shading issues – solved
Enphase micro-inverters change solar power generation.