SolarEdge vs. Enphase, A New Evaluation

Reme Meck, NABCEP-certified PV Installer

I’ve looked at Enphase vs. SolarEdge before from the view of reliability and energy production.  I also considered installation time, concluding that Enphase is superior due to not having to install a central inverter while SolarEdge does.  The reality of the situation is somewhat different, so I’m revisiting this conversation with a much-needed update utilizing new information and perspective.  My valuation focuses on three main points, installation time/labor, price of materials, and energy production.

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pairing enphase and solaredge

Enphase and SolarEdge

The most popular microinverter and the most popular DC optimizer butt heads a lot in the current solar market. Both are panel-level MPP trackers that maximize individual panel yield. Both make the array more resistant to shading and panel mismatch. And both are fully compliant with the latest Rapid Shutdown requirements for solar systems.

So, what should you use, savvy solar installer?

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Solar Array Commissioning –Production Analysis

When you install a system, sometimes you are asked to verify that it is producing the expected amount of power.  Simply reading the inverter LCD screen will give you the AC power production, but that information hasn’t been checked against the site conditions so you have no way of knowing if it’s correct or not.  To prove it using math, you have to perform a production analysis.  I am going to go over the tools and methods to analyze a solar array for power production based on temperature and solar irradiance.

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Selling Value vs. Price – The Eternal Struggle

The mark of a good salesman is identifying customer needs and providing them the products they need to fulfill those needs.  I’m taking a break from design posts to take a look at the sales side of solar and some of its unique challenges.  I’ve enlisted the expertise of our resident sales guru, Tim McGivern.  Tim is a seasoned veteran of sales with a firm grasp of customer interaction, and he comes with a wealth of knowledge and experience.  Today’s challenge is how to sell quality and value versus pure cost in a solar project.

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Grid Tied Inverter Overloading Analysis (with PVSyst)

For today’s post, I brought out the solar modeling software and gotten myself into trouble (grid tied inverter loading analysis).  System analysis and solar modeling software is a delicate subject, there are dozens of variables and different models can produce different results.  I’ve tried to keep things as simple as possible for the purpose of this analysis.

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Solar System Design – String Sizing

When designing a solar system, the most important calculation is determining the length of the string of solar panels.  Solar inverters and charge controllers have set voltage windows that have to be met by a string of solar panels whose voltage can vary as much as 40 – 60% throughout the year.  With low string voltages, operation is less efficient and the system can be in danger of shutting off during hot conditions.  Design a string voltage too high and cold sunny conditions could put the inverter into an overvoltage fault mode which shuts the inverter down.  Solar designers have to hit the “sweet spot” where their string voltage will always fall within their equipment’s voltage window while maximizing the string length for more efficient operation.  This is done by designing solar strings based on the upper voltage limit of the inverter or charge controller.

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System Design with 1000V 3-phase string inverters

Much has changed in the solar PV industry in the past five years.  Solar panel efficiency has grown incredibly while prices have dropped so much that incentives are hardly needed to make solar projects financially viable.  New solar mounting technology allows for automatic grounding from the solar panel, with universal clamps and advanced ballasted and penetrating designs.  However, by far the biggest advances have been in inverter technology.  Inverter efficiency has steadily increased, voltage windows have gotten wider, and dual MPPT inputs are now a standard.  By far the biggest advance has been the approval of a true 1000V DC voltage window.  Coupled with dual MPPT inputs and a wide input voltage window, commercial system design is more flexible and adaptable than it has ever been.

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Performing a Solar Site Analysis

The site analysis is a critical junction in the process of selling and installing a solar system.  Proper information gathering leaves your installation team with everything they need to know to prepare for the install.  Leave out the wrong piece of info and there could be last minute design changes, emergency trips for more equipment, and an overall lack of professionalism in the eyes of your customer.  A good site analysis will give you all of your design information in an efficient and structured manner and leave a lasting impression on your customer.

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Wire Sizing Tools and Resources

Hello ladies and gentlemen!  To wrap up my extended series on wire sizing and voltage drop in solar systems, I want to share some wire sizing tools/resources for calculating voltage drop that can help make your life easier.
Online Wire Sizing Configurators
This is an extremely simple voltage drop calculator that assumes no conditions of use similar to my examples for voltage drop in the previous post.  It allows you to input any voltage into it, which makes it unique among the free online voltage drop calculators I checked out.  It’s simple but easy to use and understand.
This calculator lets you select from a long list of preset voltages, and gives you the choice of DC, AC, or AC three phase.  The issue with this calculator is the list of voltages is very arbitrary, there are “holes” in its selection that prevent it from being a comprehensive tool.  Other than that, it lets you adjust conductor temperature to include derating due to conduit fill and ambient temperatures.  It also lets you select your conduit type.  All in all, it’s a very good calculator for AC voltage drop and an OK calculator for DC. 
Online Wire Sizing & Voltage Drop Information Resources
These are some of the resources I have used in the past for learning about voltage drop and wire sizing.  The information in these articles is extremely useful and in-depth.  I owe a great deal of my knowledge on this subject to them.

voltage drop formula

Episode 5 pt 3: Voltage Drop and Wire Sizing for Solar

Voltage drop and wire size are important considerations when designing and installing solar PV systems.  Voltage drop occurs in any electrical circuit carrying power. A percentage of the power running through the conductors will be lost as heat.  This amount is dependent on the size of the wire along with the voltage and current of the load, and some environmental factors as well (circuit length, temperature, conductor material).  This post will cover selecting the correct wire size based on your maximum solar current and calculating the voltage drop based on that.  I will be heavily referencing the NEC 2011 codebook throughout this post.
Conditions of Use
Before getting into selecting wire sizes, I want to cover conditions of use.  These are factors that need to be applied to the maximum current we calculated earlier based on environmental conditions.  The most obvious condition of use is ambient temperature, found in Table 310.15(B)(2)(a).  Based on most sources, the average high temperature for the location should be used.  This can be obtained from the ASHRAE handbook (the 2% value) or determined from weather data available online.  The other conditions of use are more complicated.  Conduit fill is an important consideration for large systems and long conduit runs, it is found in Table 310.15(B)(3)(a).  This is because wiring will generate heat during operation and the hotter it gets inside the conduit the worse the voltage drop will get.  Conduit distance above roof for roof mounted systems is very important as well, and can make the conductors even hotter, reference Table 310.15(B)(3)(c) for more information.  For the purpose of this post, conduit fill and conduit distance factors will both be assumed as 1 for power production.  Ambient design temperature will be fixed at 85° F, which gives a modifier of 1 on our current calculation for simplicity.
Wire Sizing
From my previous submission, maximum DC solar current is based on the solar panel’s Isc rating multiplied by 1.56.  The maximum AC solar current is the inverter’s maximum continuous current multiplied by 1.25.  Taking these maximum calculated currents, proper wire sizes for them can be selected using NEC 2011 wire sizing methods and tables.  These methods will assume conductors and terminals are rated for 75° C.  The first table to reference is 310.15(B)(16), where the 75° C Copper column will be used to select wire size based on the maximum calculated current.  For example, for a solar panel Isc rating of 8.5A, our maximum DC current is 13.26A.  Using table 310.15(B)(16), the minimum wire size #14 AWG.  For a solar inverter with a rated current of 25A, maximum AC current will be 31.25A.  This results in #8 AWG wire.  Conditions of use, if we had them, would be applied here to the overall ampacity of the wire to derate it and check against our maximum calculated current.  Conveniently, all of our conditions of use for this example are fixed at “1”.
Now that the wire size has been found for the maximum calculated current, it needs to be checked against worst case voltage drop in the conductor for the designed length of the circuit.  If the voltage drop would be too high the wire has to be upsized to the next size to correct for it.  The maximum voltage drop is 1.5% for DC and 1.5% for AC circuits based on my previous post. 
Calculating Voltage Drop
The heart of voltage drop calculations is Ohm’s law, which reads V (Voltage) = I (Current) x R (Resistance). The units are Volts, Amps, and Ohms, respectively.  Voltage drop calculations are based on values for Resistance (Ohms) per 1000 feet found in NEC tables in Chapter 9, Table 8 or 9 (DC circuits Table 8,  AC circuits Table 9). The value we need is for stranded, uncoated copper conductors.  This changes the formula from V = I x R to Voltage Change = I x (R / 1000’) x Total Circuit Length
Written another way, it can be expressed in terms of 1-way circuit length and converted into a percentage value:
Equation for Voltage Drop based on NEC 2011 Chaper 9 Tables 8 & 9

Going back to my example in Wire Sizing, let’s consider a DC and an AC wire run for a 100 foot distance and calculate the voltage drop for each.  For DC, we have:
#14 AWG Wire  300V String         I = 13.26A            L = 100 feet         R = 3.26 (NEC Chapter 9 Table 8)
VD = 2 x 100 x 13.26 x 3.14 / 1000 = 8.33V               8.33V / 300V = 2.78% Voltage Drop
Based on our maximum desired DC voltage drop of 1.5%, this obviously will not do.  We need to upsize the wire to get a better voltage drop.  Rerunning the numbers for a #12 wire gives:
VD = 2 x 100 x 13.26 x 1.98 / 1000 = 5.44V               5.44V/ 300V = 1.75% Voltage Drop
Close, but not quite what we wanted.  Obviously a #10 AWG wire is the way to go, but just to be sure:
VD = 2 x 100 x 13.26 x 1.24 / 1000 = 3.42V               3.42V / 300V = 1.10% Voltage Drop
That’s got it right there!  A #10 AWG wire will give us ~1.1% voltage drop for a 300V circuit over 100 feet (In general, I’ve found that a #10 wire is best for solar string wiring for most roofs).  Now that we’ve figured out DC voltage drop, AC voltage drop is pretty simple.  Using Table 9 of Chapter 9, we need to use the values in the column labeled “Alternating-Current Resistance for Uncoated Copper Wires”.  Going back to our earlier example:
#8 AWG Wire     240V AC               I = 31.25A            L = 100 feet         R = 0.78 (NEC Chapter 9 Table 9)
VD = 2 x 100 x 31.25 x .78 / 1000 = 4.875V               4.875V / 240V = 2.03% Voltage Drop
Again, we’re close but not quite where we want to be for this AC wire run.  Upsizing the wire to #6 results in:
VD = 2 x 100 x 31.25 x .49 / 1000 = 3.06V 3.06V / 240V = 1.28% Voltage Drop
Based on our examples, a #10 wire will give us a 1.1% voltage drop for a 300V, 8.5A string of solar panels over 100 feet.  A #6 wire will give us a 1.28% voltage drop for a 240V, 25A solar inverter over 100 feet.
Wire sizing and voltage drop calculations are an extremely important consideration for any solar designer or installer, and it’s essential to understand the fundamentals behind them.  The calculations and examples in this post have been kept simple on purpose, the subject can get complicated in a hurry when multiple conditions of use need to be applied.  In my next post, I will share some of my favorite tools and rules of thumb for calculating wire size and voltage drop.

NEC 2011
Conditions of Use: 310.15(B)(2), 310.15(B)(3)
Wire Ampacity: Table 310.15(B)(16)
Wire Resistance: Chapter 9 Tables 8 & 9