NREL is a national laboratory of the U.S. Department of Energy, Office of Energy 

Efficiency & Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

Contract No. DE-AC36-08GO28308

A Performance and Economic 

Analysis of Distributed Power 

Electronics in Photovoltaic 

Systems

Chris Deline and Bill Marion

National Renewable Energy Laboratory

Jennifer Granata and Sigifredo Gonzalez

Sandia National Laboratories

Technical Report

NREL/TP-5200-50003

January 2011 
NREL is a national laboratory of the U.S. Department of Energy, Office of Energy 

Efficiency & Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

National Renewable Energy Laboratory

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303-275-3000 • www.nrel.gov

Contract No. DE-AC36-08GO28308

A Performance and Economic 

Analysis of Distributed Power 

Electronics in Photovoltaic 

Systems

Chris Deline and Bill Marion

National Renewable Energy Laboratory

Jennifer Granata and Sigifredo Gonzalez

Sandia National Laboratories

Prepared under Task No. PVD9.1410

Technical Report

NREL/TP-5200-50003

January 2011 
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Table of Contents

Executive Summary ........................................................................................................................ 1

Introduction ..................................................................................................................................... 1

Background — Partial Shading and Mismatch Losses ................................................................... 3

DC-DC Converter Deployment and Topologies............................................................................. 5

General Benefits of Distributed PV Power Electronics .................................................................. 7

Particular Concerns for Systems Using Distributed PV Electronics .............................................. 8

Performance Analysis of Various DC-DC Topologies ................................................................... 9

Qualitative Economic Analysis of Expected Applications ........................................................... 12

Summary ....................................................................................................................................... 13

Acknowledgments......................................................................................................................... 13

References ..................................................................................................................................... 14

Additional Bibliography ............................................................................................................... 15
1 

Executive Summary

Distributed power electronics such as micro-inverters and DC-DC converters can help reduce 

mismatch and shading losses in photovoltaic (PV) systems. Under partially shaded conditions, 

the use of distributed power electronics can recover between 10%–30% of annual performance 

loss or more, depending on the system configuration and type of device used. Additional valueadded features may also increase the benefit of using per-panel distributed power electronics; 

these include increased safety, reduced system design constraints, and added monitoring and 

diagnostics. The economics of these devices will also become more favorable as production 

volume increases and integration within the solar panel’s junction box reduces part count and 

installation time. Some potential liabilities of per-panel devices include increased PV system 

cost, additional points of failure, and an insertion loss that may or may not offset performance 

gains under particular mismatch conditions. 

Introduction

A number of new products have come to the market in the field of distributed photovoltaic (PV) 

power electronics. This category of devices includes DC-DC converters and AC micro-inverters 

that are designed to either replace or work in concert with traditional central PV inverters. Recent 

improvements in the efficiency, reliability, and cost of these products have made them viable in 

many applications, from small residential installations to large commercial PV arrays. The 

breadth of options and claims among these various products shows that some differentiation 

exists between these devices. On the other hand, all of these devices share many similar benefits 

due to their distributed nature. This report intends to highlight the differences and similarities of 

these technologies and to provide some analysis of their benefits to power production and system 

economics.

In general, the use of power electronics at a per-module or per-PV string basis can reduce the 

impact of module mismatch and partial shading. A traditional central inverter will have only a 

few (typically one and rarely two or more) input channels that independently track the maximum 

power point (MPP) of the PV system. With large utility-scale inverters reaching up to half a 

megawatt (MW) in size, over 5,000 individual PV panels could potentially operate at one 

common peak power point. A reduction in the output power of one or more of these PV panels 

can lead to mismatch in the maximum power point between the various PV modules and strings. 

Possible causes of MPP mismatch include partial shading, soiling from dust, debris, and bird 

droppings, and module physical degradation. The impact of partial shading and mismatch can be 

reduced by increasing the number of independent MPP tracking channels in the PV system. The 

improvement from distributed MPP tracking depends on the amount of mismatch throughout the 

system, the size and configuration of the system, and the characteristics of its PV modules, 

among other factors. 

A DC-DC converter is one type of distributed power electronics that can provide such an 

improvement in system performance. These devices are also sometimes called power optimizers 

or power boosters. Rather than replacing a traditional central inverter, DC-DC converters work 

in conjunction with a central inverter, which is still required to convert DC power to AC grid 

power. However, the distributed electronics on each module or string help to de-couple the 

maximum power operating point of the individual modules or strings from the overall maximum 

power point of the system. A DC-DC converter will track the maximum power point of solar 

module(s) connected to it and either increase (boost) or decrease (buck) the output voltage to 
2 

match the optimum voltage requested by the central inverter. Many currently available solar DCDC converters use a separate enclosure for the power electronics at each panel, typically attached 

to the PV module frame or rack. Newer proposed versions of this technology involve partnering 

with PV module manufacturers to integrate the DC-DC electronics directly into the PV panel 

junction box. This convergence produces a so-called “smart junction box” or “smart panel” that 

provides some cost and labor savings over separate panel and power electronics. 

Another type of distributed PV electronics is the AC micro-inverter. While this technology made 

an appearance a decade ago as an integrated AC module, cost and reliability issues prevented the 

technology’s widespread adoption. The current generation of micro-inverter products appears to 

be achieving greater market penetration through improved efficiency, reduced cost, increased 

reliability, and diagnostic capabilities. AC micro-inverters are installed on each PV module, 

replacing the use of a central inverter. Each PV panel’s DC power is converted directly to AC 

120 V or 240 V and grid-tied. The output of each PV panel is therefore effectively in parallel, 

which eliminates power losses due to module mismatch. Thus the performance improvements 

that arise from independently peak-power tracking PV modules can be achieved with microinverters as well as with DC-DC converters. An additional benefit to micro-inverters compared 

with DC-DC devices is the reduction in DC balance of system components, including the central 

inverter. Also, voltages tend to be lower with micro-inverter systems, which could be a safety 

benefit for rooftop systems. Figure 1 shows some example topologies for per-panel microelectronics, including DC-DC converters and micro-inverters.

Figure 1. Schematic of conventional single-string PV system (top), DC-DC converter-equipped PV 

system (middle), and AC micro-inverter-equipped PV system (bottom).

String inverter

Electricity Grid Conventional series-connected PV panels

String inverter

PV panels connected to DC-DC converters Electricity Grid

PV panels connected to AC micro-inverters Electricity Grid

DC

AC

DC

AC

AC

Micro-inverters

DC-DC DC-DC DC-DC DC-DC
3 

There is also interest in reducing mismatch losses in larger installations, but perhaps not at the 

per-panel level. One strategy is to include DC-DC converters at the string level, which can 

reduce voltage mismatch between parallel strings. A boost converter can also provide a higher 

constant voltage to the central inverter, thereby reducing resistive losses and optimizing the DC 

operating point of the inverter. This type of string-level DC-DC equipment can be located inside 

or in place of a traditional combiner box, leading to the term “smart combiner box.” An example 

of this layout is shown in Figure 2. 

Figure 2. Schematic of a larger solar installation with multiple strings, each feeding into a “smart 

combiner box.” Maximum power point tracking is provided at the string level.

One aspect of distributed PV electronics that has yet to be addressed is the effect of their longterm reliability on the complete PV system. In general, the probability of system failure increases 

with each component in the system. Understanding the component-level reliability, failure 

modes, and effect of failure on system availability will be important in assessing the overall 

value of distributed PV electronics. 

Background — Partial Shading and Mismatch Losses 

The impact of shade and mismatch on PV systems has previously been studied, both with and 

without the use of DC-DC converters or micro-inverters [1-6]. Due to the variety of possible 

string configurations and module characteristics in PV systems, it is difficult to generalize how 

mismatch will affect a given system. However, in most PV systems with conventional silicon 

panels, the presence of shade or mismatch will have a greater than proportional impact on the 

system’s power output. This is due to the serial nature of PV modules in strings, which creates a 

“Christmas tree effect” in which current reduction in one series-connected module causes 

mismatch losses in the rest of the string. Because of this potential for greater power losses in 

mismatched systems (and for hot-spot safety concerns, which are not addressed here), solar 

module manufacturers typically include one or more bypass diodes in their modules, usually 

located in the module’s junction box. The function of the bypass diode is to allow current to flow 

past impaired sections of a module that are unable to produce as much current as the rest of the 

system. To accomplish this, the module section is shorted out by the diode, producing no power 

of its own. This bypass condition is preferable to allowing the shaded or impaired module to 

reduce the current of the entire string, thereby lowering production. Since the bypass diode shorts 

out the partially shaded section, causing its operating voltage to fall to zero, the overall operating 

voltage of the PV string will be reduced accordingly; see Figures 3 and 4. 

PV strings connected to a ‘smart combiner box’

Electricity Grid

Smart combiner box

DC-DC

Inverter

DC AC DC-DC

DC-DC
4 

+

- +-

-

+

+

-

I

+

V

-

2 N-1 N

1

+

D1

Shade 

Figure 3 (left). PV modules are shown in series, each module containing 3 bypass diodes. A 

bypass diode will typically protect 15-20 cells from shade and reverse bias. In this example, shade 

causes diode D1 to short out its substring of cells, reducing Module 1’s voltage by one third. 

Figure 4 (right). System I-V curve (red) and power [W] vs. voltage [V] curve (black) for a partially 

shaded two-string PV system. The impact of shade is shown by a reduction in operating current 

and MPP of the system at higher voltages. Note that under certain operating conditions, both local 

maxima and global maxima can arise in the power vs. voltage curve.

If the PV system in question is only composed of a single string, there is no additional impact 

due to the shaded string’s reduced voltage. However, if multiple parallel strings are present in the 

system, an additional source of mismatch loss occurs: voltage mismatch between parallel strings. 

In this situation, it’s the voltage—not the current—that needs to be equal between parallel strings 

of PV modules. Given this constraint, the voltage of a partially shaded string must remain high, 

even if bypass diodes are shorting out sections of the shaded string. This results in the MPP 

tracking inverter to force unshaded modules in the affected string to operate at a higher than 

optimum voltage to make up for the voltage drop from bypass diodes elsewhere in the string. 

This mismatch loss causes power losses in both shaded and unshaded modules. The impact of 

this voltage mismatch can range from an additional 60% power loss for unbalanced shade on 

two-string systems [5] to an additional 400% power loss for shade covering 15%–20% of a 

utility-sized PV string [7]. It is clear that partial shading and other mismatch sources can result in

performance losses much greater than the apparent scale of the shade itself. However, these 

worst-case values are not typically seen in real installations. An analysis of a shaded residential 

installation with somewhat more extensive shade than average showed a performance reduction 

of 22% due to shading from neighboring trees and other elements. Of this lost power, a majority 

(70%) was due to reduced irradiance and direct losses from shading. Only 30% of the power loss 

was due to mismatch of current and voltage [5]. Therefore, the installation of DC-DC converters 

or micro-inverters would improve this particular system’s annual production by roughly 7% 

through the elimination of mismatch losses.

The use of distributed MPP tracking equipment can also improve system performance if the PV 

panels have mismatch in their nameplate operating conditions, particularly maximum-power 

current (Imp). Although nameplate values are the same for identical module model numbers, there 

can be some variation from panel to panel, as manufacturers typically bin the modules in 5 W to 

10 W bins. There can therefore be a 5% difference or greater between the power output of 

modules with identical model numbers. This can contribute to mismatch losses between modules 

0

10

20

Volts

Current

0 50 100 150

0

500

1000 Power 

Global max [A]

Local max [B]
5 

in the same string if the mismatch is between the Imp of series-connected modules. Judging from 

datasheets of silicon PV panels in the 200 W to 240 W range, the variation in Imp within a single 

bin is typically 2%–3%. However, this does not directly indicate that the power loss within a 

series string is also 2%–3%, as seen in Figure 5. Because of the flatness of the I-V curve of a PV 

panel in the neighborhood of the maximum power point, a 2.5% change in operating current near 

the MPP only leads to a 0.5%–0.7% reduction in power for an average (0.72–0.74 fill factor) 

module. It is this 0.5%–0.7% series current mismatch loss that could be recovered through the 

installation of per-module MPP tracking equipment. Of course, not every panel in a string can 

have below-average Imp, so the real mismatch loss is likely to be lower than the previously stated 

limit, and efficiency losses in DC-DC devices may further reduce this benefit. 

Figure 5. Power vs. current curve for a typical Si PV panel (Sharp ND-208) with fill factor = 0.71. 

Note that current mismatch of 3% leads to only 0.5% power reduction.

DC-DC Converter Deployment and Topologies

Several different DC-DC converter device topologies are available for use with individual solar 

panels, each with different strengths and operating uses. The simplest DC-DC converter uses a 

single converter stage to either buck (reduce) or boost (increase) the output voltage of a PV 

panel. In either case, the PV panel output voltage is MPP tracked by the control algorithm in the 

device. A slightly more advanced DC-DC converter is the buck-boost converter, which uses 

both buck and boost stages to allow the converter to either increase or decrease the output 

voltage of a PV panel. A list of some available DC-DC converter devices and their topologies is 

given in Table I. 

7 7.25 7.5

206

207

208

Current (A)

Power (W)

- 0.5%

power

-3% current

Pmax
6 

Table I. Select commercially available distributed DC-DC devices 

Company Model Input V Power Topology Output V

Azuray AP250 14-80 V 250 W Buck 0-80 V

e-IQ energy vBoost 250 20-50 V 250 W Boost 250-350 V

Solar Edge PB250-AOB 5-65 V 250 W Buck/Boost 5-60 V

Solar Edge PB350-TFI 10-95 V 350 W Buck/Boost 5-60 V

Solar Magic SM1230-3B1 30-80 V 230 W Buck/Boost 0-86 V

Solar Magic SM3320 15-40 V 350 W Buck/Boost 0-43 V

ST Microelectronics SPV 1020 0-36 V 100 W† Boost† 0-36 V

Tigo energy MM-ES50 16-48 V 300 W Buck‡ 0-48 V

Tigo energy MM-EP35 28-42 V 280 W Boost 375 V

Xandex Solar SunMizer 15-48 V 250 W Buck 0-48 V 

† Preliminary spec, based on 3 devices per PV module.

‡Uses 'impedance matching' circuit, which is a buck converter with synchronous rectification [8]

The advantages of a buck-boost converter include an increased operating range and the ability to 

correct for a greater amount of system mismatch. Since the device includes two conversion 

stages rather than one, the increased flexibility may come at the cost of a slight efficiency 

reduction as well as possible size and cost increases relative to single-stage devices. 

In a buck-only DC-DC converter, the output voltage from a shaded panel is decreased, and the 

output current is increased to match the operating current of the unshaded modules in series with 

it. Because the current is boosted, there is no mismatch in current between the series-connected 

modules. There is no longer any need for the shaded module’s bypass diodes to begin 

conducting. Therefore, the panel equipped with the buck DC-DC converter can produce power at 

a reduced level without needing the bypass diodes to conduct. This type of converter works best 

in PV systems with limited mismatch, e.g., where shade or mismatch occurs only on a few PV 

panels. In this case, the buck DC-DC converter is installed only on those PV panels experiencing 

shade. An increase in annual production results from the partially shaded modules producing 

some limited amount of power (due to the diffuse component of irradiance that is still present 

even under shaded conditions) rather than no power at all. The amount of power that can be 

recovered depends on how diffuse the shade is, but it may account for half or more of the 

recoverable mismatch loss under certain conditions [5].

A boost-only DC-DC converter operates by taking the input PV voltage (typically at the 

maximum power voltage of the particular panel) and increasing it. This type of system is 

typically designed with every PV panel in the system equipped with a boost converter. In some 

systems, the converter boosts the voltage to a high constant value (~300 Vdc –550 Vdc), and all 

of the panels are placed in parallel. System mismatch is eliminated here because each panel 

contributes current proportional to the amount of irradiance it receives. This system will work 

even with panels facing different directions, or at different tilt angles, because all of the 

converters are placed in parallel. The high constant-output voltage from the boost converter is 

chosen to maximize the efficiency of a fixed-input voltage inverter connected to the output of the 
7 

converters. Another similar style of boost system places 10 to 20 of the PV modules and 

converters in series to achieve the high constant voltage of the inverter’s DC input. In this 

system, the amount of voltage boost is reduced for each converter, possibly improving its

efficiency or durability. Both of these boost converter systems can allow for mismatched module 

tilt and orientation—and even mismatched module size, brand, or technology (to a point). If 

multiple parallel strings are present, they can also be of different lengths within the input range 

of the fixed-input inverter.

A system using buck-boost converters enjoys most of the benefits of both buck and boost 

converters. For instance, if shade is limited to only a few modules in the system, a buck-boost 

converter can be selectively installed on only the affected modules, and it will operate in buck 

mode to reduce the current mismatch between shaded and unshaded modules. Also, if the PV 

system includes modules of different size, power rating, or orientation, a buck-boost converter 

can be placed on every module in the series string, allowing for differences in the various 

module power outputs. If parallel strings are of different lengths, buck-boost converters on the 

shorter string will increase the operating voltage of the string to match the other longer strings. 

Buck-boost converters can also be used with specialized fixed-input-voltage inverters that 

operate at a constant high input voltage. Because the converters can also buck the output voltage, 

they are also compatible with conventional input voltage inverters, which operate at a lower, 

variable voltage.

In addition to per-module DC-DC converters, which account for the majority of products 

currently available, other converter deployments are possible. On larger utility-scale arrays, perstring MPP tracking places the power electronics at the end of each series string. On a smaller 

scale, MPP tracking can be accomplished with three or more independent channels per module—

requiring access to the interconnection tabs within the PV panel. The benefit of finer (or coarser) 

MPP tracking resolution depends on the scale of mismatch within the system, balanced by the 

increased cost and complexity of additional MPP tracking channels.

General Benefits of Distributed PV Power Electronics

As stated above, there can be performance benefits to using per-panel distributed DC-DC and 

micro-inverter products based on the reduction in panel current and voltage mismatch. These 

advantages are primarily realized in residential and commercial installations, where localized 

shading and possible orientation mismatch are more common. Additional benefits include greater 

flexibility in system design and reduced time to engineer PV panel placement in complicated 

rooftop designs. This can lead to lower levelized cost of energy (LCOE) and possible reduction 

in balance of systems (BOS) wiring cost in the case of boost converters that operate at higher 

voltage and lower current, or micro-inverters that do not require string combiner boxes. 

Additional value-added features are present in many distributed PV power electronics, providing 

further benefit to the system owner and installer. System performance monitoring on a permodule basis is offered with some products, providing both PV energy production and converter 

health information. This is useful to the PV system installer as a remote diagnostic and warranty 

repair indicator, thereby helping to maximize system uptime. The owner also has more feedback 

from the PV system to understand what conditions influence PV performance, possibly leading 

to better system maintenance, cleaning, and snow removal. 
8 

In the case of micro-inverters, one specific value-added benefit is the elimination of the singlepoint failure of a central inverter. It is possible that individual independent power conversion 

devices might require more individual replacements, but since a single micro-inverter failure 

does not cause the entire system to fail, power reduction during a single failure is limited to the 

power of a single module. This advantage is balanced by the generally more difficult 

replacement of a micro-inverter on a rooftop underneath PV panels when compared with a 

typical wall-mount central inverter in a more easily accessible location. The overall lifetime of 

current micro-inverter products is also difficult to compare with central inverters, as these microinverters have not been available long enough to obtain field lifetime results. 

The safety aspects of certain distributed products may also influence their adoption. In 

conventional residential rooftop applications, a particular safety concern is the presence of high 

DC voltages (up to 600 V) even when the AC and DC disconnect are thrown. This can cause 

issues such as arc-fault damage to modules in the case of an internal module failure, and rescue 

personnel could be exposed to high voltage in the event of a rooftop fire. In the case of AC 

micro-inverters, these issues are mitigated because the system is de-energized when the AC 

disconnect is thrown. The available DC voltage is limited to that of a single module, which is 

considered benign. This arc-fault safety concern may also be mitigated by the use of per-module 

DC-DC converters, although further system tests would be required to verify this safety benefit. 

Particular Concerns for Systems Using Distributed PV Electronics

The inclusion of distributed PV electronics in a system may give rise to additional concerns or 

considerations for the system design or operation. For instance, the input voltage and current 

range of a given converter may require the use of a limited set of compatible PV panels. Also, 

the inverter that is used with a particular set of DC-DC converters might need to be specific, 

based on the output voltage of the converter and the MPP tracking algorithm of the string 

inverter. Certain combinations of inverter and DC-DC converter have been shown to lead to 

inverter input voltage instability because both the output voltage of the converters and the input 

voltage of an inverter are variable. This condition has been observed during testing at both the 

National Renewable Energy Laboratory and Sandia National Laboratories using different DCDC converter/inverter combinations. This particular condition may be mitigated by ensuring that 

at least one parallel string of PV panels is not equipped with DC-DC converters. This provides a 

stable input voltage for the inverter to track, with the parallel DC-DC converter-equipped strings 

matching this operating voltage. It may also be possible to work with equipment manufacturers 

to optimize the MPP tracking algorithms and DC-DC converter set point to improve stability. 

Output stability issues are typically discussed in a device’s operation manual. Another similar 

consideration is whether the manufacturer requires a blocking diode in series in which each PV 

string is equipped with DC-DC converters.

Regarding reliability, there is not yet enough field data nor independently measured accelerated 

testing to confidently assess the lifetime of these distributed power electronics. Currently, limited 

warranties are offered on some products for 15 to 25 years, with the longer warranties being 

offered on DC-DC converter products. This reflects the conventional wisdom that microinverters contain more life-limiting parts and also intertie to the grid, which exposes devices to 

power-surge induced failures. In general, the warranties and expected lifetime for distributed 

products are at least as good as those for traditional central inverters and are approaching the 

lifetimes of the PV modules with which they are seeking to become integrated. One aspect of 
9 

distributed PV electronics that has yet to be addressed is the effect of their long-term reliability 

on the complete PV system. In general, the probability of system failure increases with each 

component in the system. Understanding the component-level reliability, failure modes, and 

effect of failure on system availability will be important in terms of assessing the overall value of 

using distributed PV electronics. Despite these predictions of long service life, it is possible that 

the product could last longer than the manufacturer is able to replace or support it. Indeed, in the 

past year several product lines have been discontinued, with additional culling anticipated. In this 

climate, it is wise to evaluate the product’s interoperability with other similar products as well as 

what might happen if support is no longer available for a particular product in the future. For 

example, buck-only DC-DC converters that are installed on only a few PV panels in a system 

could be freely replaced with other buck-only or buck-boost converters. However, boost 

converters using a specialized central inverter might require identical replacement parts, leaving 

the system owner exposed to possible obsolescence risk. Similarly, micro-inverter interoperability may not be possible due to incompatible electrical connectors and in certain cases the 

use of proprietary powerline modem communications. In this case, a separate AC branch circuit 

would be required for the replacement micro-inverter device(s). 

Performance Analysis of Various DC-DC Topologies

Computer simulations were conducted of a variety of shade conditions and DC-DC converter 

deployments. It is difficult to account for every possible system configuration and mismatch 

condition, so a few example situations were considered. Three different “typical residential 

installations” were simulated, all based on an actual residential installation described in [5]. The 

residential installation is a 3-kW roof-mounted PV system with 14 mc-Si modules (Sharp ND208s). The system is modeled either as a single-string installation or two parallel strings. The 

shading on the system is somewhat more extensive than average, with an annual irradiance 

reduction of 20% as measured by a detailed site survey. A reduced shading condition is 

considered as well, in which the annual irradiance loss due to shade is only 10%, concentrated 

entirely on one of the two strings. 

In addition to the residential rooftop shading simulations, larger systems with inter-row shading 

were also considered. For these systems, it was assumed that rows are spaced such that 3% of the 

annual irradiance is lost due to inter-row shading. This value is consistent with industry practice 

to optimize the roof or land utilization of a PV installation. Sharp ND-208 PV modules were 

assumed for this simulation case as well. A ground coverage ratio of 0.54 and a module tilt of 

18.5o are assumed. Modules are oriented in landscape, with two modules stacked vertically and 

eight modules horizontally per row, shown in Figure 6. A total of ten rows are simulated in the 

commercial (33 kW) case.
10

Figure 6. Commercial row-row shading example with 18.5o tilt and 0.54 ground coverage ratio. The 

modules are stacked two high and eight wide for a total string length of 16. Ten strings are placed 

in parallel (figure courtesy of DOE/NREL’s Solar Advisor Model). 

To predict the performance gains from the use of distributed electronics, substring-level I-V 

curves were calculated and summed based on the predicted irradiance and shade on a given 

module substring. The performance of the DC-DC converter was modeled by a constant power 

curve, as discussed in [9]. DC-DC converter efficiency was set equal to 0.99 for all devices. This 

was done partly because detailed efficiency data for the different devices were unavailable, and 

also because the DC-DC efficiency loss will partly be offset by mismatch from soiling, aging, 

and manufacturer distribution of PV panels, which are all mismatch terms neglected in this 

simulation. Annual performance data is produced using the PVWatts engine [10], modified to 

allow for reduced irradiance due to partial shade. TMY3 meteorological data is used for Boulder, 

Colorado. The simulation results are provided below in Table II. 

 Table II. Annual performance of four PV systems, different DC-DC configurations* 

No DC-DC 

device

Buck DC-DC 

on all panels

Buck/Boost DCDC on all panelsPer-string DCDC

Rooftop system, 1 string. 20% shaded. Unshaded annual production : 4051 kWh

Annual energy: 3376 kWh 3440 kWh 3440 kWh 3349 kWh

Power lost to shade: -17 % -15 % -15 % -17 %

Shade loss recovered: N/A 10 % 10 % -4 %

Rooftop system, 2 strings. 20% shaded. Unshaded annual production : 4051 kWh

Annual energy: 3310 kWh 3427 kWh 3437 kWh 3364 kWh

Power lost to shade: -18.3 % -15.4 % -15.1 % -17 % 

Shade loss recovered: N/A 16 % 17 % 7 %

Rooftop system, 2 strings. Shade on one string only. Unshaded production : 4051 kWh

Annual energy: 3646 kWh 3693 kWh 3712 kWh 3682 kWh

Power lost to shade: -10.0 % -8.8 % -8.4 % -9.1 % 

Shade loss recovered: N/A 12 % 16 % 9 % 

Commercial system, 10 strings. 3% shaded. Unshaded annual production : 46.3 MWh

Annual energy: 44.9 MWh 44.5 MWh 44.5 MWh 44.2 MWh

Power lost to shade: -3.1 % -3 % -3 % -3.6 % 

Shade loss recovered: N/A 1 % 2 % -17 %

No DC-DC 

device

Buck DC-DC 

on all panels

Buck/Boost DCDC on all panelsPer-string DCDC

* DC-DC device efficiency = 0.99. Soiling, aging and distribution mismatch are neglected. 
11

For these particular installations, the presence of shade led to performance losses of 3%–18%. 

The addition of DC-DC converters with MPP tracking led to a recovery of 10%–20% of the 

annual loss due to partial shade, with more gain coming from systems with more parallel strings

and greater amount of shade. 

The above simulation results and additional results from literature are summarized in Table III 

below: 

Table III. Sources of mismatch loss cited in the literature

Type of mismatch System loss (est) Potential DC-DC gain* Ref 

Residential roof shade, 1 string 5-15% +15-20% of loss [5] 

Residential rooftop tree shading – 

multiple strings 

5-20% +20-30% of loss [5] 

Residential rooftop, pole shading – 

multiple strings

4-8% +40-70% of loss [6]

Commercial system with inter-row 

shading 

1-5% +0% of loss ** [Table II] 

Residential orientation mismatch 

within a string

1-20% ~100% of loss [11] 

Imp distribution mismatch 0.2 - 1% ~100% of loss [Fig. 4] 

Soiling – CA and Southwest US 1.5 – 6.2% +15-40% of loss [12]

* For typical cSi PV panels and per-panel DC-DC devices, not accounting for device efficiency and insertion loss

** In this simulation, the small amount of inter-row shading was not enough to benefit from DC-DC devices when 

device efficiency is accounted for. 
12

Qualitative Economic Analysis of Expected Applications

The relative benefit of distributed PV electronics depends on the system configuration, the 

amount of current and voltage mismatch within the system, and the cost of the PV electronics. A 

greater performance improvement could support a greater equipment cost. In some cases, the 

system is limited by the available space, and performance improvement is sought above all other 

considerations. In other cases, there is available space to include additional PV panels, and the 

increased performance provided by distributed power electronics must be compared with the 

alternative of including additional solar panels to increase production. 

In general, distributed power electronics manufacturers seek to reduce installation time and 

component expense by integrating directly with PV panels inside the junction box. This also 

streamlines distribution channels, but effectively commoditizes the “smart junction box” which 

may prevent the electronics manufacturer from differentiating. PV module manufacturers will 

likely push for more uniformity and interoperability, in addition to demanding lower margins for 

junction box electronics. This model of pre-integrated power electronics—which stands a chance 

to reduce component costs enough to become cost-competitive on a $/W basis—works best with 

new rooftop installations that include the power electronics on every panel. The compatible 

technologies include micro-inverters, high-voltage boost converters, and buck-boost DC-DC 

converters. In the case of buck DC-DC converters which are intended for installation on only a 

few PV panels experiencing isolated shade, using a module-integrated “smart junction box” may 

not make the most sense, as the amount of shade and power loss may not be apparent until after 

the system is installed. Buck converters may work the best as a standalone retrofit device, similar 

to the first-generation DC-DC converters available now. However, one advantage of selective 

installation on only a few modules is that performance improvement can be achieved with a 

much lower part count (and cost) than installing the converter devices on each module in a 

string. This configuration also minimizes reliability risks based on part count. Another 

application that may warrant the use of either the pre-integrated or standalone retrofit DC-DC 

converters could be an expansion of an existing PV installation. If the existing central inverter 

were oversized, using DC-DC converters could allow mismatched modules to be included into 

the existing array, even if the existing array’s modules are no longer manufactured. 

Alternatively, a separate AC branch circuit could be supplied to micro-inverters to increase the 

capacity of an existing PV array.

In the case of larger PV installations 100 kW or greater, the prospect of per-module power 

electronics is limited. This is because the uniformity of such large installations is usually better 

than on a residential or commercial warehouse rooftop. Also, the additional part count, 

monitoring and installation time, and desire to reduce levelized cost of electricity is a 

disadvantage to per-module power electronics in large installations. In the case of isolated 

shading from nearby obstructions, individual modules could be fitted with retrofit buck or buckboost DC-DC converters. If there is some nonuniformity on a per-string basis—for instance, if 

the installation is on rocky or otherwise un-level ground—there could be an advantage to using 

per-string MPP tracking. This technology can be integrated into the equivalent of the existing 

DC combiner boxes, so no additional installation or part count is required. In the case of boost 

DC-DC converters, higher DC voltages between the combiner box and the inverter can lead to 

reduced I2

R wire losses or lower wiring cost. Also, the central inverter can be designed to 

operate at a fixed high voltage, further reducing complexity and cost. In certain applications, this 
13

technology may be cost effective, particularly if the benefit of per-string monitoring is included, 

with its concomitant improvement in system up-time and reduced O&M costs.

Summary

Distributed power electronics have the potential to reduce PV performance loss due to partial 

shade and mismatch. Depending on the mismatch condition and system size, a variety of 

products are available to help improve system performance. The benefits of per-module power 

electronics are greatest for multi-string residential installations with close-in shade obstructions 

or mismatch from orientation or panel size. Value-added features of some devices include 

performance monitoring and emergency power-off, which may assist in market penetration, 

along with reduced cost through integration within PV panels’ junction boxes. While the 

performance aspects of these distributed electronics can be assessed, it is still unclear what effect 

the introduction of more components and complexity will have on system reliability. Larger PV 

installations may prefer to install string-level DC-DC equipment to achieve some of the 

mismatch reduction benefits of distributed power electronics without the part count and cost of 

per-panel electronics.

Acknowledgments

Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin 

Company, for the United States Department of Energy’s National Nuclear Security 

Administration under contract DE-AC04-94AL85000. 
14

References

1. M. Gross, S. Martin, N. Pearsall, “Estimation of Output Enhancement of a Partially Shaded 

BIPV Array by the Use of AC Modules,” IEEE Photovoltaic Specialists Conference, Anaheim, 

CA, 1997. 

2. A. Woyte, J. Nijs, R. Belmans, “Partial Shadowing of PV Arrays with Different System 

Configurations: Literature Review and Field Test Results,” Solar Energy 74, pp. 217-233, 2003. 

3. N. Chaintreuil, F. Barruel, X. Le Pivert, H. Buttin, J. Merten, “Effects of Shadow on a Grid 

Connected PV System,” 23rd European Photovoltaic Solar Energy Conference and Exhibition, p. 

3417, 2008. 

4. C. Deline, “Partially Shaded Operation of a Grid-Tied Photovoltaic System,” IEEE 

Photovoltaic Specialists Conference, Philadelphia, PA, 2009. 

5. C. Deline, “Partially Shaded Operation of Multi-String Photovoltaic Systems,” IEEE 

Photovoltaic Specialists Conference, Honolulu, HI, 2010. 

6. S. MacAlpine, M. Brandemuehl, R. Erickson, “Analysis of Potential for Mitigation of 

Building-Integrated PV Array Shading Losses Through the Use of Distributed Power 

Converters,” ASME 4th International Conference on Energy Sustainability, Phoenix, AZ, 2010. 

7. C. Deline, “Characterizing Shading Losses on Partially Shaded PV Systems,” PV 

Performance Modeling Workshop, Albuquerque, NM. NREL Publication PR-5200-49504. 

8. B. Burger, B. Goeldi, S. Rogalla, H. Schmidt, “Module Integrated Electronics – An 

Overview,” 25th European Photovoltaic Solar Energy Conference and Exhibition, Valencia, 

Spain, 2010. 

9. P. Tsao, “Simulation of PV Systems with Power Optimizers and Distributed Power 

Electronics,” IEEE Photovoltaic Specialists Conference, Honolulu, HI, 2010. 

10. B. Marion, M. Anderberg, “PVWatts – An Online Performance Calculator for GridConnected PV Systems,” ASES Solar 2000 Conference, June 16-21, 2000, Madison, WI.

11. S. MacAlpine, M. Brandemuehl, L.Linares, R. Erickson, “Effect of Distributed Power 

Conversion on the Annual Performance of Building-Integrated PV Arrays with Complex Roof 

Geometries,” ASME 3rd International Conference on Energy Sustainability, San Francisco, CA, 

2009. 

12. A. Kimber, L. Mitchell, S. Nogradi, H. Wenger, “The Effect of Soiling on Large GridConnected Photovoltaic Systems in California and the Southwest Region of the United States,” 

IEEE 4th World Conference on Power Conversion, Waikoloa, HI, 2006. 
15

Additional Bibliography

N. Femia, et al., “Distributed Maximum Power Point Tracking of Photovoltaic Arrays: Novel 

Approach and System Analysis,” IEEE Transactions on Industrial Electronics 55, pp. 2610-

2621. 

G. Walker, P. Sernia, “Cascaded DC-DC Converter Connection of Photovoltaic Modules,” IEEE 

Transactions on Power Electronics 19, pp.1130-1139, 2004. 

S. Rogalla, B. Burger, B. Goeldi, H. Schmidt, “Light and Shadow – When is MPP-Tracking at 

the Module Level Worthwhile?” 25th European Photovoltaic Solar Energy Conference, 

Valencia, Spain, 2010. 

S. Vighetti, J. Ferrieux, Y. Lembeye, J. Barbaroux, “Photovoltaic Module and Shadow: Study 

and Integration of a Current Balancing System,” 25th European Photovoltaic Solar Energy 

Conference, Valencia, Spain, 2010. 

B. Sadlik, T. Wardle, V. Nebusov, L. Rubin, “PV Module with Effective Shade Protection,” 25th

European Photovoltaic Solar Energy Conference, Valencia, Spain, 2010. 

H. Berger, M. Midl, A. Kaufmann, T. Sivaraman, “Module Integrated MPP Trackers: Efficiency 

Comparison of Suitable Converter Topologies,” 25th European Photovoltaic Solar Energy 

Conference, Valencia, Spain, 2010. 

N. Sridhar, D. Freeman, “Solar Module and Array Characterization under Partially Shaded 

Conditions at Low and Medium Level Illumination Conditions,” 25th European Photovoltaic 

Solar Energy Conference, Valencia, Spain, 2010. 

F. Pulvirenti, A. La Scala, D. Ragonese, K. D’Souza, “DC-DC Converters with MPPT Control 

for Photovoltaic Panels,” 25th European Photovoltaic Solar Energy Conference, Valencia, Spain, 

2010. 

N. Martin, et al., “Electrical and Thermal Characterisation of PV Modules under Partial 

Shadowing,” 23rd European Photovoltaic Solar Energy Conference, Valencia, Spain, 2008. 

N. Chaintreuil, et al., “Effects of Shadow on a Grid Connected PV System,” 23rd European 

Photovoltaic Solar Energy Conference, Valencia, Spain, 2008. 
F1147-E(10/2008) 

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A Performance and Economic Analysis of Distributed Power 

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14. ABSTRACT (Maximum 200 Words)

Distributed electronics like micro-inverters and DC-DC converters can help recover mismatch and shading losses in 

photovoltaic (PV) systems. Under partially shaded conditions, the use of distributed electronics can recover between 

15-40% of annual performance loss or more, depending on the system configuration and type of device used. 

Additional value-added features may also increase the benefit of using per-panel distributed electronics, including 

increased safety, reduced system design constraints and added monitoring and diagnostics. The economics of these 

devices will also become more favorable as production volume increases, and integration within the solar panel’s 

junction box reduces part count and installation time. Some potential liabilities of per-panel devices include increased 

PV system cost, additional points of failure, and an insertion loss that may or may not offset performance gains under 

particular mismatch conditions.

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Photovoltaic system; shading; mismatch; DC-DC converter; power optimizer

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