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Contract No. DE-AC36-08GO28308

Modeling Microinverters and DC 

Power Optimizers in PVWatts

Sara MacAlpine and Chris Deline

National Renewable Energy Laboratory

Technical Report

NREL/TP-5J00-63463

February 2015
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

This report is available at no cost from the National Renewable Energy

Laboratory (NREL) at www.nrel.gov/publications.

Contract No. DE-AC36-08GO28308

National Renewable Energy Laboratory

15013 Denver West Parkway

Golden, CO 80401

303-275-3000 • www.nrel.gov

Modeling Microinverters and 

DC Power Optimizers in 

PVWatts

Sara MacAlpine and Chris Deline

National Renewable Energy Laboratory

Prepared under Task No. SS13.5020

Technical Report

NREL/TP-5J00-63463

February 2015
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Introduction

Module-level distributed power electronics including microinverters and DC power optimizers 

are increasingly popular in residential and commercial PV systems. Consumers are realizing 

their potential to increase design flexibility, monitor system performance, and improve energy 

capture. It is becoming increasingly important to accurately model PV systems employing these 

devices, with numerous commercial [2-5] and independent research [8-11] publications 

providing modeling strategies and guidelines. This document summarizes existing published 

documents to provide uniform, impartial recommendations for how the performance of 

distributed power electronics can be reflected in NREL’s PVWatts calculator

(http://pvwatts.nrel.gov/).

PVWatts performs simplified PV system performance calculations using information about the 

array’s geographic location, orientation, and a set of user-input loss factors [1]. PVWatts’ 

default values and recommendations for these loss factors are designed for a PV system with a 

central inverter, and do not account for use of microinverters or DC power optimizers. The 

following sections provide guidelines for choosing reasonable values to use in PVWatts to model 

systems with distributed power electronics. 

Effects of Distributed Power Electronics in Simulation

While all of PVWatts’ loss factors can be tailored to represent individual system characteristics, 

a subset are particularly affected by use of distributed power electronics. Note that this 

document specifically addresses system losses used in PVWatts Version 5, which was updated in 

September 2014 and has changed slightly from previous versions of the tool [1].

Inverter: Version 5 of PVWatts allows the user to enter a nominal inverter efficiency. For 

microinverters, this factor should simply be the California Energy Commission (CEC) weighted 

efficiency; this is no different from a system with a central or string inverter. DC power 

optimizers’ loss term should include the combined (multiplied) efficiency of the central inverter 

and the optimizer devices. Efficiency in each of these cases is assumed to include the device’s 

effectiveness of maximum power point tracking. At present the CEC database does not include 

information on weighted DC power optimizer efficiency; however, this information may be 

provided by the manufacturer. If it is not, a value of 99% is recommended for newer devices 

[4,7], and a lower efficiency of 97.5% [10] is recommended for older technologies. 

Mismatch: All module level power electronics are expected to eliminate mismatch losses 

caused by performance variation between modules. This loss factor should be 0% for all 

distributed electronics technologies.

Wiring: In a PV system with microinverters, DC wiring losses are effectively eliminated. 

However, AC wiring losses will increase since there are more and longer AC wiring routes. A 

recommended guideline for the wiring loss term is 2% for the entire system [6]. Wiring losses 

remain relatively unchanged in a system with DC power optimizers.

System Availability: Due to improved system monitoring capabilities, distributed power 

electronics may decrease PV system downtime, with published reports indicating that there may 

be potential to increase availability by a total of 1-2% [4]. While module-level conversion 
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mitigates losses associated with point failures in a PV system, it may also result in longer time to 

fix these failures, as their individual impact on system performance may be too small to justify 

the expense of service calls. Without concrete evidence to support increased PV system 

availability, it is recommended that this category of losses remain unchanged, particularly if 

PVWatts is being used to model the first year of a PV system’s operation when equipment 

failures are less likely.

Shading: Both microinverters and DC optimizers have the potential to recover some of the 

power that is lost in partially shaded PV systems with central inverters, when obstacles block the 

sun’s incident beam radiation. The fraction of the losses recovered are referred to in [4] as the 

Shade Mitigation Factor (SMF). This value represents the annual percentage of shading losses 

that can be recovered through the use of distributed electronics in partially shaded PV systems.

The SMF can be used to calculate an updated shade loss term by the following equation:

Shaaa LLLL (n n) = Shaaa LLLL % ∗ (1 − SSS)

Independent research [8,9,11] indicates that a reasonable range for distributed electronics’ SMF 

is 0.25-0.4, with an individual array’s SMF depending on shade distribution and extent, as well 

as the array configuration. The shade benefit of distributed electronics can vary with the size of 

the PV system, and the type of shading. A default SMF value of 0.33 is recommended here as an 

average value. 

Soiling: Soiling is generally uniform on PV systems, so it is not anticipated that distributed 

power electronics would appreciably affect most arrays’ soiling losses. However, in very large 

arrays with regional soiling differences, or in arrays made up of modules with different 

orientations or tilts from one another, there may be an opportunity for some fraction of the losses 

to be mitigated, similar in form to the above-described Shade Mitigation Factor.
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PVWatts System Losses and Inverter Efficiency

Table 1. Recommended PV Watts System Losses and Inverter Efficiency with Microinverters or 

Power Optimizers

Soiling 2%

Soiling often mostly uniform, but 

may have regional differences at 

some sites. In these cases some 

losses may be mitigated.

Shading 3%

If shade loss% assumed to vary 

linearly with area of array shaded, 

SMF=0. 

Else SMF range = 0.25-0.4. 

Snow 0%

Mismatch 2%

Wiring 2%

2% value for microinverters 

accounts for increased AC and 

decreased DC losses.

Diodes and Connections 0.5%

Light Induced Degradation 1.5%

Nameplate Rating 1% See manufacturer's datasheet

Age 0%

Microinverters and power 

optimizers may mitigate agerelated losses caused by increased 

mismatch. 

System Availability 3%

If system repair is performed 

promptly for any failures, losses 

may be reduced to 1-2%.

Inverter Efficiency 96%

If no CEC efficiency available for 

power optimizer, use 0.99 for 2012 

and newer, else 0.975.

Microinverters: use CEC efficiency 

DC Power Optimizers: multiply CEC 

efficiencies (optimizer * central inverter)

Distributed Power Electronics 

Recommended Value

No Change

PV Watts 

Default

No Change

No Change

System Loss Type Notes

No Change

For near obstacles (not horizon blocking) 

Shade Loss% * (1-SMF) 

Default SMF=0.33 

No Change

0%

No Change

No Change

No Change
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References 

[1] Dobos, Aron P. (2014). PVWatts Version 5 Manual. Retrieved September, 2014 from 

http://pvwatts.nrel.gov/downloads/pvwattsv5.pdf.

[2] Enphase Energy, Inc. (2011). PVWatts Calculation Values with an Enphase Microinverter 

System. (Technical Brief). Retrieved from http://enphase.com/wpuploads/enphase.com/2011/08/Enphase-Technical-Brief-PVWatts-Calculations.pdf.

[3] Bartlett, V. (2013). PVWatts Derating Factors for SolarBridge Pantheon Microinverter and 

ACPV Systems. (Version 1.5). Austin, TX: SolarBridge Technologies. Retrieved from 

http://solarbridge.wpengine.netdna-cdn.com/wp-content/uploads/2013/04/PVWatts.FIN_.pdf.

[4] Solar Edge. (n.d.). Estimating Energy in PVWatts. (Application Note). Retrieved from 

http://www.solaredge.com/files/pdfs/simulating_solaredge_pv_watts_app_note.pdf.

[5] Tigo Energy. (n.d.) Modeling Guide For Smart Modules. (Technical Brief). Los Gatos, 

CA: Author. Retrieved from 

http://www.tigoenergy.com/sites/default/files/modeling_smart_modules_v2.pdf.

[6] Enphase Energy, Inc. (2013). Calculating AC Line Voltage Drop for M250 Microinverters 

with Engage Cables. (Technical Brief). Retrieved from 

http://enphase.com/global/files/EnphaseTechBrief_Vdrop_M250.pdf.

[7] Tigo Energy. (2013). Datasheet Junction Box ES Series. (Datasheet). Los Gatos, CA: 

Author. Retrieved from 

http://www.tigoenergy.com/sites/default/files/mmj_es_datasheet_11.19.2013.pdf.

[8] Deline, C.; Meydbray, J.; Donovan, M. (2014). Photovoltaic Shading Testbed for ModuleLevel Power Electronics: 2014 Update, NREL Report No. TP-5J00-62471. Retrieved from 

http://www.nrel.gov/docs/fy14osti/62471.pdf

[9] MacAlpine, S., Erickson, R., & Brandemuehl, M. (2013). Characterization of power 

optimizer potential to increase energy capture in photovoltaic systems operating under nonuniform conditions. IEEE Transactions on Power Electronics, 28(6), 2936-2945. 

[10] Deline, C., & MacAlpine, S. (2013, September). Use conditions and efficiency 

measurements of DC power optimizers for photovoltaic systems. In Energy Conversion 

Congress and Exposition (ECCE), 2013 IEEE (pp. 4801-4807). IEEE.

[11] Hanson, A. J.; Deline, C. A.; MacAlpine, S. M.; Stauth, J. R.; Sullivan, C. R. (2014). 

Partial-Shading Assessment of Photovoltaic Installations via Module-Level Monitoring. IEEE 

Journal of Photovoltaics. Vol. 4(6), November 2014; pp. 1618-1624.