Design Tools

One of the most important aspects of designing a solar array is sizing module strings to operate within the parameters of the selected inverter. Renewable Energy Associates has developed a tool to help in that process. Most inverter manufacturers offer some type of sizing tool, whether it’s simple or advanced, it’s usually limited to selecting only their products. The REA System Sizing Tool lets you select from a wide variety of products and manufacturer’s.

Our tool is free and available for download just click the image below and you will be asked to answer a few questions about yourself before downloading.

REA String Sizing Tool v8.2

Click to download the REA String Sizing Tool v8.2

– Thousands of modules.
– Hundreds of inverters.
– Add your own module or inverter.
– World Wide ASHRAE locations. 5,000+
– Create your own custom sites.
– Dual MPPT’s configuration possible.
– Voltage drop calculator.
– Performance calculator.
– Quick Printing features.

1. The tool now requires you to turn on macros. For those concerned about security we can not guarantee that the tool will work well, accurately or at all without macros enabled.
2. We will no longer be taking request for additional micro-inverters to be added to the tool as the tool is a string size calculator and there are no strings with micro inverters.
3. Request for additional equipment will require a spec sheet of the product you are requesting.

Have a question about the string sizing tool or a suggestion which might make it better? Feel free to leave a comment below. Updates will be made periodically.


Design Tools — 109 Comments

  1. Hi John – Are these sheets updated to manually input 1000 V inverters..?

    • If you mean are you able to manually enter in a 1000V inverter then the answer is yes. If your asking if the current 1000V inverters are up to date then the answer is no. We make updates periodically but don’t have the time or resources to keep the tool updated to the latest equipment. Thanks

  2. Update v8.2

    – Minor text formatting

    – SMA TL inverter updated maximum power value.

    Added Modules:
    – Itek Energy 255-260 HE
    – LG 295W
    – Solarworld 275W
    – CentroSolar E235-E250.

    Don’t forget to Donate! Thank you in advance.

  3. Ok, well you do know that the popup advertisement is so big that it obscures the data at the bottom of the page right? I’d be willing to pay for a version that doesn’t do that and which would allow the insertion of my company logo and information to use for print outs I could attach to my design submittals. I can make my own spreadsheet but I like that you’ve got all of the ASHRAE weather site data already in yours and available as pull downs. That saves look up time and manual input

    • This only appears to be an issue on your side as my printouts look fine and the data is not obscured. I would suggest adjusting the margins on your printer settings. We will take your offer of purchase into consideration.

      If you have time please consider making a donation as updates and maintenance of our sizing tool take time. Thanks!

  4. John, what I’m saying is when I try to print the calculations, there is a popup advertisement for REA that drops onto the bottom of the page and is part of the print out. On top of that, the donate button is at the top of the print out as well. I’m assuming this tool is not really for printing?

    • I believe the printer you are using is adjusting the margins. Either adjust the margins so that they are more narrow or print to PDF and then try printing the PDF document. You can also try scaling to fit the page. What operating system and or version of excel are you using? We will try to fix this on the next update. Thanks for letting us know about this issue.

      FYI: I just successfully printed from a PC using Excel 2010 onto an HP printer without making any adjustments and it printed fine.

    • If you are asking if there is a way to remove the ad then the answer is no, not at this time. If we have the String Sizing Tool developed into a paid for service or application this might be something we consider. Thanks

  5. When I print there is an advertisement at the bottom of the page obscuring part of the data. How do I get rid of that?

    • We will adjust the margins for optimizing the printing function on the next update. If you have an Adobe PDF printer installed you can use that option to create a PDF of the results for now.

    • Is this something you found by comparing the REA String Sizing Tool to SMA’s Sunny Design Tool? If you could provide a printout of the exact error that you are finding that would be great. Thanks

  6. Update v8.1

    – Minor text formatting

    – Voltage drop for three phase systems was not calculating correctly.

    – ELTEK updated maximum power value
    – Added SMA 3000TL, 4000TL and 5000TL

    – Suniva MVX group and minor text formatting

    We are now excepting donations which will help enable monthly updates. The tool is a free resource with very minimal obligations and we would appreciate all the help we can get. Thanks!

  7. It seems there is a descrepency in all THEIA_HE inverters (Theia HE-t UL 2.0kW, Theia HE-t UL 2.9kW, Theia HE-t UL 3.8kW
    Theia HE-t UL 4.4kW) between maximum input power and nominal power.

    The database is using nominal input DC power instead of maximum recommended power for input, causing “PV array input power exceeds inverter input ability”.

    For example for Theia HE-t UL 4.4kW, if I use 10 panels (IT250w) 2 strings, the array size is too big, but if I use 9 panels 2 string, array is too small, the only option is available to 13 panels max in single string, which makes this inverter way too inefficient and under power.

    Would you please correct this.

  8. can you update the file to allow for horizontal scrolling, the way it stand you cannot zoom in.

    • You should be able to zoom by holding the ctr key and scrolling with your mouse wheel. If this doesn’t work for you we will consider enabling horizontal scrolling on the next update.

  9. Hi Jon,

    Thanks for adding in the SunPower. Now that you are doing the duel MPPTs what would it take to add the SMA 3000TL, 4000TL and 5000TL

    • Andrew,
      We will add those inverters on the next update. In the meantime you can add them yourself to the custom inverters list located on the User Input sheet which you can see near the bottom of the REA String Sizing Tool v8 Excel screen.


  10. Update v8

    Major Changes:
    – Colors
    – Text
    – General look and feel changes.
    – Combined Voltage Drop and Performance tab.
    – World Wide ASHRAE locations added. 5,000+
    – Create your own custom sites.
    – Calculations using inverters with two MPPT’s now possible!
    – Simplified voltage drop calculator.
    – Quick Printing features

    New Modules:
    Suniva (All Current and available)
    SunPower (All Current and available)
    MEMC (All Current and available)

    New Inverters:
    AEG (All Current and available)
    Chint Power (All Current and available)
    ELTEK (All Current and available)
    Enphase M250 added for those wanting to do voltage drop calculations.

  11. Hello, Please Add AEG Inverters data in your Tool Data base




    Please it will greatly help us…. & also how add Manually Location, Country, city since My location is not there in your data base (India – Asia Region)


    • Looking at the spec sheets for those inverters I don’t see why you couldn’t add them to the custom equipment list on your own. If for some reason you are unable to do so send us an email with the spec sheets of the inverters you wish to have added and we will try to include them on the next update.

      You can also use the manual temperature data entry mode to use data that is not already included in the data set. For ASHRAE locations this is a great resource:

      We will be releasing a new version soon that will include ASHRAE locations world wide and several other modifications.

  12. Please add these inverters
    Theia HE-t UL 2.0kW
    Theia HE-t UL 2.9kW
    Theia HE-t UL 3.8kW
    Theia HE-t UL 4.4kW


    • From first glance I don’t know why you couldn’t add these to the custom equipment list. If for some reason you are unable to do this send us a spec sheet and we will try to include them on the next update.

  13. I’m using your tool on a mac and it works but says visual basic macros don’t open. Do you have a different version for mac users, or can I just use it as I am now?

    • I have not used our tool on a mac and am unfamiliar with using excel on mac’s in general. Most of the VBA in the tool has to do with warnings and errors that might occur with specific equipment or scenarios selected. There might be a way to enable macros on Mac’s, I am not sure. If the tool is working for you, I think it’s probably fine.

  14. Update v7.5.6

    Added Inverters:
    SMA 6000TL,7000TL,11000TL
    note: The SMA 3000TL, 4000TL and 5000TL will not be added to our tool as they have two MPPT.

    Many central inverters were showing “0” for the maximum number of strings in parallel.

  15. Updated v7.5.5

    Added Modules:
    Solarworld 265W Module
    Solarworld 270W Module
    Yingli YL285P-35b
    Yingli YL290P-35b
    Yingli YL295P-35b
    Yingli YL300P-35b
    Silicon Energy SiE160
    Silicon Energy SiE165
    Silicon Energy SiE170

    Added Inverters:
    Altenergy Power Systems Micro Inverters

    1. You can now select the ASHRAE .4% and 2% or manually enter your own temperature.
    2. There is now a tab to input your own custom equipment. You can manually enter nine additional modules and inverters.

    1. The string sizer is not setup for inverters with multiple MPPT. There might be a couple inverters (Power-One) included in our tool that have this feature but our results will vary from what the manufacturers will recommend.
    2. We will no longer be taking request for micro-inverters to be added to the tool as the tool is a string size calculator and there are no strings with micro inverters. If you wish to have a micro inverters added so that you can calculate voltage drop we would suggest following the manufacturer recommended install procedure typically found in the install manual or on the manufacturers website.
    3. Request for additional equipment will require a spec sheet of the product you are requesting.

    This is a service provided by us at no cost to you so please let us know what you think of it and if you find errors or have problems do not hesitate to contact us.

    • After you provide your information and the download window opens you are presented with two options: Run and Save. Save the file to an appropriate location on your computer and you just have to open the file again from that location to use the sizing tool.

    • It states “Resize System” now, no matter what variables are entered in the spreadsheet. Is this an error in the program?

    • Be sure to read the Disclaimer on the “Disclaimer” tab and select “Yes” and enable editing and macros on the spreadsheet. If there is not a message bar near the top of the screen that shows when you open the file which asks if you would like to to enable macros check in File/Options/Trust Center/Trust Center Options and make this file a trusted location.

      If this doesn’t resolve the issue email me the file that is showing the “Resize System” error with all of the parameters which result in this error selected.

  16. Hi,

    While measuring max Vmp at the given ambient temperature, why does the E17 cell value has different temperature? I used 37.8 C as my locations maximum ambient temperature. Design tool uses 72.8 C which is wrong in my opinion. Is there any reason for doing this?

    • The ambient temperature of your location is not the same as the module cell operating temperature. The difference between the two temperatures is dependent on your selection of the mounting method. Since the difference is 35°C you have selected a “Roof” mounted PV system. This is a standard industry practice and is discussed in the String Sizing Instructions section at the bottom of the String Sizing tab in our tool.

    • We could not find the Sunpower 435W module. Please provide us with a spec sheet and we can add it. The latest version of the tool has a custom equipment tab which enables the user to add their own equipment. If you have any questions about this process don’t hesitate to ask.

  17. Great tool, I do a lot of Excel programming for solar volage drop for large arrays and I applaud the effort. Have you though about adding conduit type for voltage drop. In the DC there are only slight changes between Steel and ALuminum/PVC, but in the AC voltage drop there is a difference between Magnetic and Non-Magnetic conduits which affect the voltage drop. Checkout the Bussmann charts on long runs it does make a difference. Ken

    • Hi Ken, I use Excel a lot for Solar PV design and would appreciate some advice on your excel sheets and programming if you are willing to share? I’m based in Durban South Africa so won’t be affecting your sales turf. My email address is Going to trial the blue software now too.
      Thanks again

  18. Would you please add APS Micro-inverter YC-200-NA made by Altenergy Power Systems Inc in your next revision.


  19. i would like to add a module that’s not listed, why can’t you unprotect the sheet so i can edit it.
    please yinli 300w poly, thanks

    • I will add that module on the next update. The data sets contained in the tool belong to us and considering how the data has been integrated into the tool you risk incorrectly entering the new data and corrupting subsequent sections of the tool. A future option might be a section where you can enter you own equipment specifications. Thanks.

  20. John,
    Try suntechSTP290 + solaron5001KV in Vegas rack mounted.  Min size String is larger than Max

  21. Updated v7.5.1

    LTD Modules

    Samsung Modules

    Advanced Energy Inverter
     -AE 500

    SMA Inverters
     – The REA tool was not displaying an error when the inverters were overloaded.

  22. Great tool.. thanks for all the work on this!!

    Quick question: if a SunnyBoy 2500HF-US is selected, along with SW245 polys, and the string size is set to 14, with 1 string in parallel, the total STC watt rating is 3,430, which is over the Maximum Input Power (WDC) of 3,125 W – is this to be expected, or is this a data check missing from the spreadsheet, or…?


    • Thanks for finding that! We had some numbers stored as text and the calculations were not showing up as an error. I will make that change on the next update.

  23. Updated version 7.4.3
    – Global Solar PowerFLEX BIPV 250/275/300W

  24. Updated version 7.4.2
    Solarworld poly modules TC values updated
    Solarworld 245 & 250 mono black modules added
    Solarworld module Rated Power Tolerances updated

  25. SolarWorld module Vmp coefficients need to be corrected to match the latest data sheet values for 220 poly, 225 poly, 230 poly, 235 poly, 240 poly and 245 poly modules. 245 mono black and 250 mono black modules could be added. Also all module Rated Power Tolerances are -0/+5Wp.

  26. Updated version 7.4.1
    – Schuco MS 08 series added
    – REC PE series electrical specifications updated

  27. Updated version 7.4
    Aurora Power One Inverters
    – Fixed the minimum MPPT value.

    Note: At this time our tool is unable to simulate multiple MPPT configurations.

  28. SolarWorld module data needs to be updated. Minimally TC values do not reflect those reported within the latest data sheets available on the SolarWorld website.

  29. Updated version 7.2
    Silicon Energy inverters
    – SiE3840
    – SiE5300
    REFUsol inverters
    – 012K-UL
    – 016K-UL
    – 020K-UL
    – 024K-UL
    itek energy modules
    – IT 225-255W

  30. Please add module SOLAR SLW0005 SIZE 63.9 X 32.0 X 1.6 INCH, 40LB, ELECTRICAL RATING 120WP 35VMP 4.72IMP, AMPERAGE 4.72 VMP 5.4 ISC.

    And Xantrex TR1512-230-50 inverter module.


    • What is the manufacturer of the module you are requesting? Could you send us a spec sheet? The Xantrex inverter is a charge controller and battery based inverter and unfortunately our string sizing tool is not set up for battery based PV systems.

  31. Updated version 7
    – Added and updated hundreds of modules and inverters. To many to list.
    – Formatted for easy printing.

    Note: We do our best to make sure there are no errors with the tool but if you find one please notify us right away so we can fix it. Thanks!


  32. Can you add ET Solar Modules M572185,190 & 195 and P660235 – 250?
    I didnt really make sure to see if you have the Conext TX Series Inverters 2800,3300,3800,and 5000?

    • The latest update includes the ET modules you requested and the Context Inverters can be found under Schneider Electric.

  33. Is there a way to add inverters to the spread sheet? I’m trying to add a Xantrex XW6048 but can not modify the worksheet. This is a neat tool you created though. Thanks.


    • We do not have a way for users to add equipment but you can make a request for equipment to be added. Unfortunately the Xantrex XW series inverters do not work with our string sizer because they are battery based inverters.

  34. Updated as of 10/11/11
    -Lumos module series
    -SolTech module series
    -Two word state (New Jersey, New York, New Mexico ect.) drop down location list.
    -Voltage drop calculations to be based off of 12AWG or larger wire.

  35. Can you add the 250-watt 1SolTech STH-250 panel, please?
    Great tool – thanks for the effort!

  36. Thanks for your help. It is possible in tap “string size” add Puerto Rico in State area. In this tap also in “array design options” area, what you can do for more than one inverter or multiple inverters?

    • From what I can tell there is no ASHRAE data for Puerto Rico but if you find any data I can get it into the tool. If you are looking to size a PV string located somewhere outside the united states the manual temperature input area is your only option. At this time the REA string sizing tool has no option for multiple inverters.

  37. Updated as of 8/3/11
    -Minor text changes
    – Sanyo 225A
    – Kyocera KD 200-60 series

  38. It is possible to add Kyocera KD 200-60 series (230,235,240,245 watts) solar panels?


  39. Please add the Sanyo HIT Power 225A module to the stringing tool.

    Thank you very much – this is a great tool!

  40. Updated as of 7/12/11
    – Voltage drop calculations
    – Solarworld 240 & 245 poly modules

  41. Updated as of 6/23/11
    – Temperature correction is based off of ASHRAE values
    – Voltage drop calculator has an additional AC section
    – Sunpower 320’s
    – Westinghouse (Andalay) WHS_FL175
    – Canadian Solar CS6P-M & CS6P-P Series
    – Suntech 205
    – AUO Optronics Corp PM Series
    – Fronius CL Series
    – Nexus Energy DJ Series
    – UniSolar PVL144
    – Enphase 215

  42. Is it possible to update the “System Sizing Tool” with the new Enphase 215 microinverter as well as the Uni-Solar PVL144’s?

    Thank you,


    • We have added Enphase inverters to this tool more as a convenience then anything else. Like you point out, there is no string sizing involved with micro inverters which is why when you select an Enphase inverter and a Enphase inverter compatible module you only have the option of selecting one module per inverter. If there is a discrepancy somewhere in the tool it is likely because the module selected is not compatible with Enphase inverters. Please check the Enphase site for the module compatibility list. A future string sizing tool revision might include this compatibility list. Using the voltage drop calculator to calculate voltage drop with Enphase inverters is the main reason why Enphase inverters were included in this tool. Thanks.