Sources

USED ON: DESIGNER

Tool validation

• Designer Validation Benchmark, v264 (PDF, 4 pages) Primary
  Sun Safe LLC · rerun October 8, 2026 · Designer revision v264 (baseline v207, October 7; earlier runs: v244, v207) · rerun as the tool changes
  ResultSame weather, same systems: Designer within −3.8% to +2.5% of NLR SAM (PVWatts v8) open-sky, and within 1.0% of SAM with the same traced shade. Sales-style calculators match only an ideal south-tilted panel; for a vertical balcony panel they read 29–142% high.
  Compared: NREL SAM (open-source core, release 2025.4.16), the PVWatts V8 web service, and the PES Supply, Voltflow and The Green Watt calculators. Three climates (Des Moines, Phoenix, and Rutland, VT on real NSRDB weather), four systems, with and without shade, plus a Rutland payback check. v264 matches v244 exactly, and v244 matched the v207 baseline; Rutland reads +2.5% because its weather file is unusually calm (explained in the PDF). Methods, inputs and limits are in the PDF.

USED ON: PLUG-IN SOLAR MAP

State laws

Each link goes to the bill’s page on the state legislature’s own site, where the full text and every version are kept. The excerpts on each state page were checked word for word against the official text on October 13, 2026.

• Plug-in solar legislation tracker, all 50 states Secondary
  Bright Saver · checked October 8, 2026
  Used forBills not yet confirmed on a legislature site: Ohio HB 755, D.C. B26-0602, and the 2026 bills deferred to 2027 in Arizona, Georgia, Oklahoma, Delaware, Alaska and Washington (SB 6050). Marked “per Bright Saver” on the map.
• “Where does balcony solar stand in your state?” Secondary
  Canary Media · Sarah Shemkus · updated October 2, 2026
  Used forCross-check of the map: 10 states signed, New York passed both chambers, and the states with bills under consideration or deferred.

• Governor Sherrill signs the Garden State Balcony Solar Act Primary
  Office of the Governor of New Jersey · September 1, 2026
  Used forNew Jersey’s signing date, the 1,200 W cap, and the limits on landlords, HOAs and towns.
• New Jersey legalizes plug-in solar up to 1,200 W Secondary
  pv magazine USA · Ben Zientara · September 1, 2026
  Used forNew Jersey’s March 1, 2027 effective date and the 400 W exemption from the UL 3700 listing requirement, checked against the bill text linked above.

News coverage of plug-in solar lives on the News page.

USED ON: PLUG-IN SOLAR · DESIGNER

Safety standards, listings & certificates

• UL white paper: “Interactions of Plug-In PV with Protection of Existing Power Systems” Primary
  UL Solutions, 2025 · UL’s announcement of the UL 3700 program, January 8, 2026 (also on Business Wire)
  Used forThe three risk cards in the UL 3700 section: a live plug, an overloaded circuit and a GFCI that trips for no reason.
• CSA Group certified product listing Primary
  CSA Group · search by certificate number
  Used forConfirming the plug-in inverters are certified to UL 3700: certificate 80310017 (APsystems EZ1-LV) and certificate 80310016 (Hoymiles HiFlow Pro LV, HMS-360 to 500-1WM-LV). CSA certifies the inverters. The solar module data comes from the CEC list below.
• Vermont Act 149, as enacted (PDF) Primary
  Vermont General Assembly
  Used forVermont’s entry on the map, the conditions in the UL 3700 section, and the note in the payback step that exported power isn’t paid for.
• CSA certificate 80310017: APsystems EZ1-LV, UL 3700 (PDF) Primary
  CSA Group, July 15, 2026 · copy hosted by SanTan Solar
  Used forThe certificate’s conditions: a dedicated circuit on a 10 A breaker, a locking plug-in-solar plug and matching outlet, and a listed two-way Class A GFCI.
• CSA certificate 80310016: Hoymiles HiFlow Pro LV, UL 3700 (PDF) Primary
  CSA Group, July 15, 2026 · copy hosted by Bright Saver
  Used forThe certificate’s conditions: a 15 A circuit that may be shared, with the unit’s built-in fuses, a locking plug-in-solar plug and matching outlet, and a listed two-way Class A GFCI.
• UL Solutions follow-up testimony on Vermont S.202 (PDF) Primary
  Ken Boyce, UL Solutions · Vermont House Energy and Digital Infrastructure · April 14, 2026
  Used forWhy UL 3700 calls for a special plug and outlet and a two-way GFCI.
• “Mango Power S Earns UL 3700 Certification” Secondary
  Mango Power press release · PR Newswire · September 18, 2026
  Used forMango’s UL 3700 claim. Mango’s own page calls it component recognition for the microinverters, not the whole kit. No certificate number published yet, so the Designer shows it as announced only.
• UL 1741 safety testing of inverters includes an anti-islanding requirement Secondary
  MET Labs (Eurofins), August 3, 2023 · UL 1741, tested to IEEE 1547 / 1547.1
  Used forThe Designer’s Outage shutoff (UL 1741 / IEEE 1547) check: a grid-tied inverter must sense the grid is down and stop feeding it within 2 seconds.
• UL 3700 explained Secondary
  PlugInSolarUS
  Used forWhy UL 1741 alone isn’t enough for plug-in solar: the 2-second window leaves plug blades live, so UL 3700 adds touch safety, shared-circuit protection and two-way GFCI. The Designer requires both listings.
• Intertek directories (ETL listings) Primary
  Intertek · search by company name
  Used forWhere to look up Mango Power’s UL 3700 record once it’s published: search MANGO POWER TECH INC.
• Mango Power S Plug-In PV datasheet (PDF) Primary
  Mango Power · 120 V / 1200 W kit
  Used forThe Mango kit in the Designer: 280 VA at 120 V per unit, MPPT 16–60 V, 25 A short-circuit, 300 W panels, NEMA 1-15P plug, and its listings (UL 1741, UL 1741-SB, IEEE 1547, Intertek ETL mark). UL 3700 isn’t on it.
• Mango Power S PIPV installation guide (PDF) Primary
  Mango Power
  Used forThe Designer’s Mango flags: four units on one cord and plug, “no additional grounding,” and export limiting on by default (gateway required in states without a plug-in law).
• “Mango Power S Pro Achieves UL 3700 Certification” Primary
  Mango Power, September 17, 2026
  Used forThe scope: Intertek issued the Authorization to Mark on September 7, 2026, as component recognition for the microinverters; it doesn’t certify every complete kit.
• “Balcony PV Enters the Code Conversation” Secondary
  Howard Liu, Intertek · IAEI Magazine, July 1, 2026
  Used forWhat inspectors look at: devices sharing a branch circuit, shock and grounding, mounting loads and anti-islanding.
• Statement on the first UL 3700 certifications Secondary
  Environment America · July 2026
  Used forBackground: no whole-system kit was certified when the first inverter certificates were issued.
• “Plug In Balcony Solar Power Moves from Hack to Household” Secondary
  IEEE Spectrum · Andrew Moseman · July 20, 2026
  Used forBackground on how UL 3700 came about and how utilities view it.

USED ON: DESIGNER · DESIGN PACKAGE

Electrical code

NFPA 70, the National Electrical Code. Section numbers below are from the 2023 edition; a few may sit elsewhere in other editions. It’s copyrighted, so we cite section numbers only. NFPA offers a free read-only copy online once you register. States enforce different editions (2014 to 2026); the Designer, the map and each state page show yours, and your local inspector has the final word.

• The NEC and plug-in PV: “The 2026 NEC, like all previous editions, does not recognize the use of plug-in photovoltaic (PIPV) modules” Secondary
  UL Solutions, The Code Authority (accessed October 2026) · NFPA in IAEI Magazine, Summer 2026: proposals for this technology were submitted in the 2026 cycle and “further consideration is expected in the 2029 edition.”
  Used forThe “What you can build today” box on each state page: where a law requires the NEC and gives no waiver, the inspector decides.
• NEC edition each state enforces, checked October 8, 2026 Primary
  State electrical boards, building-code agencies and adopted rules: Vermont 2023 · Utah 2023 · Maine 2026 · Virginia 2020 · Maryland by county · New Hampshire 2023 · Connecticut 2020 · Colorado 2026 · California 2023 · New Jersey 2023 · New York 2023 · Massachusetts 2026 · Pennsylvania 2020 · Michigan MRC 2015 · Ohio 2023 · North Carolina 2023 · District of Columbia 2014
  Used forThe “NEC 20xx” tag on the one-line diagram’s state icon, the electrical-code line in each state’s rules (check screen and report), the map and the state pages. Other states: ask your local inspector.
  Pennsylvania and New York name the edition through their adopted building codes; Colorado’s August 1, 2026 date is from the Electrical Board’s rulemaking notice; New Jersey allows NEC 2020 during its grace period. Maryland has no statewide edition.

• 690.7Maximum PV voltage. This is the cold-morning check in the Designer’s step 5.
• 690.8Circuit current: module short-circuit current × 1.25.
• 690.9Overcurrent protection for PV circuits.
• 690.43Equipment grounding of module frames and racking, with listed bonding devices (UL 2703). The basis for the bonding path drawn on the single-line diagram.
• 250.138Cord-and-plug equipment grounded through the equipment grounding conductor in its cord. How a plug-in inverter is grounded.
• 406.4(D)(2)A GFCI on a circuit with no ground wire is marked “No Equipment Ground”. Why the Designer asks for a working ground at the outlet.
• 705.12Where the solar connects to your home’s electrical system, and how much it can add to a panel.
• 400.12Where flexible cords may not be used.
• 110.3(B)Install and use equipment as its listing and label say.
• 250.50The grounding electrode system at the service.

USED ON: DESIGNER

Equipment & module data

• Solar Equipment List: PV Modules Primary
  California Energy Commission (CEC)
  Used forThe 51 module presets from 200 to 600 W, three makers per size: Aptos, Auxin, Axitec, Boviet, Canadian Solar, Chint Astronergy, Heliene, Hyundai, JA Solar, Jinko, LG, LONGi, Mission Solar, Philadelphia Solar, Phono Solar, Qcells, REC, Risen, Silfab, Solar4America, SunPower, Trina and Waaree: Voc, Vmp, Isc, Imp, power, NOCT, and the temperature coefficients for voltage (βVoc), power (γPmax) and current (αIsc). Each module on the list has been independently tested and listed.
  We pulled the data from the copy of this list packaged with NLR’s System Advisor Model: SAM libraries, “CEC Modules.csv” (copy of September 30, 2026).
• APsystems EZ1-LV user manual, Rev 1.1 (PDF) Primary
  APsystems · May 2026 · copy hosted by Solartown
  Used forGrounding: “The external protective earthing conductor is connected to the inverter protective earthing terminal through AC connector.”
• APsystems installer FAQ Primary
  APsystems USA
  Used forThe bonding washer: it bonds the inverter to the rails; grounding per your local code and inspector.
• Hoymiles HiFlow Pro LV user manual, Rev 1.1 (PDF) Primary
  Hoymiles · July 2026 · HiFlow Pro page
  Used forGrounding through the cord’s equipment grounding conductor, and the 14 AWG ground point. Ratings for the HiFlow Pro presets.
• NEC 690.43 explained (PDF) Secondary
  Mike Holt Enterprises
  Used forPlain-language summary of bonding module frames and racking. Written to the 2020 NEC; 690.43 keeps its number in 2023 and 2026.
• Bright Saver balcony solar kits Primary
  Bright Saver · FAQ
  Used forThe Bright Saver 180 W and 360 W kit presets: contents, price, wiring (Y-cable) and certification status. Panel ratings aren’t published.
• Abundance Solar 390 W plug-and-play kit Primary
  Abundance Solar · panel ratings from the ECO-WORTHY 195 W listing
  Used forThe Abundance kit preset. The seller doesn’t state the panel wiring; the Designer shows it in series, the only way the panels fit the inverter.
• US Solar Supplier 1080 W and 1180 W kits Primary
  US Solar Supplier · 1180 W kit · Talesun 540 W datasheet · HanerSun 590 W datasheet
  Used forThe two kit presets and their panel ratings.
• NAZ Balcony Solar Roof Kit 001 Primary
  altE Store · Mission Solar MSX10 datasheet
  Used forThe NAZ kit preset and its Mission Solar 435 W panel ratings.
• EZ1 plug-in microinverter (EZ1-LV) Primary
  APsystems USA · EZ1 documents
  Used forInput voltage window, MPPT range, maximum input current and output rating. The wiring rule in step 5, up to two modules per input and in parallel only, comes from APsystems’ PV Module Selection Guidelines and Quick Install Guide.
• HiFlow (360–500) Pro LV Primary
  Hoymiles · HiFlow Pro overview
  Used forInput and output limits for the five HiFlow Pro models.
• ZNShine ZXM7-UHLDD144-580/N and Suniva Optimus OPT255-60-4-100 datasheets Primary
  Manufacturer datasheets
  Used forThe two extra module presets.
• 110 W folding panel (default)
  Typical label values, not one specific product
  Used forA starting example. Enter your own panel’s label before trusting any result.

USED ON: DESIGNER (SUN, PAYBACK)

Sun & weather data

• NSRDB solar resource data (Solar Resource API) Primary
  National Laboratory of the Rockies (formerly NREL), U.S. Department of Energy · developer.nlr.gov
  Used forMonthly average sunlight on flat ground for your location, from the same dataset PVWatts uses. Fetched live; NASA POWER is the backup. Used under the NLR Developer Network terms; not endorsed by DOE or NLR.
• NASA POWER climatology for Rutland, Vermont (JSON) Primary
  NASA Langley Research Center · 2001–2020 averages · CC BY 4.0
  Used forThe offline fallback: when neither sun data service can be reached, the Designer shows these Rutland numbers, labeled as the Rutland example.
• NASA POWER climatology API Primary
  NASA Langley Research Center · how to cite (CC BY 4.0)
  Used forLong-term monthly sunlight on flat ground (total and diffuse) for your location. It’s fetched live from NASA each time you look something up. Nothing is stored here.
• NLR PVWatts V8 API: NSRDB typical meteorological year (TMY) Primary
  National Laboratory of the Rockies (formerly NREL), U.S. Department of Energy · developer.nlr.gov · not endorsed by DOE or NLR
  Used forThe hourly weather behind the production estimate, the tracking-range hours and the clear-day temperatures: 8,760 hours of beam, diffuse and air temperature from the NSRDB TMY year nearest your location, the same weather the PVWatts calculator uses. Also PVWatts’ own open-sky annual kWh for your tilt, direction and size, shown beside the Designer’s estimate. NASA POWER (below) is the backup.
• NASA POWER hourly API Primary
  NASA Langley Research Center · CC BY 4.0
  Used forStep 5’s tracking-range count. The last full year of hourly air temperature and sunlight (total, direct and diffuse), 8,760 hours, for your location. Each hour is turned into sunlight on your panel, a cell temperature from the NOCT model with your mounting, and a working voltage. The Designer counts the producing hours (over 50 W/m² on the panel) where that voltage falls below the inverter’s tracking floor, and warns when it’s more than 5%.
  No wind cooling is applied, so the count leans high. Where NASA has no direct and diffuse split for an hour, it’s estimated with the Erbs correlation.
• ASHRAE climatic design conditions Primary
  ASHRAE via ashrae-meteo.info
  Used forThe extreme annual minimum temperature you enter for the cold-voltage check.
• U.S. Census Bureau Geocoder
  U.S. Census Bureau · API documentation
  Used forTurning a street address into coordinates in step 3, so the site map and sun calculations land on the house, not the middle of town.
• OpenStreetMap Nominatim
  © OpenStreetMap contributors (ODbL)
  Used forA backup address lookup when the Census Bureau can’t match an address.
• Open-Meteo Geocoding API
  Open-Meteo (place data from GeoNames)
  Used forTurning “City, ST” into latitude and longitude. A short built-in list of towns is the fallback.
• NOAA Solar Calculator
  NOAA Global Monitoring Laboratory
  Used forChecking our sun-path math. Sun positions on the chart agree with NOAA’s to within a fraction of a degree.

USED ON: DESIGNER

Math & methods

• Cold-morning voltage: NEC 690.7(A)(1), temperature-coefficient method Primary
  NFPA 70, 2023 · see Electrical code
  Used forVoc at your coldest temperature = label Voc × (1 + Voc coefficient × (T_min − 25 °C)).
  What we don’t use: Sandia PV Array Performance Model (PVPMC)
• CEC PV module database, as shipped with NLR’s System Advisor Model Primary
  California Energy Commission · NLR SAM library “CEC Modules.csv”, updated Sep 30, 2026 · 21,677 modules
  Used forVetting a module typed in by hand. Limits come from the 17,592 monofacial crystalline modules in the database: hard stops at or just past its extremes, “double-check” warnings outside its middle 99%. Voc 5–92 V, Vmp 4–78 V, Isc 1–20 A, Imp 1–18 A, Pmax 15–740 W; βVoc 0.13–0.50, γPmax 0.20–0.60, αIsc 0–0.15 %/°C; NOCT 38–60 °C. Cross-checks: Vmp below Voc and 75–90% of it; Imp below Isc and at least 85% of it; Pmax within 5% of Vmp × Imp (the maximum power point at STC); fill factor Pmax ÷ (Voc × Isc) 0.65–0.85.
  Tested: 1,500 real modules from the database entered by hand, 2 blocked (0.13%, both with inconsistent ratings in the database itself); every nonsense entry tried (100 V, −1 A, 0 W, 900 W, Vmp above Voc, Pmax that doesn’t multiply out) blocked. Thresholds are Sun Safe check rules built on this data.
• Hot-afternoon voltage and current: NOCT cell-temperature model
  NOCT Cell Temperature (Sandia PVPMC)
  Used forStep 5 of the Designer. Cell temperature = hottest air temperature + (NOCT − 20 °C) × 1000 ÷ 800, with NOCT raised for mounting. From that: the lowest working voltage (Vmp, using γPmax) checked against the inverter’s tracking floor, and the highest current (Isc and Imp, using αIsc) checked against its input ratings. Parallel modules add current; series don’t.
  Defaults when a label doesn’t list them: NOCT 45 °C; γPmax 0.47 %/°C, the PVWatts V5 “Standard” module (Dobos 2014, Table 3, “determined from a statistical analysis of over 11000 modules in the CEC module database”). PVWatts V8 lowered Standard to 0.37 %/°C (SAM Help, PVWatts); the Designer keeps 0.47 as the more conservative blank-label default; αIsc 0.06 %/°C for monofacial crystalline modules: PVEducation (“0.06% per °C for silicon”), inside PVsyst’s typical crystalline range (“usually between 0.04 and 0.08 %/°C”). Mounting raises NOCT by the gap behind the panel: over 3.5 in, 0 °C; 2.5–3.5 in, +2; 1.5–2.5 in, +6; 0.5–1.5 in, +11; under 0.5 in, +18. That is the table in NLR’s System Advisor Model, from Fuentes at Sandia. Balcony rails get no credit for running cooler, which keeps the voltage check on the safe side.
• Mounting and panel temperature: the research behind the table
  SAM standoff adjustments (NLR) · Kurtz et al., 2009 (NREL) · King et al., 2004 (Sandia) · Kempe et al., 2024 (NREL, ASU) · TNO vertical PV (pv magazine)
  Used forThe “How it’s mounted” choices in step 3. NREL measured roof-mounted panels about 20 °C hotter than rack-mounted ones in the same conditions, and Sandia’s coefficients show the same gap. Heat falls off quickly as the gap behind a panel grows, with most of the penalty under 2 to 4 inches.
  TNO found vertical panels run with about half the temperature rise of flat ones. The Designer doesn’t take that credit for balcony rails, so its hot-voltage figure stays conservative.
• Sun position: Spencer (1971), “Fourier series representation of the position of the sun,” Search 2(5):172
  Basic Solar Position Models (Sandia PVPMC)
  Used forSolar declination and the equation of time behind the sun-path chart and the photo overlay. Cross-checked against the NOAA calculator above.
• Dobos (2014), PVWatts Version 5 Manual Primary
  NREL/TP-6A20-62641 · PVWatts Calculator
  Used forThe production model in step 7, which follows PVWatts hour by hour: sunlight on the panel with the Perez sky model and 0.2 ground reflectance; glass reflection on direct sun (Fresnel, glass index 1.526); DC power at the module’s temperature coefficient; 14% system losses; the PVWatts inverter part-load curve at 96% nominal; and the inverter’s output limit.
  Weather is the same as PVWatts: the NSRDB typical meteorological year (TMY) from NLR’s PVWatts V8 web service (hourly beam, diffuse and air temperature; GHI = beam × cos zenith + diffuse at the middle of each hour), and the Designer shows PVWatts’ own open-sky annual estimate for the same tilt, direction and size as a cross-check. If that service can’t be reached, the backup is one NASA POWER hourly year scaled month by month to NASA’s long-term averages. Differences from PVWatts: cell temperature uses the NOCT model with the mounting adder instead of the Fuentes heat balance, with no wind cooling. Checked against an independent implementation of the Sandia equations: identical to three decimals across six tilts and bearings. When hourly weather can’t load, the monthly backup runs the same chain (glass loss on the beam, cell temperature, temperature coefficient, 14% losses, inverter curve, output limit) using NASA POWER’s monthly mean daily maximum air temperature, or your design high temperature when none is available.
• Perez et al. (1990), “Modeling daylight availability and irradiance components from direct and global irradiance” Primary
  Solar Energy 44(5):271–289 · Perez sky diffuse model (Sandia PVPMC)
  Used forHow much sky light reaches the panel, split into the even sky, the bright ring around the sun, and the brighter band near the horizon. The same model PVWatts and SAM use. Coefficients from the 1990 all-sites table as published by Sandia.
  The circumsolar term uses the beam’s own angle of incidence (a = max(0, cos AOI)), so it is shaded together with direct sun when the sun is behind the skyline. As in PVWatts V5, sky light is treated as isotropic when the sun is within 2.5° of the horizon (zenith 87.5–90°).
• Solar Pathfinder Co. (2016), Assistant User’s Guide Primary
  Solar Pathfinder Assistant software · Solar Pathfinder
  Used forThe shading trace: solid obstructions traced as one skyline, left to right; deciduous trees outlined as separate closed shapes.
  Not applied: the software’s default credit for bare trees (“all deciduous areas will be calculated as receiving 50% of the available solar radiation from the months of October through March”). The Designer stays conservative and counts leafy trees as solid all year, as HelioScope does.
• Dozier & Frew (1990), “Rapid calculation of terrain parameters for radiation modeling from digital elevation data” Primary
  IEEE Transactions on Geoscience and Remote Sensing 28(5):963–969 · USDA-ARS topocalc (viewf)
  Used forHow much sky light the traced skyline takes away. The sky view factor of the tilted panel (equation 7b, 72 directions, horizon limited by the panel’s own plane) is compared with its open-sky value, (1 + cos tilt)/2; the ratio scales the even-sky and horizon-band sky light.
  Checked: open sky at 0°, 20°, 35°, 60° and 90° tilt matches (1 + cos tilt)/2 to four decimals; a level surface ringed by a 30° skyline gives cos²30° = 0.75. SAM treats diffuse horizon shading the same way, integrating over the shaded part of the sky dome (Solargis, PV simulators comparison).
• Meinel & Meinel (1976), Applied Solar Energy: An Introduction; Kasten & Young (1989), “Revised optical air mass tables and approximation formula”
  Addison-Wesley · Applied Optics 28(22):4735–4738 · doi:10.1364/AO.28.004735
  Used forThe clear-day chart only: direct sun on a cloudless day = 1,367 W/m² × 0.7^(AM^0.678) (Meinel), with air mass AM from Kasten & Young. The yearly estimate uses measured weather, not this.
  The clear-day diffuse share (10% of direct) is still being sourced.
• Marion et al. (2005), “Performance Parameters for Grid-Connected PV Systems”
  NREL · IEC 61724 performance parameters
  Used forSpecific yield on sheet 1: the IEC 61724 final yield, Yf = net energy E ÷ DC nameplate P0, in kWh per kW per year.
• HelioScope Help Center, “Create Tree Keepouts”
  Aurora Solar / Folsom Labs · Shade Modeling
  Used forLeafy trees in leaf, and all solid objects, block like any obstruction: “Tree Keepouts are solid objects and do not account for any seasonal variation due to leaf change.” A shaded panel “loses its direct irradiance, though it still receives diffuse irradiance.”
• NLR System Advisor Model (SAM) Help, “Shading”
  NREL · the shading inputs behind PVWatts and SAM
  Used forHow shade enters the energy model: direct (beam) sun is reduced when it is behind an obstruction, and sky (diffuse) light by a separate sky-shading loss. SAM accepts shade data from Solar Pathfinder and Solmetric SunEye, the same kind of horizon trace the Designer makes.
• NASA POWER hourly data
  NASA Langley Research Center · CC BY 4.0
  Used forHourly sunlight (total, direct and diffuse) and air temperature for the production model and the tracking-range count.
• Duffie, Beckman & Blair, Solar Engineering of Thermal Processes, Photovoltaics and Wind, 5th ed.
  Wiley, 2020 · Chapters 1–2
  Used forA quick monthly estimate shown for a few seconds while the hourly weather loads, or if it can’t load. Built from the three papers below, with ground reflectance set to 0.2.
• Liu & Jordan (1960), “The interrelationship and characteristic distribution of direct, diffuse and total solar radiation”
  Solar Energy 4(3):1–19 · Isotropic sky model (Sandia PVPMC)
  Used forHow much diffuse (cloudy-sky) light a tilted panel sees, and the hourly diffuse ratio.
• Collares-Pereira & Rabl (1979), “The average distribution of solar radiation”
  Solar Energy 22(2):155–164
  Used forSpreading each day’s total sunlight across the hours, so tilt and facing direction are weighed hour by hour.
• Erbs, Klein & Duffie (1982), “Estimation of the diffuse radiation fraction for hourly, daily and monthly-average global radiation”
  Solar Energy 28(4):293–302
  Used forEstimating the diffuse share when NASA doesn’t return one, in both the hourly and the monthly form.
• Payback: NLR System Advisor Model (SAM) Help, “Payback”
  NREL · simple payback, not discounted
  Used forPayback = the year cumulative savings equal the cost, SAM’s simple payback: “the time in years that it takes for cumulative annual savings to equal the cumulative annual costs.” Savings = energy used at home × your price, with the price rising at the escalation rate and output falling at the aging rate. Energy sent to the grid earns nothing. Future savings are not discounted (SAM reports a separate discounted payback).
• Jordan & Kurtz (2013), “Photovoltaic Degradation Rates — An Analytical Review” Primary
  Progress in Photovoltaics 21(1):12–29 · NREL
  Used forThe 0.5% a year panel-aging default: the median of nearly 2,000 measured degradation rates. The PVWatts V5 manual uses the same figure (“typical long-term degradation … (∼0.5 %/year)”).

USED ON: DESIGNER (PAYBACK, IMPACT)

Cost & emissions

• Standard warranties, as of Oct 8, 2026 Primary
  Makers’ pages and datasheets: APsystems EZ1 · Hoymiles HiFlow Pro · HanerSun · JA Solar · REC Alpha · Mission Solar (EnergySage)
  Used forThe warranty rows in the Designer, the single-line diagram and the design package, and the warranty-end lines on the payback chart. Standard terms only; paid extensions aren’t counted. Unknown terms are marked “check before purchase.”
• U.S. Electric Utility Companies and Rates: Look-up by Zip Code (2024) Primary
  NREL / OpenEI, from U.S. EIA Form 861 and ABB Velocity Suite · released Nov 2025
  Used forThe Designer’s suggested utility rate: each utility’s 2024 average residential price, matched to your ZIP code. Includes fixed charges; your bill gives a more accurate payback.
• Equipment prices, as of Oct 8, 2026 Secondary
  Retailer listings: APsystems EZ1 at the APsystems shop (apshop.apsystems.com) · APsystems EZ1-LV at altE (second option) · Hoymiles HMS-360-1WM-LV at US Solar Supplier · Hoymiles HMS-391-1WM-LV at altE · US Solar Supplier solar panels
  Used forThe suggested costs in the Designer’s Financial modeling step: inverter and kit prices from each seller’s page, panel prices from listings where the model is still sold, and $0.50 per watt otherwise. US Solar Supplier’s single-panel prices had a median of about $0.44/W (42 listings). Prices change; each one links to the seller.
• State Energy Data System, Table ET3: residential electricity prices by state Primary
  U.S. Energy Information Administration · Vermont · California · Colorado · Connecticut · Maine · Maryland · New Hampshire · New Jersey · New York · Utah · Virginia
  Used forThe Designer’s price history chart, its price-rise buttons (last 10 years, last 5 years, full span) and the default yearly price rise, for each state with a plug-in solar law. Retail electricity, dollars per million Btu, converted to cents per kWh (× 0.3412). Vermont is shown when no state is picked or the state has no plug-in law.
• Residential rates Primary
  Green Mountain Power · Rate 1 tariff, bills from Oct 1, 2026 ($0.23066/kWh, $0.673/day)
  Used forThe Vermont example price, and why the fixed daily customer charge is left out of the price per kWh.
• How much coal, natural gas, or petroleum is used to generate a kilowatthour of electricity? Primary
  EIA FAQ 667
  Used forPounds of coal and cubic feet of gas per kWh in the impact picture.
• How much carbon dioxide is produced per kilowatthour of U.S. electricity generation? Primary
  EIA FAQ 74
  Used forA cross-check on the CO₂ rate.
• Greenhouse Gas Equivalencies Calculator: calculations and references Primary
  U.S. Environmental Protection Agency
  Used forCO₂ avoided per kWh (the AVERT national rate) and the “tree seedlings grown for 10 years” measure.

USED ON: PLUG-IN SOLAR MAP · DESIGNER

Maps & imagery

• Free US SVG map
  Simplemaps
  Used forThe state outlines on the legislation map.
• Leaflet
  Open-source map library (BSD-2)
  Used forDisplaying the satellite map.

All links checked October 8, 2026. If one breaks or a figure looks out of date, tell me through the contact section.

FROM THE SUN SAFE TEST STAND

USED ON: PLUG-IN SOLAR · DESIGNER

Change log