Balcony Solar Designer

Pick your equipment and panels. See the wiring, and whether it fits.

Preloaded: Sun Safe’s test stand, Rutland, VT. Accept each step as the example, or enter your own.

The diagram fills in as you go. Tap it to enlarge.

STEP 1

Name your system

Shown top-left on the diagram and on the design package.

A finished 6-page package, with notes on every part.

STEP 2

Where will it go?

Sets your state’s plug-in rules and your sun data.

A street address is looked up with the U.S. Census Bureau (or OpenStreetMap if the Census can’t find it); a city with Open-Meteo. The coordinates then go to NASA POWER for sunlight and weather. Nothing is saved or tied to you.
Street address, city and state, or coordinates, e.g. 20 Main St, Rutland, VT 05701. Press Load.

STEP 3

What are you plugging into?

Pick a microinverter or a complete kit.

Loaded: Sun Safe test stand · EZ1-LV + 2 × Suniva OPT255.

Text in orange is the Rutland example. Change it, or press OK to keep it.

STEP 4

Which module?

From the label on the back of the panel.

Open-circuit voltage. The highest voltage the panel makes, with nothing connected.
Voltage at maximum power, where it normally runs.
Short-circuit current. The most current it can push.
Current at maximum power.
Rated watts.
Voltage rise per °C of cold. Not printed? Leave blank: NEC Table 690.7(A) is used.
Power loss per °C of heat. Blank: 0.47 (PVWatts V5 standard module; V8 uses 0.37).
Current rise per °C. Typical: 0.05.
Cell temperature in sun (NOCT or NMOT).
Metal frame = bonded to ground. Set automatically for listed modules.

Text in orange is the Rutland example. Change it, or press OK to keep it.

STEP 5

How the panels connect

One panel model per input. No mixing on a Y-connector. Input B can differ (Module step).

Series: panel to panel · adds voltage
Parallel: Y-connectors · adds current
Same power either way.

Per input. Max 3 × 3.

STEP 6

Your site

Direction, tilt, temperatures and mounting set sun hours and panel heat.

Compass bearing, true north: 90 E · 180 S · 270 W.

0 = flat · 90 = vertical, on a railing.

Hottest day of a typical year: ASHRAE red Max.
Coldest morning of a typical year: ASHRAE blue Min. Used for NEC 690.7.
Where to find your two numbers

Find your nearest station at ashrae-meteo.info. Scroll to Extreme Annual Design Conditions, find Extreme Annual Temperature, then Mean. Pick the newest edition (2025).

Rutland, Vermont (the example here)
Extreme Annual Design Conditions
Extreme Max WBExtreme Annual Temperaturen=5 years
MeanStandard deviation
MinMaxMinMaxMinMax
27.4−25.832.04.31.6−28.833.2

Layout as it appears on the ASHRAE page; values from the 2009 ASHRAE Handbook. Tables redrawn here, not copied.

Los Angeles, California
Extreme Max WBExtreme Annual Temperaturen=5 years
MeanStandard deviation
MinMaxMinMaxMinMax
24.04.434.31.42.53.436.1

Max → Max hot · Min → Max cold. Skip the standard deviation.

Less air behind = hotter panels, lower voltage.
How to pick the right one

Measure roof or wall to panel back. Adds 0–18 °C to NOCT (NREL SAM method).

STEP 7

System compatibility check

Checked: inverter input limits (cold and hot), wiring rules, state limit, outlet, grounding, outage shutoff (UL 1741) and plug-in listing (UL 3700). Any flag stamps WARNING on the diagram.

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Cold voltage: NEC 690.7(A)(1). Hot cell temperature: NOCT model. Tap a result for the math.

PLEASE READ Verify against your equipment documents before buying or installing. A planning guide, not a site assessment.

Your system

Swipe sideways to see the full diagram →

Source: Equipment limits from manufacturer datasheets. Module ratings from the California Energy Commission PV module list or the module label. Cold-morning voltage per NEC 690.7(A)(1). GFCI per NEC 210.8. Outage shutoff per UL 1741 / IEEE 1547; plug-in listing per UL 3700, from each certificate. State output limits from each state’s plug-in solar law, listed on the Plug-In Solar page. Not for construction – example only.

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STEP 8

Expected production*

Locked until the compatibility check passes. Go to the check
* Open-sky estimate. Any shade means less energy. Optional shading profile: last step.

Sun path: calculated from your location. Sun hours: NSRDB or NASA.

Swipe sideways to see the full chart →

Source: Sun positions from Spencer (1971) solar geometry, checked against the NOAA Solar Calculator (gml.noaa.gov/grad/solcalc). Standard sun path diagram: sun height against compass direction, solar time.

  • The sun’s track on the 21st of each month. Heavy lines are the longest and shortest days.
  • Hour lines, solar time. 12 = sun due south (about noon–1:30 on the clock).
  • Sun behind the panel. No direct light reaches the face.

Press Load above to fetch monthly sun hours for this spot.

Full breakdown: how the sun hours and kWh were worked out
About the sunlight data

Irradiance data from the NASA Langley Research Center (LaRC) POWER Project, funded through the NASA Earth Science/Applied Science Program. Long-term monthly averages, not a forecast. Turned into sun on the panel with the Duffie & Beckman isotropic-sky method, ground reflectance 0.2. Output assumes 14% system losses before the inverter limit.

STEP 9

Financial modeling and payback

Locked until the compatibility check passes. Go to the check

Example: Sun Safe test stand, Rutland, VT. Replace with your numbers.

What it cost

What you paid for the inverter.
0 if you already own them, like the example.
Rack, brackets, hardware.
Recommended for every system. Include it.
Cords, connectors, anything else.
Total
$525.00

Text in orange is the Rutland example. Put in what you paid, or press OK to keep it.

What it saves

Filled in from the production estimate once your location loads.
Share of the solar your home uses as it’s made.
More
Power goes to whatever is running in the house first. Anything extra flows to the grid and isn’t paid for: plug-in systems get no net metering. A small system rarely makes more than a home’s daytime use, so 95% is typical.
Load your location for your utility’s rate, or use your bill (below).
Pick a pace below.

How fast have prices risen? Vermont homes, per kWh:

Source: Prices: U.S. Energy Information Administration, State Energy Data System (SEDS), Vermont residential electricity prices.

Vermont average home price, U.S. EIA, 2003–2024.

More accurate: work out your price from your bill

Bill total, minus the fixed daily charge, ÷ kWh used.

The amount due for one month.
Printed on the bill, usually near the usage chart.
The length of the billing period.
“Customer charge.” GMP: $0.673/day on bills from Oct 1, 2026.

Your price: —

Text in orange is the Rutland example. Put in your own production, use and price, or press OK to keep them.

Simple payback, not a financial forecast.

What this payback leaves out

Past price rises don’t guarantee future ones; the frozen line shows the result if prices never rise again. Panel output assumed to fall 0.5% a year. No tax credits, financing or replacement parts included. Dashed lines mark where known standard warranties end; a replacement after that isn’t counted. Plug-in systems generally don’t qualify for net metering, so exported power is counted as worth nothing.

System offset

STEP 10

Design package

Locked until the compatibility check passes. Go to the check

Updates with every change. Print double-sided.

What’s on each page
  1. Electrical: the single-line diagram, the checks and this system’s yearly offsets
  2. Sun and production, and how the energy was worked out
  3. Cost and payback
  4. 25 years of offsets
Tap to share your design:
How
LinkedIn and Facebook share a link to the Designer. Instagram has no web link: this saves page 1 as an image to post from the app (on a phone, pick Instagram in the share sheet).

Thanks for using the Designer.

I hope you learned something along the way, and that this design helps you on your solar journey.

If you’re willing, share your design: Share my design with Sun Safe (above) emails a copy of your packet, with the street address left out by default. With enough of them, Sun Safe can map plug-in systems so others can learn from real installs.

Have a sunny day!

Shade from trees, roofs or railings? Trace it on a panorama: updates energy and payback, adds page 5.

STEP 11 · OPTIONAL

Shading profile

Locked until the compatibility check passes. Go to the check
Your site’s shade, from a panoramaOptional

Sun paths over your photo; anything above them blocks the sun. The photo stays on your device.

How to take the panorama
  • Stand at the middle of the panel, with the phone at the height of the middle of the panel.
  • Use Pano mode on the 0.5× lens, phone held upright. It reaches about 50° up; 1× only about 30°.
  • Point the middle of the sweep the way the panel faces, and sweep from east to west.
  • Tilt the phone slightly up as you sweep: get every treetop and roofline in. Some ground can drop out; you set the level line afterward.
  • Keep the arrow on the line so the photo stays level, and turn in place around the phone.
  • Shoot between 10 am and 2 pm with the sun in the frame: the tool lines the drawing up on it.
  1. Load your photo
  2. Line it up with the sliders
  3. Lock it in place
  4. Trace the skyline
Panorama from the panel location with the sun’s yearly paths drawn over it

Swipe sideways to see the whole photo →

2. Line it up from the sun. Automatic.

  • Best: photo taken 10 am–2 pm, sun in the frame.
  • Aim the middle of the photo due south (180°). If it faced another way, enter that bearing.
  • Set when it was taken, then tap the center of the sun.
  • Hour marks are solar time, not clock time: the sun can sit before or after its clock hour.
°Default 180°, due south. Compass bearing, true north (iPhone: Settings, Compass, Use True North). Shifts the sun chart only; the panel’s orientation is set in its own step.

All line-up settings

Or by hand. Put the sun on its path, or use the horizon and a known bearing.

Panel bearing in the photo. 0 = centered.
Level the horizon if the phone tipped.
Degrees covered, edge to edge.
Level line, % from the top.
Unlocked. Line the photo up, then lock it.
4. Estimate the shade
  • Skyline (blue line): roofs, walls, the far tree line. Everything below the line blocks.
  • Outline (green shape): tap around a tree or bush. Only inside the shape blocks; open sky around it stays clear. Counted as solid all year: conservative.
Where these numbers come from

Tracing method after Solar Pathfinder Assistant: solid skyline, with deciduous trees outlined separately. Shade math after HelioScope: trees are solid objects with no seasonal leaf change. Solid shade removes direct sun. Sky light is reduced by the panel’s sky view factor under the trace (Dozier & Frew 1990), as SAM integrates over the shaded part of the sky dome. Solar Pathfinder’s optional 50% winter credit for bare trees is not applied.

STEP 12

Design package, with shading

Includes your shading profile. Shaded energy, payback and offsets, plus page 6. The open-sky 5-page package stays on the previous step.
Tap to share your design:
How
LinkedIn and Facebook share a link to the Designer. Instagram has no web link: this saves page 1 as an image to post from the app (on a phone, pick Instagram in the share sheet).

Thanks for using the Designer.

I hope you learned something along the way, and that this design helps you on your solar journey.

If you’re willing, share your design: Share my design with Sun Safe (above) emails a copy of your packet, with the street address left out by default. With enough of them, Sun Safe can map plug-in systems so others can learn from real installs.

Have a sunny day!