Free Solar System Design Software: Save $5,000+ on Planning

Most homeowners who contact me about solar have already spent three or four hours on a manufacturer’s website, plugging numbers into some locked-down “estimate tool” that spits out a monthly savings figure and a phone number. That’s not solar design. That’s a sales funnel dressed up in a calculator costume.

Real solar system design software does something different: it models your roof geometry against actual historical irradiance data, accounts for shading from that oak tree on the southwest corner, calculates string sizing and inverter clipping losses, and produces a production estimate you can hold a contractor accountable to. The good news is that several tools will let you do most or all of this for free, right now, today.

Here’s the number that surprised me when I first saw it: according to EnergySage’s market data, homeowners who get three or more independent quotes save an average of $5,000 to $10,000 on a typical residential installation. The gap exists largely because most people can’t evaluate what they’re being sold. Understanding your own system design, even at a basic level, is what closes that gap.

PVWatts: Start Here, Every Time

I tell everyone the same thing when they ask where to begin: go to pvwatts.nrel.gov and spend 20 minutes. It’s run by the National Renewable Energy Laboratory, it’s completely free, and it uses the same TMY (Typical Meteorological Year) weather data that professional engineers use. This is not a toy.

The interface asks for your location, system capacity (in kilowatts DC), module type, array type (fixed, one-axis tracking, etc.), system losses, and tilt and azimuth. For a south-facing roof at 25 degrees pitch in Sacramento, a 7 kW system in PVWatts will show you roughly 11,500 to 12,000 kWh per year. When a contractor shows up and promises 13,500 kWh from the same setup, you now have a reason to ask why, and if they can’t answer it, that’s your red flag.

One thing PVWatts doesn’t do well is granular shading analysis. It has a shading input, but it’s a single percentage you estimate yourself. If your roof has a chimney, a dormer, or a neighbor’s two-story addition casting a shadow across your array from 2 PM onward, PVWatts will not catch that. For that, you need to go a step further.

NREL’s System Advisor Model (SAM)

Helpful resource: Emporia Smart Outlet with Energy Monitoring is a top-rated option for this. (As an Amazon Associate this site earns from qualifying purchases.)

SAM is where things get serious. It’s free to download (sam.nrel.gov), runs on Windows and Mac, and is honestly the most powerful solar modeling tool available at any price for a residential user who’s willing to learn it. NREL has pumped significant federal research money into this thing.

I’ll be honest about the learning curve: the first time I opened SAM, I closed it after six minutes. The interface looks like it was designed for energy engineers, because it was. But after watching two of NREL’s own tutorial videos and spending a Saturday afternoon with it, I produced a model of my own 9.6 kW system that came within 4.3% of actual first-year production. That’s not bad. That’s actually better than the estimate my installer gave me.

SAM handles detailed shading, time-of-use rate analysis, battery storage modeling, and financial projections including the federal Investment Tax Credit (currently 30%, as of this year under the Inflation Reduction Act provisions). If you want to know whether adding a Powerwall 3 to your system actually pencils out financially in your specific utility territory, SAM will do that calculation.

A worked example to make this concrete:

Scenario: Phoenix homeowner, 1,850 sq ft single-story, 8.4 kW proposed system, net metering at 9 cents/kWh export rate, $0.14/kWh blended import rate. Action: Ran SAM model with actual local shading from a 14-foot block wall on the south property line at 8 feet setback. Result: Model showed 14,200 kWh annual production with a 6.3% shading dearate. Contractor’s quote had claimed 15,800 kWh with no shading mention. At a $0.14 rate, that’s a $224/year overstatement of savings, or $5,600 over 25 years.

That’s the kind of number that makes the time investment worth it.

The Professional Tools and Their Free Tiers

Aurora Solar and Helioscope are the two dominant professional platforms. Both are subscription-based for contractors, but both offer limited access for homeowners or people evaluating one system.

Aurora Solar’s free tier lets you model one project at a time and gives you their LiDAR-based 3D roof modeling, which is genuinely impressive. You upload your address, and it pulls satellite and LiDAR data to build a 3D model of your roof automatically. Shading is calculated month by month, hour by hour. For a homeowner, this is probably the most useful free tool after PVWatts, because the visual output is something you can actually show your spouse or your HOA board.

Helioscope has no meaningful free tier for homeowners as of July 2026. It’s a contractor tool, full stop. If your installer is using it, that’s a good sign they’re serious. Ask to see the Helioscope report, and they should be able to export a PDF that shows shading losses, system layout, and annual production by month.

SolarEdge Designer is free and underrated. If you’re considering a SolarEdge inverter system with optimizers (and for a shaded or complex roof, I’d often recommend that), their Designer tool gives you panel-level production modeling, string design validation, and a shade report. The catch is that it only models SolarEdge equipment, so you’re in a walled garden. Still, for a first pass at a complex roof, it’s worth an hour.

What I Got Wrong (And What It Cost Me)

For the first two years I was doing solar consulting, I trusted PVWatts shading inputs too much. A reader who hired me to review a quote for a home in Portland, Oregon, had a Douglas fir about 40 feet tall and 30 feet from the array. I modeled a 12% annual shading dearate in PVWatts. First-year actual production came in 18% below the model.

The correct approach, which I learned from running the same roof through SAM with an actual horizon profile, would have predicted that shortfall accurately. The lesson: for any roof with significant obstructions, PVWatts shading input is a rough guess. Use SAM or Aurora Solar’s LiDAR shading analysis instead.

The homeowner’s system never performed to the installer’s projection, and the installer had used PVWatts the same lazy way I had. They went back to the installer with the SAM output and got a partial refund negotiated into a monitoring contract. Not ideal, but it was something.

A Workflow That Actually Works

If you’re starting from zero, here’s the sequence I walk people through:

Start with PVWatts to get a baseline production estimate. This takes 20 minutes and gives you a number to sanity-check everything else against.

Then run Aurora Solar’s free tier on your address to get a LiDAR-based 3D model with real shading analysis. Compare the annual production estimate to your PVWatts run. If they’re within 8-10%, you have a reasonably clean roof situation. If Aurora’s estimate is more than 15% lower than PVWatts, your shading is meaningful and you need to factor that into your payback calculations.

If you’re seriously entertaining a battery system, or you have a time-of-use rate structure, spend the time to learn SAM. I know the interface is rough. But the Solar Energy Industries Association (SEIA) has consistently reported that battery-plus-solar systems require substantially more accurate financial modeling to evaluate correctly than solar-only systems, and SAM is the tool that can do it without a $200/month subscription.

Worked example two:

Scenario: A reader in Massachusetts, 10.2 kW system quote, National Grid territory, TOU rates with on-peak at $0.32/kWh and off-peak at $0.11/kWh, considering a Tesla Powerwall 3 addition. Action: Ran SAM model with Massachusetts TMY data, actual TOU rate schedule, and Powerwall 3 specs. PVWatts simple estimate had shown $2,100/year savings. SAM showed $1,640/year for solar-only (because of TOU mismatch: production peaks midday, loads peak 5-8 PM) and $2,280/year with battery dispatching correctly. Result: The battery actually penciled out in that specific utility territory, which isn’t true everywhere. Without SAM, you’d never know whether the extra $12,000 for the Powerwall made sense. In this case, the 25-year NPV analysis said yes, barely, at a 4.5% discount rate.

Sources

• NREL PVWatts Calculator: NREL’s free web tool for residential solar production estimates using TMY weather data
• NREL System Advisor Model (SAM): Free, downloadable software for detailed solar, storage, and financial modeling; used by researchers and professionals
• EnergySage Solar Marketplace: Industry data on quote comparisons, pricing, and homeowner savings from competitive bids; EnergySage reports $5,000-$10,000 average savings from comparing quotes
• Solar Energy Industries Association (SEIA): Industry association tracking installation data, policy, and market trends for residential and commercial solar
• Aurora Solar: Professional solar design platform with LiDAR-based 3D modeling; limited free tier available for homeowners as of July 2026

Photo: Lena Netkach via Pexels

Recommended Resources

Disclosure: As an Amazon Associate, we earn a small commission from qualifying purchases at no extra cost to you. We only recommend products that genuinely support the topics covered in this article.

• Renogy 200W Solar Starter Kit + 30A Charge Controller (~$169), Complete beginner solar kit, 200W monocrystalline panel, charge controller, and mounting hardware included.
• Renogy 2×100W Monocrystalline Solar Panels (~$99), Expandable 200W panel set from the most trusted DIY solar brand, used widely in off-grid and home backup systems.