Google Project Sunroof told a homeowner in my area she’d save $1,400 a year with solar. She called me after getting three installer quotes, all projecting closer to $800. That gap isn’t nothing. It’s the difference between a system that pays off in 8 years and one that pays off in 14.

So let’s talk about what Sunroof actually gets right, where it quietly fails you, and how to use it as a starting point without letting it make your financial decisions for you.

What Google Sunroof Is Actually Doing Under the Hood

The tool uses aerial and satellite imagery, combined with 3D modeling of your roofline, to estimate how many hours of usable sunlight your roof receives each year. It factors in your local weather patterns, roof pitch and orientation, and shading from trees or neighboring structures. Then it layers in local utility rates and average system costs to project savings.

That’s genuinely impressive for a free tool. And for a lot of roofs, it’s in the right ballpark.

Here’s what most people don’t realize though: Sunroof’s accuracy is heavily dependent on the age and resolution of the imagery in your area. Google updates its aerial data on no fixed schedule. In fast-growing suburbs or rural counties, I’ve seen imagery that was clearly three or four years old. A tree that got taken down in 2023, a new addition on the west side of your house, a neighbor’s second story that now shades your array in the afternoon. None of that shows up if the satellite photo predates it.

The 3D modeling is also a generalization. It works better on standard gable roofs than on complex hip-and-valley configurations. I’ve tested this myself on two similar homes: a simple two-pitch ranch house and a four-bedroom with a complicated roofline and three dormers. On the ranch, Sunroof’s usable square footage estimate was within 8% of what our design software calculated. On the complex roof, it was off by over 20%, which translated to a meaningfully different panel count and projected output.

The Savings Estimates: Where the Numbers Go Soft

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ScenarioSunroof EstimateActual Installer QuotesDifferencePayback Period Gap
Homeowner case study$1,400/year~$800/year$600/year (43% overestimate)8 years vs. 14 years
Nevada client (pre-PUCN change)7.5 years payback11 years payback3.5 year differenceNet metering policy change
Simple ranch roofWithin 8% accuracyBaseline±8%Minimal impact
Complex hip-and-valley roof20% overestimate usable areaBaseline-20%Significant panel count/output variance

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This is where I’d ask you to be most skeptical.

Sunroof pulls electricity rate data, but it’s often working with state or regional averages, not your specific utility’s rate structure. If you’re on a time-of-use plan with Pacific Gas & Electric, or if your co-op in rural Georgia has below-average rates, the generic number will be wrong in ways that matter. As of July 2026, residential electricity rates vary from around 10 cents per kWh in parts of the South to over 30 cents in Hawaii and California. A tool averaging those together will produce wildly different accuracy depending on where you live.

The net metering assumption is the bigger problem. Sunroof tends to assume relatively favorable net metering (selling excess power back at or near retail rates). But net metering policies have been changing fast. Several states have moved to “net billing” at wholesale rates, which cuts the effective value of your excess generation by 50% or more. If your state made that change recently and Sunroof’s model hasn’t caught up, your savings estimate could be overstated by thousands of dollars over the life of the system.

I made this mistake myself early in my consulting work. I was using Sunroof projections as a sanity check on quotes without verifying the net metering assumption. A client in Nevada got a projection based on old rate structures before the PUCN modified net metering rules there. His real-world payback period was about 11 years, not the 7.5 Sunroof had suggested. I’ve been more careful since.

What It Gets Right (and I Don’t Want to Be Unfair)

Look, I’m being critical here, but I don’t want to leave you thinking Sunroof is useless. For roof orientation and basic solar potential, it’s genuinely solid. If Sunroof says your north-facing roof in Seattle is a bad candidate for solar, believe it. That conclusion is almost certainly right.

The shading analysis is also better than it looks at first glance. By modeling the sun’s path across your specific address at different times of year, it can flag situations where morning or afternoon shading would cost you meaningful production. I’ve compared Sunroof’s shading estimates to reports generated by professional tools like Aurora or Solargraf on a handful of projects, and on roofs with straightforward obstructions (one large tree to the southwest, for example), the estimates were within 10-15% of the professional tool’s output.

EnergySage’s market data confirms that Sunroof’s general “is this roof a good solar candidate” verdict lines up with professional assessments in the majority of cases. It’s the financial modeling layer where you need to add your own due diligence.

How to Use It Without Getting Burned

Here’s a practical workflow. Run Sunroof first to get a rough sense of your roof’s solar potential and an order-of-magnitude savings estimate. Then do three things before you take any of those numbers seriously:

Check your actual electricity rate, including your specific tier or time-of-use schedule. Log into your utility account and look at the last 12 months of bills. Total the kWh used. That number is what matters for sizing.

Verify your state’s current net metering policy. The National Renewable Energy Laboratory maintains resources on state-by-state policies, and your state public utilities commission website will have the current rules. This takes 20 minutes and can save you from a four-figure miscalculation.

Get at least two quotes from installers who will produce a detailed shade analysis using actual design software, not a Sunroof screenshot. Any installer who hands you a proposal built entirely on Sunroof data without running their own roof design is cutting corners.

Three worked examples from my experience:

Seattle homeowner, north-facing primary roof with a south-facing garage → Sunroof flagged it as low potential → Professional assessment confirmed the garage roof alone could support a 4.2 kW system, enough to cover 70% of usage. Sunroof was right to be cautious, but the real installer saw an option the tool missed.

Suburban Phoenix home, simple south-facing roof, SRP electric customer → Sunroof projected $1,100/year savings → Actual first-year savings after a correctly sized 8.5 kW system: $1,050. That’s one of the closest matches I’ve seen, partly because Arizona’s solar resource is so consistent and SRP’s rates fit neatly into the modeling.

New Jersey homeowner, complex Victorian roofline → Sunroof projected 14 panels and $900/year → Professional design identified only 9 usable panels due to shading and roof configuration → Real projected savings: $560/year. That’s a 37% gap. Big enough to change the whole financial case.

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