When you search your address on Solrova, the number that comes back — your roof's estimated annual kWh production — anchors every dollar figure we show you. That number isn't a guess. It comes from satellite roof imagery, an NREL production model, and a set of documented assumptions we publish on this page. Here is exactly how the calculation works, where it shines, where it falls short, and how to read the result before you talk to a Solar Partner.
How the satellite roof analysis works
We start with Google Solar API, which turns aerial imagery of your home into a usable 3D model of the roof. Google has trained machine learning models on satellite imagery of nearly every U.S. roof, and the API returns four pieces of information we care about.
First, it identifies the individual roof planes on your home, since most houses have several slopes facing different directions. Second, it estimates the usable area of each plane after subtracting skylights, vents, HVAC equipment, and roof edges that can't hold panels. Third, it estimates annual sun hours on each plane, factoring in your region's typical cloud cover. Fourth, it maps shading from nearby trees and buildings across the day and across the seasons.
The output is essentially a high-resolution survey of your roof's solar exposure built entirely from satellite imagery. No one has to climb a ladder, and the cost is low enough that we can include this analysis for free in every Solrova estimate. The trade-off is what satellite can't see — interior structural details, very recent tree growth, or obstacles tucked behind chimneys. That is why your Solar Partner's site visit is still the final check. Google's API also flags roofs the model isn't confident about, typically very small homes, complex multi-tier roofs, or addresses where the satellite tile is old, and when that happens our estimate carries a wider confidence range.
How PVWatts converts sunlight into kWh
Knowing your roof gets sun isn't the same as knowing how many kWh you will produce. To make that conversion, we use PVWatts V8, the National Renewable Energy Laboratory's reference model for residential solar systems. PVWatts is the industry standard because it's open, validated against decades of real-world data, and refreshed regularly by NREL staff. Solar designers, utilities, and government agencies all use PVWatts as a baseline because no commercial alternative offers the same combination of accuracy, transparency, and free public access.
PVWatts takes the solar radiation data for your location, runs it through a simplified photovoltaic system, and accounts for the real losses every system experiences. Hot panels are slightly less efficient than cool ones. Dust and pollen reduce output. Wiring losses, inverter conversion losses, and small panel-to-panel mismatch each take a few percent off the theoretical maximum. Combined, these losses bring a modern residential system to roughly 80 to 85 percent of its theoretical output.
PVWatts gives us an annual kWh figure per kilowatt of installed capacity. We multiply that by the panel capacity that fits on your usable roof area, and that's the kWh number you see on Solrova. For a typical 10-12 kW system in the continental United States, the model lands between 12,000 and 18,000 kWh per year depending on local sunlight. Cloudier markets like the Pacific Northwest produce closer to 10,000 kWh. Sunnier markets like Arizona or southern California push past 16,000 kWh.
The assumptions we publish and why we picked them
Every solar estimate sits on top of assumptions, and we publish ours so you can sanity-check them against your situation. Panel degradation runs at 0.5% per year, the figure backed by NREL field data and standard manufacturer warranties. A panel making 100 kWh in year one makes about 99.5 kWh in year two, and so on for the warrantied 25-year life.
System cost is set at $3,500 per installed kilowatt, the national median price for a residential cash purchase in Lawrence Berkeley National Laboratory's Tracking the Sun (2025 data update). Regional cost varies, with quotes landing near $3,000 per kilowatt in low-cost markets and above $5,000 in high-cost ones. Your Solar Partner's quote replaces this assumption with a binding number for your address.
For financed systems we model a 7.49% APR over 25 years, a mid-market loan rate in 2026. Better credit will lower the rate. We assume a $2,000 inverter replacement around year 13, since residential inverters typically last 10 to 15 years. Utility rate escalation runs at 3.5% per year, the long-run average across U.S. markets. If your local utility raises rates faster, your kWh savings come out higher than our estimate. If they raise more slowly, savings come out lower. Lease pricing is modeled regionally — 14¢/kWh in California, 9¢/kWh in Texas, 10¢/kWh elsewhere — to reflect how lease providers price.
One assumption we don't carry by default: the 30% federal solar tax credit. Section 25D, which covered cash and loan purchases, expired December 31, 2025, so we model owned systems without it unless you tell us otherwise. Section 48E, the credit that applies to leases and PPAs, remains available under current law if the project starts construction by July 4, 2026, or is in service by December 31, 2027, and lease providers price that benefit into your monthly rate rather than as a paper credit you claim. The Solar Design Studio reflects both rules in the financing comparison.
What we get right and where we're conservative
Solrova's estimate is most reliable at the annual scale. If we project 13,500 kWh per year, the system's actual first-year output is usually within 500 to 1,000 kWh of that figure. Satellite imagery captures roof geometry accurately, and PVWatts is well-validated against thousands of real residential systems. The relative ranking between cash, loan, lease, and PPA financing is also reliable — the option that looks best in our analysis is almost always the option that looks best after a Solar Partner walks the math with you. On a month-to-month basis the variance is larger because weather is variable, but the annual total smooths out the swings.
We chose the conservative end of every assumption deliberately. The 0.5% degradation rate is the standard, not an optimistic outlier. The $3,500/kW cost sits at the national median rather than the lowest regional quote. The 3.5% utility escalation reflects the long-run average rather than the higher rates many markets have actually seen the past five years. The result is that real savings often beat our estimate, particularly in markets where utility rates have been climbing fast. We would rather understate the kWh value of your roof and let your Solar Partner's quote pleasantly surprise you than overstate it and disappoint. The same conservatism shapes our payback calculations, which is why Solrova rarely tells you solar will pay back in three or four years even when sales materials in your area promise it.
What satellite analysis can't see
Remote analysis has hard limits, and a Solar Partner site visit is how those gaps get closed. Satellite imagery can be months or years old, so a newly planted tree, a deck added since the last flyover, or recent solar on a neighbor's roof won't appear in our model. Shading estimates assume average tree height for the season — if your yard has unusually mature canopy, real production may run 10 to 20% below the estimate.
We also can't see structural details that affect what fits. A second-story dormer, a chimney shadow, or unusual roof pitch can reduce the panel area Google flagged as usable. Microclimates aren't captured either: a home in a coastal fog belt produces less than the regional weather data suggests, even if a home two miles inland does fine.
For all these reasons, we treat the Solrova number as a screening estimate. It is accurate enough to tell you whether solar deserves a serious look, and accurate enough to compare financing options at a glance. Your Solar Partner's on-site measurement is the document that becomes binding. If our estimate and the Solar Partner's diverge by more than 15%, that's worth a conversation before you sign anything.
What this means in practice: Solrova's job is to give you a credible enough estimate that you know whether to keep going. If our number says solar pays back in 8 years on your roof, that's a strong signal worth pursuing. If it says the payback is 19 years, you may still want a Solar Partner's perspective — local incentives or your utility's particular rate structure can shift the math more than a satellite model can pick up.