What actually differs between the two options
Ground mounts need real, meaningful extra space
For a 10 kW system, this calculator estimates about 650 square feet of open land for a ground mount against about 500 square feet of roof. The ground mount needs more because tilted rows cast shadows and need gaps. At latitude 40, panels 5.6 feet long and tilted 30 degrees need roughly 10.5 feet from the front of one row to the front of the next to clear the winter-solstice noon shadow. The 650 square foot figure assumes a compact one- or two-row layout, and every added row takes more land. Before committing to a ground mount, confirm the roof really cannot carry the system with the roof space estimator and the usable roof area calculator. A ground mount also usually needs its own permit, footing design, and setback check, and some jurisdictions require engineering review.
A ground mount's energy advantage depends on your roof
A ground mount can sit at the ideal tilt and direction, so its advantage is the gap between that ideal and your roof. Modeled with NREL typical-year weather data, the gain over a south-facing roof of typical pitch is only about 0 to 3% at mid-latitudes, roughly 5 to 10% over southeast- or southwest-facing roofs, and about 20 to 30% over east- or west-facing roofs. Near-flat roofs fall in between, at roughly 14 to 25%. The calculator uses 2%, 6%, 20%, and 14% for those four cases. Escaping roof shade is a separate gain on top. To test your own roof against the ideal, run both through NLR's PVWatts calculator, and the tilt angle optimizer shows the best tilt for your latitude.
The cost premium is real, but smaller per watt than many guides claim
A ground mount costs more because it adds racking, concrete footings or ground screws, and trenching for underground conduit back to your electrical panel. This calculator uses $0.35 per watt for that premium, which is $3,500 for a 10 kW system before any extra output offsets it. Installer guides commonly quote $0.30 to $0.50 per watt for the same gap, and the premium is worth checking against current quotes in your area, since racking and labor prices move with the market. For a grounded view of what residential systems actually cost, Berkeley Lab's Tracking the Sun report publishes installed prices from real projects across the country.
When a ground mount actually makes sense
Ground mounts make the most sense when your roof is shaded, aging, poorly oriented, or too small for the system size you need, and you have open, unshaded land available. A roof mount remains the more affordable default for most homeowners, since it avoids the foundations, trenching, and separate structure a ground mount needs. For a side-by-side output estimate on your own numbers, see the output comparison between ground mount and roof mount.
Ongoing costs differ too
Ground-level panels are easier to reach for cleaning and inspection, but a ground array adds vegetation control around the array. The maintenance cost for ground mount systems estimator puts numbers on those ongoing costs.