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How the estimate is made

Methodology updated October 10, 2026. These are planning estimates, not measured production, a site survey or a guaranteed quote.

1. A real building, or no estimate

For a supported address, our analysis worker matches a Microsoft building footprint and reconstructs roof planes from the public USGS LA_2021GNO_1_C22 LiDAR point cloud. Initial coverage is the Greater New Orleans survey, acquired November 12–20, 2021, not all Louisiana. Nearby ambiguous buildings, insufficient point support, unsupported complexity and source failures return an unavailable result rather than a generic roof. Plane fitting rejects unsupported areas; candidate panel rectangles must fit within a buffered, supported roof face. The interface displays face boundaries, point counts, fit residuals and coverage diagnostics, not a guaranteed accuracy grade.

Roof fitting uses three-dimensional neighborhoods and laser return metadata. Local contradictory lower-height returns are excluded from panel placement; a distributed lower layer can reject an apparent canopy surface. Roof-support triangles have edges at most 1 m and a maximum interior empty-radius bound of 0.5 m; larger survey gaps remain excluded. These rules cannot reliably identify every small vent, skylight or opaque smooth canopy; an installer must inspect those conditions.

North-up 2025 Louisiana Cities 6-Inch aerial imagery is supplied by Louisiana DOTD / NORPC and uses the same Web Mercator viewport as the overlays. It is context, not an inspection or the roof-height source. Orthophoto processing can displace elevated surfaces because of building lean even when ground features and coordinate extents match. LiDAR and aerial dates also differ; roofs and trees may have changed since acquisition. Microsoft building footprints are attributed under ODbL. Downloads, reconstruction and energy modeling run as a queued job; stopping browser polling does not cancel a shared job. No paid mapping credentials are needed.

2. Roof shading and weather-derived AC output

PVGIS supplies an hourly typical meteorological year for the location from its ERA5 weather source. These are historical typical-year conditions, not a live forecast or measurements at your roof. The model uses the supplied irradiance time offset and solar position, transposes direct and diffuse irradiance onto each reconstructed face, and applies local LiDAR horizons to direct sunlight. Diffuse-sky obstruction is an approximation. The weather request disables PVGIS terrain-horizon shading so the local correction is not applied twice.

pvlib applies a physical incidence-angle modifier to direct sunlight and Marion-integrated modifiers to isotropic sky and ground-reflected light. Generic uncoated glass uses refractive index 1.526, extinction coefficient 4/m and thickness 2 mm; actual coatings differ. Incident irradiance drives the module-temperature model, while optically effective irradiance drives DC output. Inverter conversion follows. Each candidate has twelve monthly AC totals, summed directly with no extra annual derate. Both shaded and unshaded paths include the same optical losses.

The comparison without local LiDAR shading uses the same weather, roof orientation, equipment and losses; it is not independent validation or a promise about tree removal. PVGIS’s separate generation tool can provide a cross-model check using the roof orientations and comparable equipment losses, but related ERA5 weather is not independent rooftop measurement. The local shade radius is 80 m; foliage, future growth, distant obstructions and recent roof changes remain uncertainties.

Default equipment assumptions are 430 W panels measuring 1.134 × 1.722 m, 14% system losses, a 1.2 DC/AC ratio and 96% rated inverter efficiency. Generic module temperature coefficients and a distributed per-panel share of inverter capacity simplify the model; this is not an electrical string design or a specified product. Panel gap and obstacle-buffer assumptions are shown with each result. The 0.5 m roof-edge margin is a modeling allowance, not a universal code-compliant setback. Candidates are ranked by modeled annual AC, with at most 200 panels and 12 supported faces; those limits are not a recommendation to fill the roof.

3. Savings depend on your tariff and usage

You must choose the utility shown on your bill; a New Orleans mailing address does not automatically select a tariff. For Entergy New Orleans, the planner models NM-4 monthly net metering: net surplus kWh carry forward to offset later monthly deficits. The modeled bank starts empty in January, continues across all 25 years, and never pays a bill from a previous month using future generation. Fixed charges remain. Unused credits are not valued as investment income; NM-4 pays a remaining balance at the applicable avoided cost when utility service terminates, not every month. No service-termination date or final payout is assumed here.

NM-4 requires a signed interconnection agreement and utility eligibility. We conservatively exclude modeled DC sizes above 25 kW from residential recommendations; the utility must confirm the applicable capacity-rating basis, network restrictions and actual approval. Full-roof scenarios remain visible but are flagged when they exceed this screen. Entergy Louisiana is a separate utility. Its selector, other utilities and unknown accounts use an explicitly generic net-billing scenario: direct use avoids the entered retail rate and exports receive the entered compensation. This is not a determination of any account’s current or legacy agreement. Negative generic bills assume export compensation is actually payable.

Twelve January–December consumption entries take precedence over an annual total. Annual-only input is spread evenly, which can change credit timing; incomplete monthly input remains preliminary. Direct-use share is an assumption capped by each month’s usage, not a measured hourly load simulation. It affects net billing but not monthly kWh netting. Choosing Entergy New Orleans starts with $0.13991/kWh in variable charges and an $8.07 customer charge, derived from Entergy’s October 2026 published 1,000-kWh example. Choosing Entergy Louisiana starts with its published $0.0259331/kWh avoided-cost export rate effective April 1, 2025; its retail rate remains an illustrative input. All remain editable starting points, not verified account prices. The confirmation boxes record that the user checked the current inputs and eligibility; they do not change a rate, billing rule or calculation. One average retail rate simplifies seasonal rates and variable riders. Taxes, minimum bills and account-specific adjustments are excluded.

4. Compare bill economics, not every reason to install

The planner evaluates every supported first-N panel layout, including zero panels, using production-ranked candidates. It shows roof capacity, the size closest to annual consumption, and the size with the highest 25-year bill-savings net present value (NPV). Exact ties choose the smaller size. This is not a search of every possible panel arrangement or a certified installation design. No installation is the zero-cost baseline; when it ranks highest, no modeled solar size has positive bill-savings NPV under the entered assumptions.

Installed price initially equals DC watts times an illustrative $3/W, not an installer bid. The starting maintenance allowance is $20/kW/year, with one $200/kW equipment-replacement allowance in year 15. These are editable budgeting assumptions, not measured failure rates or service prices. The model assumes 0.5% annual production degradation, constant household usage and an editable 2% annual retail-rate change. Export prices, fixed charges, maintenance and replacement budgets remain constant in nominal dollars.

Cash purchase deducts the price upfront. Optional financing uses the entered down payment and a fixed nominal annual interest rate with monthly amortization over 1–25 years. Include dealer and origination fees in the entered price; no lender offer or fee schedule is inferred. First-year operating cash flow excludes the separately displayed upfront cash. The 25-year net solar cash flow includes upfront cost, loan payments and interest, upkeep and the scheduled replacement. NPV discounts each year’s bill savings minus those costs back to today using an editable rate starting at 5%, then subtracts upfront cash. Positive NPV means discounted bill savings exceed modeled solar costs; negative NPV means they do not. The discount rate represents the user’s value for receiving money sooner, not a separately simulated investment or the loan rate.

Cash-flow payback is the last interpolated recovery from cumulative deficit that remains recovered through year 25. A later replacement can invalidate an earlier apparent break-even. No-solar has no solar payback period or solar costs; ordinary electricity bills still continue. The financial comparison excludes incentives, property value, insurance changes, emissions, battery backup value and tax effects.

Standard grid-tied solar normally disconnects during a utility outage because anti-islanding protection must keep a customer system from energizing a failed utility circuit. Outage power requires an intentionally islandable design with compatible inverter and isolation or transfer equipment, typically paired with battery storage and a defined set of backed-up loads. Under the modeled Entergy New Orleans NM-4 rules, monthly exported kWh are already banked for later monthly use, so battery storage does not automatically add bill savings and conversion losses can reduce usable energy. Under net billing, storage may replace low-value exports with later avoided purchases, but a defensible estimate needs an actual battery quote and specifications plus hourly household load, solar output, dispatch and tariff data. This monthly model does not have those inputs. Battery size, critical-load demand, outage duration, generator integration and the value of resilience are not modeled here.

Production sensitivity varies generation by ±20%; generic net billing also varies direct use by ±15 percentage points. The lowest and highest resulting first-year bill savings hold rates and costs constant. These are chosen stress scenarios, not measured annual weather variability, a statistical confidence interval, or a complete financial-risk range. Survey/shading uncertainty and editable financial assumptions are disclosed separately.

5. Calculate a supported layout or manual proposal

The optional proposal calculator accepts either a supported modeled panel count or user-entered facts from an installer or independent design. A manual scenario requires a whole panel count, DC system size, first-year AC production and itemized installed prices. It does not add rectangles to the roof image, assert that extra panels fit, or infer missing production. To run monthly billing, the entered annual AC total is distributed using the address’s modeled seasonal production shape; different orientations, shading or equipment can produce a different monthly pattern. A blank cost is not silently treated as zero. Both scenarios use the household, tariff, financing and upkeep assumptions selected above. Proposal drafts stay in page memory and clear when the property changes or the page reloads; they are not saved, sent to analytics or submitted as inquiries. Installer introductions use a separate form and explicit sharing permission.

6. Current incentives excluded from the estimate

The planner does not subtract a federal residential clean energy tax credit for a 2026 installation. The IRS states that the credit is unavailable for property placed in service after December 31, 2025. New Orleans adopted Resolution R-26-421 on October 1, 2026, setting initial Neighborhood Power Plan battery incentives at $550 per installed kWh for residential customers and $850 per installed kWh for qualifying low- and moderate-income customers, capped at $15,000. The resolution directs Entergy to propose implementation no later than the first quarter of 2027, so availability is not assumed. Battery price, backup value and demand-response revenue are outside the solar calculation. The City’s August 2025 Solar for All NOLA page described a planned $5,000 income-qualified subsidy but said application details were forthcoming; it is not counted as available funding.

Sources and limitations

Before making a purchase, verify roof condition, structural and electrical suitability, fire setbacks, permitting, utility interconnection, net-metering/export rules and a written installer price.