What a Planning Estimate Cannot Replace
SolarRoof.asia Team · 9/29/2026
What a Planning Estimate Cannot Replace
You’ve just run a quick satellite‑based solar calculator. The numbers look promising: a 7 kW system, $1,200 / year in savings, a 6‑year payback. It feels like a green light to call an installer and sign a contract.
Hold on.
A planning estimate is a starting point, not a final answer. It tells you what might fit on your roof and what could be produced under idealized conditions. It does not replace the on‑site work that a qualified installer must do before any equipment is ordered, any permit is filed, or any money changes hands.
Below is a practical breakdown of what a satellite‑derived estimate can show you — and, more importantly, what it cannot. Use it as a checklist when you move from “curious” to “ready to install.”
1. What a Planning Estimate Actually Gives You
| Output | How It’s Derived | Typical Accuracy |
|---|---|---|
| Roof‑area & usable‑space | 2‑D satellite imagery + roof‑outline detection | ±10‑15 % (depends on image resolution, roof complexity) |
| Orientation & tilt | GIS‑derived azimuth & pitch from the same imagery | ±5‑10° for azimuth; tilt often assumed (e.g., 15°) |
| Annual irradiance | Historical satellite irradiance datasets (e.g., NASA POWER, Solcast) | ±5‑10 % for yearly totals |
| System size & energy yield | Simple DC‑size × specific yield (kWh/kWp) | ±10‑20 % for first‑year production |
| Rough financial snapshot | Local electricity rate × estimated production × incentive assumptions | Wide range; incentives, net‑metering rules, and rate structures vary widely |
Bottom line: The estimate is a screening tool. It helps you decide whether a deeper look is worth the time and money.
2. Structural Reality — The Roof You Can’t See From Space
2.1 Load‑bearing capacity
Satellites see the roof’s shape, not its strength.
- Dead load – existing roofing material, insulation, decking.
- Live load – snow, wind, maintenance personnel.
- Added load – racking, modules, wiring, inverters (≈ 15‑20 lb/ft² for a typical flush‑mount system).
A structural engineer or a certified solar installer must verify that the roof framing can handle the combined loads plus the required safety factor (usually 1.5×).
2.2 Roof age & condition
- Remaining service life – If the roof needs replacement in 3‑5 years, it’s cheaper to re‑roof before installing solar.
- Penetrations & flashings – Existing vents, skylights, chimneys, and HVAC curbs affect where racking can be attached.
2.3 Roof material & attachment method
| Material | Typical attachment | Special considerations |
|---|---|---|
| Asphalt shingle | Flash‑mounted lag bolts or rail‑less clamps | Must avoid damaging granules; use sealed flashings |
| Standing‑seam metal | Clamp‑on rails (no penetrations) | Verify seam spacing matches rail pattern |
| Tile (clay/concrete) | Hook‑and‑batten or tile‑replacement mounts | Fragile; requires experienced crew |
| Flat membrane (TPO/EPDM) | Ballasted or mechanically attached | Ballast weight must stay within structural limits |
| Wood shake | Not recommended for direct mount | Usually requires a new roof deck first |
Action: Ask the installer for a structural assessment report (often a one‑page letter) before you sign any contract.
3. Electrical Infrastructure — More Than Just “Plug‑and‑Play”
3.1 Service panel capacity
- Main breaker rating (e.g., 100 A, 200 A).
- Available breaker spaces for a new solar breaker (usually a 2‑pole 30‑60 A).
- Busbar rating – the panel’s busbar must handle the combined load of existing circuits + solar backfeed (NEC 705.12).
If the panel is full or undersized, you’ll need a panel upgrade (often $1,500‑$3,500) or a sub‑panel dedicated to solar.
3.2 Wiring & conduit runs
- Distance from array to inverter to panel.
- Voltage drop calculations (target < 3 % for DC, < 2 % for AC).
- Conduit type (EMT, PVC, flexible) and support requirements per NEC 300.
3.3 Grounding & bonding
- Equipment grounding conductor (EGC) sized per NEC 250.122.
- Ground‑fault protection (GFCI/GFPE) for DC circuits > 30 V (NEC 690.41).
3.4 Utility interconnection requirements
- Net‑metering vs. feed‑in tariff – each utility has its own application, study, and possible interconnection agreement fees.
- Anti‑islanding – inverter must be UL 1741‑SA listed.
- Revenue‑grade meter – some utilities require a separate meter for solar production.
Action: Request a pre‑interconnection checklist from your utility early. Many installers will handle the paperwork, but you should know the timeline (often 30‑90 days).
4. Shading, Micro‑climate & Real‑World Irradiance
4.1 Near‑object shading
Satellite imagery captures large shadows (trees, adjacent buildings) at the time of the pass. It misses:
- Seasonal leaf‑on/leaf‑off cycles.
- New construction or tree growth since the image date.
- Small‑scale obstructions: chimneys, vent pipes, satellite dishes, parapets.
A shade analysis (e.g., using a SunEye, Solmetric, or a drone‑based LiDAR scan) quantifies hourly shading throughout the year. Even 10 % annual shading can cut production 5‑15 % depending on string layout.
4.2 Soiling & soiling loss
- Dust, pollen, bird droppings, pollution.
- Local climate (dry vs. humid) changes cleaning frequency.
- Satellite models usually assume a clean surface (0‑2 % loss). Real‑world loss can be 3‑7 % in dusty regions.
4.3 Temperature effects
Modules lose ~0.4‑0.5 %/°C above 25 °C. Roof‑mounted arrays can run 20‑30 °C hotter than ambient. A planning estimate often uses a standard temperature coefficient; an installer will model the actual cell temperature based on mounting type, airflow, and local climate data.
Action: Ask for a PVsyst or SAM simulation that includes measured shading, soiling, and temperature profiles.
5. Permitting, Code Compliance & HOA Rules
5.1 Building & electrical permits
- Local jurisdiction – each city/county has its own permit forms, plan‑review fees, and inspection schedule.
- Plan set – typically includes site plan, structural details, single‑line diagram, equipment specs, and a fire‑access plan (NFPA 1).
5.2 Fire‑code setbacks
- IRC/IRC‑R (International Residential Code) often requires a 3‑ft clear zone around the array for firefighter access.
- Setbacks from roof edges, ridges, and penetrations can shrink usable area by 10‑20 %.
5.3 HOA / historic district restrictions
- Some HOAs limit visible panels, require low‑profile racking, or ban installations altogether.
- Historic districts may require design review and specific mounting aesthetics.
Action: Get a permit readiness checklist from the installer. If you live in an HOA, request the architectural guidelines before you order equipment.
6. Roof Geometry Nuances That Satellites Simplify
| Satellite simplification | Real‑world impact |
|---|---|
| Single azimuth per roof plane | Complex roofs have multiple facets; each facet gets its own azimuth/tilt. |
| Uniform tilt assumption | Actual pitch may vary (e.g., dormers, skylights). |
| No parapet or equipment curbs | These create “dead zones” where modules cannot be placed. |
| Flat‑roof treated as 0° tilt | Many flat roofs use tilt‑up racking (10‑30°) to boost yield; ballast limits may restrict tilt angle. |
An installer will walk the roof (or use a drone) to map every usable module‑placement zone and then run a layout optimization (often with software like Aurora, HelioScope, or OpenSolar).
7. System Sizing & Performance Modeling — Why “One Size Fits All” Doesn’t Work
7.1 DC‑to‑AC ratio (inverter loading)
- Typical ratios: 1.1‑1.3 : 1.
- Oversizing the DC array can improve capacity factor but may trigger inverter clipping during peak sun.
- The optimal ratio depends on local irradiance distribution, shading, and utility demand‑charge structure.
7.2 Module‑level power electronics (MLPE) vs. string inverters
| Feature | String inverter | Microinverter / Power optimizer |
|---|---|---|
| Shade tolerance | Low (whole string affected) | High (module‑level) |
| Monitoring granularity | String‑level | Module‑level |
| Cost per watt | Lower | Higher |
| Reliability (single point of failure) | Inverter failure = whole system down | Module‑level failure isolated |
A planning estimate usually assumes a string inverter. If you have any shading, MLPE often pays for itself in extra production.
7.3 Degradation & warranty modeling
- Linear degradation ~0.5 %/yr (typical).
- Performance warranty guarantees 80‑90 % at year 25.
- Financial models should use a degradation curve rather than a flat 25‑year production number.
Action: Request a cash‑flow model that includes degradation, inverter replacement (usually year 10‑12), O&M costs, and realistic incentive timing.
8. Financial Incentives — The Moving Target
| Incentive | Typical source | Common pitfalls |
|---|---|---|
| Federal Investment Tax Credit (ITC) | 30 % of system cost (2024‑2032) | Must have tax liability; claim on IRS Form 5695 |
| State rebates / performance‑based incentives | State energy offices | Often capped, first‑come‑first‑served |
| Utility rebates | Local utility | May require specific equipment or installer certification |
| Property‑tax exemption | County assessor | Not automatic; file paperwork |
| Sales‑tax exemption | State | Varies; sometimes only for equipment, not labor |
| Net‑metering / feed‑in tariff | Utility commission | Rates can change; some utilities moving to net‑billing |
A planning estimate usually plugs in today’s incentive values. By the time you sign a contract, some may have expired or changed.
Action: Have the installer provide a current incentive snapshot and a contingency plan if a program ends before interconnection.
9. Installer Expertise & Design Optimization — The Human Factor
9.1 Why the same hardware can yield different results
- Array layout – maximizing usable roof area while respecting fire setbacks.
- String design – balancing voltage/current limits, minimizing mismatch losses.
- Racking selection – low‑profile vs. tilt‑up, ballasted vs. penetrated, impact on roof warranty.
- Commissioning – proper IV‑curve testing, grounding verification, and performance baseline.
9.2 Red flags in installer proposals
- No site visit before quoting a firm price.
- Single‑page quote with no breakdown (equipment, labor, permits, engineering).
- Guaranteed production numbers without a performance guarantee clause.
- Pressure to sign before you’ve reviewed the structural report or utility interconnection status.
9.3 Questions to ask every installer
- Can you show me a recent structural assessment for a roof like mine?
- What is your process for shade analysis — drone, SunEye, or software only?
- Which inverter topology do you recommend for my shading profile, and why?
- What is the expected timeline for permitting, interconnection, and installation?
- Do you provide a performance guarantee (kWh/yr) and what happens if it’s missed?
- Who handles the utility paperwork — you or me?
- What is your warranty coverage for labor, racking, and roof penetrations?
10. Practical Next‑Step Checklist for Homeowners & Property Managers
| Step | Who Does It | Typical Timing | Deliverable |
|---|---|---|---|
| 1. Run a planning estimate | You (online tool) | 15 min | Rough kW size, annual kWh, ballpark savings |
| 2. Gather roof info | You / property manager | 1‑2 days | Roof age, material, recent inspection photos |
| 3. Request 2‑3 installer site visits | Installers | 1‑2 weeks | Structural letter, shade report, layout PDF |
| 4. Compare proposals | You | 1 week | Side‑by‑side matrix (cost, equipment, warranties, timeline) |
| 5. Verify utility interconnection | Installer (or you) | 2‑4 weeks | Interconnection application receipt |
| 6. Secure permits | Installer (usually) | 2‑6 weeks (jurisdiction dependent) | Permit approval |
| 7. Finalize financing / incentives | You / financier | Parallel with permits | Signed loan/lease, incentive reservation |
| 8. Schedule installation | Installer | 1‑3 days on‑site | Completed system, commissioning report |
| 9. Post‑install inspection & PTO | AHJ / utility | 1‑2 weeks after install | Permission to Operate (PTO) |
| 10. Activate monitoring & O&M plan | You / installer | Immediate | Online portal access, cleaning schedule |
Tip: Keep a project folder (digital or paper) with every email, PDF, and signed document. It saves hours if you need to reference a warranty claim or incentive proof later.
11. Quick “What‑If” Scenarios — Seeing the Gaps in Real Time
| Scenario | Planning estimate says… | On‑site reality | Resulting change |
|---|---|---|---|
| Older asphalt roof (15 yr left) | 7 kW fits, $1,200/yr savings | Roof needs replacement in 3 yr | Re‑roof first → add $8‑12k, but avoids future removal/re‑install |
| Large oak tree 12 ft south | Minor shading (5 %) | Seasonal leaf‑on shading 22 % | Add microinverters or redesign layout → +$0.15/W |
| 100 A panel, full breaker slots | No issue noted | No space for |