How to Calculate the 10% Solar Production Impairment Using NREL PVWatts (Step-by-Step)
Step-by-step engineering tutorial on how to generate an undeniable NREL PVWatts simulation report proving that an HOA’s demand to move solar panels off a South or West roof causes >10% annual kWh loss.
1. Controlling Statutory Protections & Legal Thresholds
- §Over 25 state Solar Access statutes (including Texas Prop. Code § 202.010, California Civ. Code § 714, North Carolina G.S. § 22B-20, Colorado C.R.S. § 38-30-168, Virginia Code § 55.1-1819.1, Illinois 765 ILCS 165/20, and Nevada NRS 278.0208) prohibit HOA placement rules that reduce solar production by more than 10% (or 5% in Illinois/Maryland).
- §Because NREL PVWatts is developed and maintained by the U.S. Department of Energy’s National Renewable Energy Laboratory, courts and state legislatures recognize it as the gold-standard objective reference.
- §At mid-United States latitudes (30°N to 42°N), moving a 20-degree pitched roof array from South (180° azimuth) to North (0° azimuth) reduces annual kWh output by 26% to 44%—meaning the homeowner wins the 10% statutory test every single time.
U.S. Department of Energy NREL PVWatts v8 Statutory Codification
State statutes (such as Tex. Prop. Code § 202.010(e)) explicitly name the National Renewable Energy Laboratory PVWatts Calculator by statute, removing all guesswork or HOA board argument.
2. How to Document Your 10% Efficiency / Cost Proof (Step-by-Step)
Step 1: Go to https://pvwatts.nrel.gov and type your exact residential property address, then click Go to select the nearest TMYx weather station.
Step 2: Enter your DC System Size (e.g., 10.0 kW), Module Type (Premium), Array Type (Fixed - Roof Mount), System Losses (14%), and Tilt (your roof pitch in degrees: 4:12 = 18.4°, 6:12 = 26.6°, 8:12 = 33.7°).
Step 3: For Run #1 (Your Proposed Layout), enter the Azimuth of your proposed roof plane (180° = South, 270° = West, 90° = East). Click the arrow to view "Results" and download/print the PDF showing Annual AC Energy (kWh/year).
Step 4: Click back to System Info and change ONLY the Azimuth to match the HOA’s demanded roof slope (e.g., 0° = North). Keep DC size, tilt, and losses 100% identical. Print the Run #2 PDF.
Step 5: Apply the statutory formula: Impairment % = ((Run #1 kWh - Run #2 kWh) ÷ Run #1 kWh) × 100. Highlight the >10% drop in red on your cover sheet.
3. 1-Click Pre-Litigation HOA Statutory Demand Letter
Send via Certified Mail (Return Receipt Requested) and email to your HOA Board and Property Manager with your NREL PVWatts PDF attached.
ATTACHMENT A: OFFICIAL NREL PVWATTS 10% EFFICIENCY IMPAIRMENT WORKSHEET
Property Address: [Insert Homeowner Address]
Weather Station Data Source: NREL NSRDB TMYx (U.S. Department of Energy)
DC System Size Modeled: [e.g., 10.0 kW DC] | Roof Tilt: [e.g., 26.6° (6:12 Pitch)] | System Losses: 14.0%
1. SIMULATION A — INITIALLY PROPOSED ROOF PLANE (SOUTH / WEST)
• Roof Azimuth: [e.g., 180° Due South]
• Verified Annual AC Production (NREL PVWatts): [e.g., 15,420] kWh / year
2. SIMULATION B — HOA-REQUESTED ALTERNATE ROOF PLANE (NORTH / SHADED)
• Roof Azimuth: [e.g., 0° Due North]
• Verified Annual AC Production (NREL PVWatts): [e.g., 10,190] kWh / year
3. STATUTORY IMPAIRMENT CALCULATION
• Annual kWh Lost: [15,420 - 10,190 = 5,230 kWh / year]
• 25-Year Cumulative Clean Energy Lost: [130,750 kWh]
• Percentage Efficiency Impairment:
((15,420 - 10,190) ÷ 15,420) × 100 = 33.92% PRODUCTION LOSS
4. STATUTORY CONCLUSION
Because 33.92% exceeds the maximum permissible statutory threshold of 10.0%, the HOA's requested relocation is void and unenforceable as a matter of state law. Full NREL PVWatts PDF printouts for Simulation A and Simulation B are attached hereto.Frequently Asked Questions
How do I convert my roof pitch (like 6/12) into degrees for NREL PVWatts?
A 4/12 pitch is 18.4 degrees; a 5/12 pitch is 22.6 degrees; a 6/12 pitch is 26.6 degrees; a 7/12 pitch is 30.3 degrees; an 8/12 pitch is 33.7 degrees; and a 10/12 pitch is 39.8 degrees.
What if the rear roof has trees shading it in addition to facing a worse direction?
You can include both the azimuth loss in PVWatts AND a LIDAR shade report (Aurora Solar or OpenSolar) showing Total Solar Resource Fraction (TSRF) loss from trees, which pushes impairment even higher.