1. Roof surface area
When you enter a measured roof-surface area, the calculator uses that figure directly and does not apply a second pitch adjustment. In the combined calculator's home-area route, the entered above-ground finished floor area is divided by the number of stories to approximate a building footprint. That footprint proxy is then adjusted by the selected pitch factor:
Pitch factor = √(1 + (rise ÷ run)²)
Examples: 2:12 ≈ 1.014; 4:12 ≈ 1.054; 6:12 ≈ 1.118; 9:12 = 1.250; 12:12 ≈ 1.414. A 1,600 sq ft footprint at 6:12 produces approximately 1,789 sq ft of sloped surface before overhangs, dormers, hips, valleys, attached structures or other geometric adjustments. The standalone roof-area calculator uses the selected exact pitch factor. Neither tool measures roof planes or complex geometry.
Dividing finished floor area by stories is only an approximation, not a measured footprint. Do not use it as a survey or order quantity. Measure separate roof planes, overhangs, porches, additions and complex features independently.
2. Roof replacement arithmetic
Estimated low/high cost = measured roof surface area × visitor-entered low/high installed price per roof sq ft.
There are no built-in national or local price assumptions, material premiums, pitch premiums, complexity multipliers, waste allowances or automatic contingencies. The visitor-supplied rates must already reflect comparable material and project scope, including local labor, tear-off, disposal, decking, flashing, access, permits, warranty and other relevant work. The website cannot determine whether an entered rate is realistic or competitive.
The material selector in the standalone roof-cost calculator is a descriptive label only; it does not change the formula. The calculator does not diagnose roof condition or decide whether repair or replacement is appropriate.
Arithmetic example, not a market benchmark: 1,800 sq ft × $5.50–$8.00 per sq ft = $9,900–$14,400. A roofing square is 100 sq ft; do not confuse dollars per roofing square with dollars per sq ft.
3. Solar system size and production
Target annual generation = annual household electricity use (kWh) × visitor-entered target percentage.
Approximate DC system size (kW) = target annual generation (kWh) ÷ visitor-entered annual production per DC kW (kWh/kW/year).
Estimated annual generation = estimated DC system size × visitor-entered annual production per DC kW.
Both solar calculators require annual household consumption in kWh from utility bills. They do not estimate consumption by dividing total bill dollars by a retail rate; fixed customer charges, demand charges, taxes and other components can distort that shortcut.
The visitor must supply a production-per-kW assumption from a location-specific model such as NLR PVWatts or another credible source. For PVWatts, divide modeled annual AC energy output in kWh by modeled DC system capacity in kW. The website does not call PVWatts, use ZIP-code irradiance, measure roof orientation or shading, or determine whether the array physically fits.
The accepted 100–4,000 kWh/kW/year input range is a broad typo check only. It is not a U.S. benchmark or recommended production range.
4. Annual solar energy-flow scenario
No direct-use share, import rate or export credit is prefilled. These fields are visitor inputs. A blank field means unknown; it is not silently treated as zero. An explicit $0 rate is accepted as a visitor-entered value when the applicable tariff supports it.
Self-consumed kWh = min(annual solar generation × visitor-entered annual direct-use share, annual household kWh).
Exported kWh = max(annual solar generation − self-consumed kWh, 0).
Modeled annual energy value = (self-consumed kWh × marginal import rate) + (exported kWh × utility export credit).
The import rate is needed only when modeled direct-use kWh are positive. The export credit is needed only when modeled exports are positive. If all modeled generation is used directly, an export credit is not needed. If no direct-use share is entered, the calculators do not split generation into direct use and exports or calculate total energy value. If a required tariff input is unknown, total annual dollar value and simple payback are withheld.
The visitor-entered annual direct-use share is a scenario assumption, not a measured hourly load profile. Actual self-consumption depends on when the household uses electricity and when the array produces it. This annual-share calculator does not simulate hourly or interval-meter data, seasonal variation, time-of-use periods, fixed charges, demand charges, minimum bills, taxes, battery dispatch, export caps, curtailment or utility-specific monthly credit rules.
5. Simple cash payback and financing illustration
Modeled cash system price = system DC watts × visitor-entered installed dollars per DC watt.
Simple cash payback = modeled cash system price ÷ first-year modeled annual energy value.
This is an undiscounted first-year arithmetic ratio, not ROI, NPV, IRR, a forecast or a financing recommendation. It does not include degradation, maintenance, inverter replacement, insurance, taxes, incentives, financing fees, future rate changes or the time value of money. The combined calculator's optional fixed-rate loan payment uses a standard monthly amortization formula from the entered principal, APR and term. It excludes dealer fees, origination charges, escalators, liens and lender underwriting; no loan approval or offer is implied.
6. Solar panel count
Panel count = ceiling(target DC system watts ÷ visitor-entered module nameplate watts).
If module area is entered, total module surface area is panel count multiplied by that area. The result excludes spacing, setbacks, pathways, fire access, obstructions, shade, roof-plane geometry, attachment design and structural constraints. It is not a roof-fit plan.
7. Worked arithmetic examples
- Solar scenario: 10,800 kWh annual use, 80% target generation, 1,200 kWh/kW/year visitor-supplied production, 60% visitor-supplied annual direct-use share, $0.18/kWh marginal import rate and $0.04/kWh export credit. Target generation = 8,640 kWh; system size = 7.2 kW DC; direct use = 5,184 kWh; exports = 3,456 kWh. Modeled annual energy value = (5,184 × $0.18) + (3,456 × $0.04) = $1,071.36. At a visitor-entered $3/W installed cash price, modeled price = $21,600 and simple payback = 20.2 years. These are deliberately chosen example inputs, not typical or recommended assumptions.
- Roof cost: 1,800 sq ft × $5.50–$8.00 per sq ft = $9,900–$14,400. No additional factor is applied.
- Roof area: a 1,600 sq ft assumed footprint at 6:12 pitch = approximately 1,789 sq ft before unmodeled geometry.
- Panel count: an 8 kW DC target with 400 W modules requires 20 modules; at 22 sq ft per module, module surface area totals 440 sq ft before spacing and access.
8. Sources and verification limits
- NLR PVWatts — a visitor-directed planning model with uncertainty and site-input limitations.
- U.S. Department of Energy: Evaluating Your Utility Rate Options — explains energy, demand and fixed-charge differences.
- U.S. Department of Energy: Homeowner's Guide to Going Solar — roof suitability and project-planning information. Its live financing section includes historical federal tax-credit figures; do not use that section to determine current federal tax eligibility.
- IRS 2025 law-change FAQ — current Section 25D homeowner-credit timing information as checked October 31, 2026. This page does not determine individual eligibility.
- National Roofing Contractors Association homeowner resources — roof service-life context and project-scope guidance.
None of these sources validates a visitor's particular roof, electricity tariff, load profile, solar proposal, financing contract or tax position. The formulas have passed local scenario tests, but independent roofing/PV professional review and real-world bill/proposal validation have not yet occurred.
Version and change control
Methodology revision: October 31, 2026. The owner should log later formula, assumption and source changes with the date and affected calculators. Technical review has not yet occurred. Do not display an independent-review claim or reviewer name until an actual review is completed.