Methodology
How SolarCalcNow estimates solar size, cost, and payback
The calculator is designed for early planning. It uses transparent formulas and visible assumptions instead of claiming installer-grade precision.
Methodology updated 2026-07-16.
Source stack
Quick answer
What does the estimate represent?
It is a first-pass planning range built from your usage, a production assumption, visible system losses, direct solar use, export value, and gross cost benchmarks. It is not an engineering design, utility approval, tax determination, production guarantee, or installer quote.
Core formulas
- annualUsageKWh = monthlyUsageKWh * 12
- monthlyUsageKWh = monthlyBillUSD / electricityRateUSDPerKWh, when usage is not provided
- targetAnnualProductionKWh = annualUsageKWh * targetOffset
- rawSystemSizeKW = targetAnnualProductionKWh / effectiveSpecificYieldKWhPerKWYear
- adjustedSystemSizeKW = rawSystemSizeKW / (1 - systemLossRate)
- panelCount = ceil((adjustedSystemSizeKW * 1000) / panelWattage)
- roofAreaSqFt = panelCount * panelAreaSqFt * spacingFactor
- grossCostRange = adjustedSystemSizeKW * costPerWattRange * 1000
- selfConsumedKWh = min(targetAnnualProductionKWh * selfConsumptionShare, annualUsageKWh)
- exportedKWh = max(targetAnnualProductionKWh - selfConsumedKWh, 0)
- uncappedSavings = selfConsumedKWh * retailRate + exportedKWh * exportRate
- annualSavings = min(uncappedSavings, annualUsageKWh * retailRate)
- paybackYears = grossCost / annualSavings
Default assumptions
- 14% system loss.
- 400W panel default.
- About 21 sq ft per panel plus a 15% spacing factor.
- 55% direct solar use before export as an editable planning default.
- State-level production defaults unless ZIP/PVWatts is configured and available.
- Gross cost range before incentives, batteries, roof work, or financing.
- Energy-only savings capped at the modeled annual energy charge; no assumed cash payout for excess credits.
Where estimates can be wrong
- Shade, roof orientation, roof pitch, and usable roof planes.
- Utility rate plan, export rate, net metering, and time-of-use rules.
- Fixed charges, minimum bills, demand charges, and interval-level self-consumption.
- Installer pricing, equipment, permitting, interconnection, and roof condition.
- Battery size, backup loads, and financing structure.
Evidence labels used across SolarCalcNow
| Label | Meaning | Use before a decision |
|---|---|---|
| Calculated estimate | A formula result based on entered and default assumptions. | Change assumptions and compare a range. |
| Manufacturer specification | A value published for a named equipment model. | Confirm the exact model and datasheet. |
| Government incentive | A program claim tied to an official source and eligibility rules. | Verify current status and personal eligibility. |
| Utility information | Tariff, export, interconnection, or billing terms from the serving utility. | Confirm the exact account and rate plan. |
| Installer quote | Project-specific price, equipment, scope, and production proposal. | Compare written assumptions and contract terms. |
Incentive handling
Incentives are not applied by default unless they have a source URL, updated date, eligibility note, active status, and explicit inclusion in the data model. This is especially important for 2026 homeowner estimates after federal credit-status changes.
View sourcesWhat to do next
Once the assumptions make sense, run the calculator and keep the source notes handy when comparing installer proposals.