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Calculator Methodology

Reviewedby RenewableCalc Data Team

Every RenewableCalc estimate shows its formulas, assumptions, and data-source context. Nothing is hidden behind a black box. This page documents the shared principles behind all our tools.

Core Principles

  1. Visible formulas.Every calculator displays its calculation method in a “Methodology” or “Breakdown” section — never hide the math.
  2. Attributed data. All default values cite their source (EIA, NREL, IRS, DSIRE, EPA, ENERGY STAR). Users can override any default with their own numbers.
  3. Conservative by default. For 2026+ residential projects, no federal credit is assumed unless the user confirms eligibility. State incentives are modeled where data exists, with clear caveats.
  4. Planning-grade, not quote-grade. Outputs are educational estimates — not installer quotes, tax advice, or utility tariff approvals. Always verify with qualified professionals.

Solar ROI Calculator

The Solar ROI calculator estimates payback period, net savings, and return on investment for a grid-tied residential solar system.

Formula

payback_years = (installed_cost − federal_credit − state_incentives) / annual_savings

Key Assumptions

  • System size estimated from monthly bill ÷ state electricity rate ÷ production factor (0.78 × annual sun hours).
  • Production factor accounts for inverter losses, wiring, temperature, and soiling (~22% total derate).
  • Annual bill savings = pre-solar bill × bill offset ratio, capped at 100%.
  • Federal credit: 0% by default for 2026+ (Section 25D expired Dec 31, 2025). User can override.
  • State incentives pulled from DSIRE data where available; user can override.
  • Annual utility rate escalation: 2.5% default. System degradation: 0.5%/year.

Data Sources

  • NREL Q1 2026 — $2.80/W national median installed cost
  • EIA-861M (Dec 2025) — State electricity rates
  • NREL PVWatts v8 — Solar production factors
  • DSIRE — State incentive amounts
  • IRS Section 25D / Section 48E — Federal tax credit guidance

Battery Storage Calculator

The Battery Storage calculator sizes a home battery system based on daily usage and desired backup duration.

Formula

recommended_capacity_kWh = (daily_usage × backup_hours / 24) × 1.2 (safety factor)

Key Assumptions

  • Economy tier: Lead-acid, $150/kWh, 50% depth of discharge.
  • Standard/Premium: Li-ion, $400–$600/kWh, 90% depth of discharge.
  • Annual value: $0.15/kWh with solar (TOU arbitrage + backup), $0.08/kWh without.
  • Safety factor of 1.2 applied to recommended capacity.

EV Charging Cost Calculator

Compares home and public EV charging costs against gasoline for a specific vehicle and annual mileage.

Data Sources

  • EPA fueleconomy.gov — Vehicle efficiency ratings (MPGe, kWh/100mi)
  • EIA — State electricity rates and average gas prices

Energy Consumption Calculator

Estimates household energy consumption by appliance, using ENERGY STAR benchmarks and user inputs.

Data Sources

  • ENERGY STAR — Appliance energy consumption benchmarks
  • EIA — Residential energy consumption survey

Quote Review Tool

Benchmarks installer quote $/W against state-level and national medians from NREL data.

Benchmarks

  • National median: $2.80/W (NREL Q1 2026)
  • State medians: CA $2.95, TX $2.55, FL $2.65, NY $3.05, MA $3.15, AZ $2.45, CO $2.70, NV $2.50
  • Red flag: > 15% above benchmark
  • Warning: 5–15% above benchmark
  • Green: within 5% of benchmark

State-Specific Data

Each state page uses the following structured data points:

  • Electricity rate — From EIA Electric Power Monthly, latest available month.
  • Solar cost per watt — NREL benchmark with state-level adjustment.
  • Payback range — Estimated using Solar ROI calculator with state defaults.
  • Net metering — Policy status from DSIRE and state utility commission filings.
  • Incentives — State tax credits, rebates, exemptions from DSIRE.

Update Frequency

  • Electricity rates — Updated quarterly following EIA releases.
  • System cost benchmarks — Updated semi-annually from NREL and LBNL Tracking the Sun.
  • Incentive data — Updated quarterly from DSIRE.
  • Federal policy — Updated as legislation passes or IRS guidance changes.
  • State policy — Updated as state legislatures pass or modify solar laws.

Limitations

  • All estimates are planning-grade. Actual costs vary by roof, installer, equipment, financing, and local permitting.
  • Tax credit eligibility depends on individual tax liability, project-year timing, and current IRS rules.
  • Utility rate structures (TOU, demand charges, buyback plans) vary by provider and can materially change results.
  • System production depends on roof orientation, shading, tilt, inverter selection, and local weather patterns.
  • Financial projections assume constant-dollar utility rate escalation and do not account for inflation, financing costs, or opportunity cost of capital unless specified.

Policy last reviewed: 2026-06-15. Questions or corrections? Contact us through the contact page.