How We Calculate

The Real Math Behind Solar Payback & ROI

If you've searched for solar panel savings, you've likely seen extreme claims: that solar is entirely free or that it pays for itself in just two years. We built HowMuchSolarCost to provide independent, math-driven answers.

We are an independent editorial project. We do not sell solar panels, batteries, or installer services. Our models use physics-based and financial equations populated with public sources so you can test scenarios transparently.

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1. Solar Energy Production

Estimating annual solar generation requires mapping your geographic average sunlight hours to your total solar panel capacity (kW) and adjusting for systemic energy losses.

Annual Gen (kWh): kW × Sun Hours × 365 × 0.82

Year 1 Savings: Generation × Utility Rate

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2. Panel Efficiency Degradation

Solar panels lose efficiency over time. Our models assume a standard degradation rate of 0.5% per year, meaning generation decreases slightly each year over a typical 25-year panel lifetime.

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3. Federal Solar Tax Credit (ITC)

The Residential Clean Energy Credit (Section 25D) provides a tax credit for 30% of the total installation and equipment cost. Our calculations apply this baseline credit to lower the net project cost.

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4. Utility Rate Inflation

Utility rates rise over time. Historically, US electric rates rise by ~3% to 4% annually. Our calculations compound utility rates annually (default 3.5%) to reflect growing future savings.

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5. Storage Integration (Batteries)

Adding battery backup storage (like a Tesla Powerwall) increases the upfront installation cost but provides grid outage resilience and stores excess solar energy for evening usage.

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Data Integrity

We pull default variables quarterly from public authorities: state average electricity tariffs from the EIA, regional sunlight parameters from NREL, and national solar installer averages.

We always recommend overriding defaults with your exact local utility bill rates.

What Our Models Cannot Calculate

While our models are powerful, some variables depend on individual circumstances:

Roof Structure & Shading

Shading from nearby trees or structures and complex roof orientations (e.g., North-facing) reduce efficiency and require a physical site assessment.

Local Net Metering Rules (1:1 vs. Avoided Cost)

While we support net metering calculations, some local utilities pay far less than retail rates for exported power (avoided cost or Net Metering 3.0), which alters ROI.

Financing Interest (APR)

Loans or solar leases add interest costs over time. We calculate cash purchases by default but provide interest rate adjustments for financed systems.

Suggestions for Accurate Results

1
Run a Worst-Case Scenario: Test what happens if utility rates inflate slower (e.g., 1.5% per year) and net metering buybacks are reduced by your utility.
2
Get Three Installer Quotes: Solar installation costs vary by local competition. Always compare multiple quotes instead of relying on averages.
3
Evaluate Battery Storage Needs: Analyze if your local utility charges high peak rates (Time-of-Use). If so, batteries can shift power usage and increase your ROI.

Ad-Light Policy: We do not accept manufacturer sponsorship or affiliate commissions to bias calculations. The site is supported by standard AdSense slots positioned away from main tool controls.

Have questions about our calculations?

Our analyst team regularly updates the models. If you have feedback or source suggestions, let us know.

Contact Analysis Team