Solar Panel Payback Calculator accuracy and testing

Formula source

• Annual savings = annual production kWh * electricity rate, escalated by utility rate increase and reduced by panel degradation. Payback is when cumulative savings exceed net cost.

Test cases

Known limitations

• This page documents the checked-in calculator logic and does not certify results for every real-world edge case.
• Use the result as a planning estimate unless the calculator page states a stricter purpose.

Assumptions

• Incentives are modeled as a direct reduction of cost.
• Savings are modeled from annual production and electricity value rather than utility tariff details.
• This is not tax, financial, or solar design advice; calculations run locally.

Unit handling

• Installed Cost accepts values in $.
• Incentives/Rebates accepts values in $.
• Annual Production accepts values in kWh.
• Electricity Value Preset accepts one of the configured options.
• Electricity Value accepts values in $/kWh.
• Utility Rate Escalation accepts values in %/yr.
• Solar Degradation accepts values in %/yr.
• Analysis Period accepts values in years.
• Estimated Payback is displayed in years.
• Net System Cost is displayed as currency.
• First-Year Savings is displayed as currency.
• Lifetime Net Savings is displayed as currency.

Rounding rules

• Number displays use up to 2 decimal places in the calculator UI.
• Currency outputs are displayed with standard US dollar formatting.
• Input precision follows configured step values where provided.

Version history

Change log