Calculator
Fields support decimals. Currency is a label only.Example data table
These sample values illustrate a typical homeowner scenario. Replace them with local quotes and tariff details.
| Item | Example value | Notes |
|---|---|---|
| System size | 5.0 kW | Rooftop grid-tied array. |
| Installed cost | 9,000 | Before rebates and credits. |
| Rebate | 500 | Applied upfront. |
| Tax credit | 30% | Often realized in the next tax cycle. |
| Annual consumption | 8,500 kWh | From recent utility bills. |
| Annual production | 7,800 kWh | From installer simulation. |
| Self-use share | 70% | Higher with daytime usage or storage. |
| Utility rate | 0.18 /kWh | Blended energy charge estimate. |
| Escalation | 3%/year | Conservative planning assumption. |
| Export credit | 0.08 /kWh | Check net metering rules locally. |
If you run these values in the calculator, your results will reflect every chosen parameter, including financing and replacement timing.
Formula used
- Net upfront cost = Installed cost − Rebate − (Credit applied upfront).
- Credit amount = Installed cost × (Tax credit %).
- Production in year y = P₁ × (1 − degradation)^(y−1).
- Utility rate in year y = R₁ × (1 + escalation)^(y−1).
- Self-used kWh = min(Production × self-use %, Consumption).
- Exported kWh = max(0, Production − self-used).
- Gross savings = Bill without solar − Bill with solar.
- Net cashflow = Savings − O&M − Loan − Major costs + Bonuses.
Break-even occurs when cumulative net cashflow reaches zero. Discounted break-even uses discounted cashflows: CFᵧ / (1 + discount rate)ᵞ.
How to use this calculator
- Enter installed cost, rebate, and credit percentage from your quote.
- Fill annual usage and production from bills and the installer model.
- Set self-use share based on daytime usage or storage plans.
- Confirm utility and export rates, then adjust escalations if needed.
- Add annual O&M and any expected replacement year and cost.
- Choose cash or loan financing, then press Calculate.
- Review break-even, NPV, and the yearly cashflow table.
- Export results to CSV or PDF for sharing.
Installed cost drivers and incentives
Upfront pricing varies by roof complexity, equipment tier, and permitting. Many quotes are expressed per kilowatt, while this calculator accepts a single installed total. Enter rebates as immediate reductions, then set the credit percentage to match local incentives. If credits arrive later, apply them in Year 1 to model delayed cash. Also include any upfront interconnection, inspection, or electrical upgrade fees for completeness. This creates a baseline for comparing offers and timelines.
Production, degradation, and self-consumption
Annual output depends on location, tilt, shading, and inverter sizing. Use your installer simulation for Year 1 production, then apply degradation to estimate long term performance. Modern modules often degrade around 0.3% to 0.7% per year, so 7,800 kWh can trend toward about 6,700 kWh by Year 25. Self use matters because self consumed energy offsets the highest retail rate first. Storage can raise self use significantly too.
Rates, escalation, and export credits
Tariffs drive savings. Set the retail rate from your bill, then choose an escalation rate for future price growth. A change from 2% to 4% escalation can materially shift cumulative savings. Export credits are usually lower than retail, so exported kilowatt hours contribute less value. If your plan provides full net metering, set export rate equal to utility rate and keep escalations aligned. Time of use rates can boost self use value.
Operating costs, replacements, and financing
Operating costs keep projections realistic. Add annual maintenance for cleaning, monitoring, and minor repairs, plus insurance if your lender requires it. Apply O and M inflation to reflect rising service prices. Plan at least one major replacement, often an inverter around years 10 to 15, and enter its expected cost. Financing changes cashflow shape: loans reduce early net cash but can improve affordability. Shorter terms increase payments but reduce total interest paid.
Interpreting payback, NPV, and IRR
Break even occurs when cumulative net cashflow becomes positive. The calculator reports both simple and discounted break even, where discounted values account for the time value of money. Net present value summarizes overall benefit in today's terms; positive NPV suggests the project beats the selected discount rate. The IRR estimate provides another lens, but it may be unavailable if cashflows never change sign. Use the CSV to audit assumptions and share results.
What does break-even mean in this tool?
Break-even is the first point where cumulative net cashflow reaches zero. It means total savings and incentives have recovered all upfront, operating, financing, and replacement costs included in your inputs.
Why is discounted break-even different from simple break-even?
Discounted break-even applies a discount rate to future cashflows, so near-term savings count more than distant savings. It is useful when comparing solar returns to alternative investments or borrowing costs.
How should I estimate annual solar production?
Use an installer proposal, a monitoring report from similar systems, or a reputable solar resource model. Enter the expected first-year kilowatt-hours, then use degradation to reflect gradual output decline over time.
How is self-use percentage used?
The calculator treats self-use as the share of solar production consumed on-site and caps it by annual consumption. Higher self-use generally increases savings because it offsets retail-priced energy instead of lower export credits.
How does loan financing affect results?
With financing, Year 0 includes only the down payment and other upfront costs. Annual loan payments are subtracted during the loan term, which can delay payback but reduce initial cash required.
Should I include battery storage here?
Yes, if storage changes self-use, costs, or replacement timing. Increase self-use percentage, add storage cost to upfront costs, and include expected replacement as a major cost in the appropriate year.