Calculator
Example data table
Sample inputs and outputs for reference.
| Scenario | System (kW) | Net Cost | Annual kWh | Rate | Self-use | Break-even | ROI |
|---|---|---|---|---|---|---|---|
| Urban rooftop | 5.00 | $9,000 | 7,500 | 0.20 | 70% | ~6–9 yrs | Often 80–180% |
| High-tariff home | 7.00 | $12,500 | 10,800 | 0.28 | 65% | ~4–7 yrs | Often 120–250% |
| Export-heavy | 6.00 | $10,200 | 9,200 | 0.18 | 35% | ~7–12 yrs | Often 60–150% |
Your actual results depend on weather, tariffs, and usage patterns.
Formula used
1) Production with degradation
Prod(y) = Prod₁ × (1 − d)^(y−1)
Where d is annual degradation rate.
2) Electricity rate escalation
Rate(y) = Rate₁ × (1 + e)^(y−1)
Where e is annual tariff growth.
3) Effective value per kWh
Value(y) = Rate(y) × [S + (1−S)×M]
S is self-consumption share, and M is export value factor.
4) Annual net cashflow
Net(y) = Prod(y)×Value(y) − O&M(y) − Loan(y) − Repl(y) + Credit(y)
5) Break-even and ROI
Break-even is when cumulative cashflow crosses zero. ROI is modeled as Total Net / Upfront Cash over the period.
6) NPV and IRR
NPV discounts each year: NPV = Σ CF(t)/(1+r)^t. IRR is the rate where NPV becomes zero.
How to use this calculator
- Enter your system size, installed cost, and incentives.
- Input Year 1 production and expected annual degradation.
- Set electricity rate, escalation, and export value factor.
- Choose self-consumption based on your usage profile.
- Add O&M and any expected inverter replacement.
- Select cash or loan, then enter loan terms if needed.
- Click Calculate to see payback and ROI.
- Use CSV or PDF to save your results.
Installed cost and incentives shape the cash you actually deploy
This calculator separates total installed cost from rebates, tax credits, and cash incentives so you can see the true out-of-pocket investment. Upfront cash is calculated as installed cost minus incentives and any financed portion. If incentives arrive later (such as a credit claimed at tax time), the model places them in the first-year cashflow so payback timing remains realistic for many homeowners and businesses, for most rate structures.
Year‑1 production anchors every future-year savings estimate
Annual energy production is the primary driver of value. The model starts with your Year‑1 kWh estimate and applies an annual degradation rate to reflect module aging. Because tariffs can rise faster than inflation, the same kWh can be worth more each year; the escalation input captures this. When you update production with a realistic site estimate, ROI and break-even become far more actionable.
Self‑consumption and export value explain why net metering matters
Savings depend on how much solar you use onsite versus export to the grid. The self‑consumption percentage weights the full retail rate for used energy, while exported energy is valued by an export factor (for example, avoided-cost or a partial credit). A small change here can shift payback by years, especially where feed‑in credits are lower than retail tariffs or time-of-use pricing applies.
Operating costs and replacements protect you from “too-good” payback
Solar is low maintenance, but not zero maintenance. The calculator includes annual O&M that can grow with inflation, plus one-time replacement costs such as an inverter at a chosen year. By modeling these costs explicitly, cumulative cashflow remains conservative and comparable across system sizes. This improves decision quality when you are choosing between bids or considering warranties.
Financing, discount rate, and IRR help compare solar to alternatives
If you finance the project, loan payments reduce annual net cashflow until the loan ends. The discount rate converts future cashflows into today’s money to compute NPV, while IRR summarizes performance as an annualized return. Together, these metrics let you compare solar to other uses of capital, such as paying down debt, investing, or upgrading efficiency measures before sizing the system.
FAQs
1) What does “break-even” mean in this calculator?
Break-even is the first year when cumulative net cashflow becomes positive. It reflects savings, incentives, O&M, replacements, and financing effects combined.
2) Why are my results sensitive to self-consumption?
Onsite usage is valued at the full retail rate. Exported energy may receive only partial credit, so higher self-consumption typically improves payback and ROI.
3) How should I estimate Year‑1 production?
Use an installer proposal, an online solar estimator, or historic production from a similar nearby system. Include shading, orientation, and soiling assumptions for accuracy.
4) What discount rate should I use for NPV?
A common approach is your expected return from alternative investments or your borrowing cost. Higher discount rates reduce NPV and usually lengthen economic payback.
5) Does the calculator include battery storage?
This version models solar-only cashflows. You can approximate storage impact by increasing self-consumption, adding battery cost as a replacement line, and adjusting export value.
6) Why can ROI be high while NPV is modest?
ROI measures total net gain versus upfront cash over the period, while NPV discounts future gains. Long-term savings may look large in nominal terms but smaller in today’s value.