Calculator Inputs
Enter realistic values for your site, rate plan, and incentives.
Example Data Table
Use these sample values to test the calculator quickly.
| Input | Example | Why it matters |
|---|---|---|
| System size | 5.00 kW | Sets annual energy output scale. |
| Installed cost | $9,000 | Controls upfront investment level. |
| Rebate | $500 | Immediate cost reduction before credits. |
| Tax credit | 30% | Reduces net cost under the assumption shown. |
| Annual output | 1,400 kWh/kW | Captures location, tilt, shading, and climate. |
| Retail rate | $0.18/kWh | Higher rates generally increase savings. |
| Self-use share | 70% | Onsite use often earns the best value. |
| Export credit | 100% | Models full net metering or reduced credit. |
| O&M cost | $150/yr | Accounts for upkeep and small repairs. |
| Analysis period | 25 years | Defines your financial horizon. |
Formula Used
- Tax credit value: Credit = credit% × (installed cost − rebate).
- Net upfront cost: Net = installed cost − rebate − credit.
- Annual production: Prody = Prod1 × (1 − degradation)y−1.
- Retail rate escalation: Ratey = Rate1 × (1 + escalation)y−1.
- Bill savings: Savings = Self kWh × Retail rate + Export kWh × Export rate.
- Export rate: Export rate = Retail rate × (export credit%).
- Net cashflow: Net = Savings − (O&M + optional costs + loan payment).
- NPV: NPV = Σ Nety / (1 + discount)y + Net0.
- Loan payment (annual): Payment = P×r / (1 − (1+r)−n) (or P/n if r=0).
These are simplified planning formulas; local rules can differ.
How to Use This Calculator
- Enter system size and total installed cost.
- Add incentives like rebates and tax credit percent.
- Choose a production method and fill the energy estimate.
- Set your retail rate, escalation, and export credit level.
- Adjust self-consumption, degradation, and yearly upkeep costs.
- Enable financing if you plan to use a loan.
- Click Calculate Net Savings to view results above.
- Download CSV or PDF for sharing and recordkeeping.
Notes and Assumptions
- Incentives are applied as a simplified upfront adjustment to net cost.
- Export credit is modeled as a percentage of the retail rate.
- Yearly loan payments are treated as annual cash outflows.
- IRR is computed only when cashflows cross from negative to positive.
System sizing and production baseline
A clear baseline drives every savings forecast. This calculator accepts system size in kW and converts it to annual output using kWh per kW, or a direct annual kWh value. For quick planning, 5.0 kW at 1,400 kWh/kW yields about 7,000 kWh in year one, before degradation. Use site shading, tilt, and climate to refine that figure.
Consumption split and export credit
Net savings depend on how much solar is used onsite. Self-consumption is valued at the retail rate, while exports are valued at an export credit that can be lower than retail. Example: with 70% self-use and 100% export credit, all kWh earn retail value; with 70% self-use and 50% export credit, exported kWh earn half, reducing bill savings materially.
Rate escalation and degradation modeling
Two opposing trends shape long-term cashflows. Utility rates can escalate each year, increasing savings per kWh, while module degradation reduces production each year. If escalation is 3% and degradation is 0.5%, the value per kWh rises faster than output falls, often improving late-year savings. Set negative escalation to stress-test a conservative tariff outlook.
Financing, cashflow, and payback
When financing is enabled, the tool treats loan payments as annual cash outflows and tracks cumulative net cashflow from year zero. A down payment reduces initial outlay but increases total payments less than borrowing the full amount. Payback is reported when cumulative cashflow becomes positive, helping compare cash purchase versus loan scenarios under identical energy assumptions.
Decision metrics for construction projects
Alongside total net savings, the calculator reports NPV using your discount rate, plus estimated ROI and IRR when cashflows change sign. NPV supports capital budgeting and multi-project ranking, while IRR helps compare returns against hurdle rates. Use the yearly schedule to document assumptions for stakeholders and to validate that replacement costs and O&M are properly included. For permits and bids, keep a copy of inputs, especially incentive timing, billing structures, and load profiles, so revisions remain traceable during design changes later.
FAQs
1) What does “net savings” mean here?
Net savings is the total bill savings minus all modeled costs, including maintenance, optional insurance, property tax add-ons, replacement costs, and loan payments, across the selected analysis period.
2) Why do my savings change each year?
Savings change because the tool escalates the utility rate by your annual escalation input and reduces production by your degradation input. Costs like O&M and loan payments can also vary by year.
3) How should I choose the self-consumption percentage?
Use interval data if available. Otherwise estimate based on daytime loads, HVAC schedules, and any storage. Higher self-use typically increases value when export credit is below the retail rate.
4) What if my export credit is a fixed price, not a percent?
This version models export credit as a percentage of the retail rate. To approximate a fixed export price, adjust the retail rate and export percent so their product matches your expected export credit.
5) Why is IRR sometimes “not available”?
IRR requires cashflows to cross from negative to positive at least once. If cumulative results never recover the initial outlay in the analysis window, the tool reports IRR as unavailable.
6) Are incentives always applied correctly?
The calculator applies rebates and a percent-based credit as a simplified upfront reduction. Real programs can include caps, carryforward rules, or timing delays, so confirm your local policy and adjust inputs conservatively.