Inputs
Example data table
| Scenario | DC Size (kW) | Module (W) | Modules | Optimizers | Incentive | Net Cost | Payback |
|---|---|---|---|---|---|---|---|
| Roof, standard | 7.5 | 410 | 19 | 19 | 10% | $10,800 | 8–11 yrs |
| High-rate utility | 7.5 | 410 | 19 | 19 | 10% | $10,800 | 6–9 yrs |
| Lower yield site | 7.5 | 410 | 19 | 19 | 10% | $10,800 | 10–14 yrs |
These are illustrative examples, not guarantees.
Formula used
- Modules needed = ceil((Target kW × 1000) ÷ Module W)
- Optimizers needed = ceil(Modules × Optimizers per module)
- Modules per string = ceil(Modules ÷ String count)
- Estimated string Voc = Modules per string × Module Voc
- Year 1 production = Actual kW × (kWh per kW)
- Year y production = Year1 × (1 − degradation)^(y−1)
- Electricity rate in year y = Rate × (1 + escalation)^(y−1)
- Year y savings = Production(y) × Rate(y)
- Annual O&M = Net cost × O&M%
- NPV = −Net cost + Σ [Net cashflow(y) ÷ (1 + discount)^y]
How to use this calculator
- Enter your target DC size and the panel wattage you plan to use.
- Set optimizers per module, then choose a rough number of strings.
- Add unit costs for optimizers, modules, inverter, and other system items.
- Fill in expected energy yield and your local electricity rate.
- Adjust incentive, degradation, and discount rate for realistic finance outputs.
- Click Calculate to view sizing, cost, savings, and payback above.
- Download results as CSV or PDF for sharing or recordkeeping.
Sizing targets and hardware counts
Start with the DC target and selected module wattage. The calculator rounds up to whole modules, producing an actual DC size close to your target. For example, 7.50 kW with 410 W modules becomes 19 modules, or 7.79 kW DC. Optimizer quantity is driven by the optimizer-per-module ratio, so module-level monitoring or partial optimizer coverage can be modeled quickly for planning.
Voltage planning and string design
String count affects modules per string and the estimated open-circuit voltage. Comparing estimated string Voc to your voltage limit highlights risk before procurement. Ten modules at 49.5 V Voc estimates 495 V, leaving margin under a 600 V cap. If the estimate exceeds the limit, increase strings to reduce modules per string and lower voltage exposure.
Cost model and incentive sensitivity
Total installed cost combines module hardware, optimizer hardware, optimizer labor, inverter cost, balance-of-system, and permitting. Separating line items helps you see where spend concentrates, especially optimizer hardware plus labor. Incentives are applied as a percentage to estimate net cost; a 10% incentive on a $12,000 gross cost reduces net cost by $1,200. Use conservative assumptions when incentives are capped or uncertain.
Energy yield and savings forecast
Annual production is based on kWh per kW, then reduced each year by degradation. A yield of 1,400 kWh per kW produces about 10,900 kWh in year one for a 7.79 kW system. Savings use your electricity rate and optional escalation to reflect tariff changes. Operations and maintenance is estimated as a percentage of net cost, producing net cashflows for each year.
Interpreting NPV, IRR, and payback
Payback shows when cumulative net cashflow turns positive, while NPV discounts future cashflows using your discount rate to reflect time value. Higher discount rates reduce the value of distant savings and can change the decision. IRR is computed from the full cashflow series and can be compared to alternative investments or financing rates. Use all three metrics together to balance cost, resilience, and long-term return.
FAQs
Do I always need one optimizer per module?
No. Use the ratio field to model partial coverage, selective shading areas, or design preferences. The calculator scales optimizer counts and related costs while keeping module count driven by your DC target.
Why does the module count round up?
Modules are discrete units, so the tool rounds to the next whole module to meet your target size. That creates a slightly higher actual DC size, which then drives production and savings estimates.
Is the string voltage estimate a safety check?
It is an initial screening only. It multiplies modules per string by module Voc and compares it to your limit. Real designs should consider temperature corrections, equipment specifications, and local electrical requirements.
How are savings calculated?
Savings equal annual production times the electricity rate, with optional annual escalation. Production declines by the degradation rate. The model then subtracts annual O&M to create net cashflow used for payback and NPV.
What does NPV tell me?
NPV converts future net cashflows into today’s value using your discount rate. Positive NPV suggests the project beats the discount-rate benchmark over the selected years, while negative NPV indicates weaker financial attractiveness.
Why might IRR show as unavailable?
If net cashflows never offset the initial cost within the analysis window, the cashflow series may not cross zero. In that case, the tool cannot bracket a solution and returns “n/a” for IRR.