Inputs
Example Data Table
| Scenario | System (kW) | Sun hrs/day | PR (%) | Self‑use (%) | Tariff | Export | Cost | Year‑1 kWh | Year‑1 Net |
|---|---|---|---|---|---|---|---|---|---|
| Small site | 5.0 | 5.2 | 78 | 75 | 0.18 | 0.06 | 7,000.00 | 7,402 | 960.33 |
| Warehouse roof | 30.0 | 5.5 | 82 | 65 | 0.22 | 0.08 | 36,000.00 | 49,385 | 8,294.75 |
| Office retrofit | 15.0 | 4.8 | 80 | 85 | 0.25 | 0.10 | 19,000.00 | 21,024 | 4,632.96 |
Formula Used
- Year‑1 Production (kWh) = System kW × Sun hours/day × 365 × Performance ratio
- Self‑Used (kWh) = Production × Self‑consumption%
- Exported (kWh) = Production − Self‑Used
- Year‑1 Gross Savings = Self‑Used×Tariff + Exported×Export rate
- Year‑1 Net Savings = Gross Savings − Annual O&M
- Net Installed Cost = System cost − Incentives
- Simple Payback (years) = Net Installed Cost ÷ Year‑1 Net Savings
- NPV = Σ(Net savingsᵧ ÷ (1+Discount rate)ᵧ) − Net installed cost
- Cashflow assumptions: production decreases by degradation each year, and tariff/export/O&M escalate by the escalation rate.
How to Use This Calculator
- Enter system size, sun hours, and performance ratio to estimate generation.
- Set self‑consumption to represent how much solar your site uses.
- Enter tariff and export credit to value self‑use and exports.
- Add system cost, incentives, and annual O&M for realism.
- Optional: adjust escalation, degradation, discount rate, and years.
- Press Calculate Solar Savings to view results above.
- Use the download buttons to export CSV or PDF.
Project Notes
Baseline energy profile
Start with a realistic site load pattern, because savings depend on how much solar is consumed onsite. Daytime loads from lighting, HVAC, pumps, and tools typically align well with solar output, improving self-consumption. If your site is mostly night-shift, exported energy will dominate and the export rate becomes the key driver.
Expected production and losses
Annual energy is estimated from system size, average sun hours, and a performance ratio that captures temperature, soiling, inverter efficiency, and wiring losses. A 30 kW array with 5.5 sun hours/day and an 82% performance ratio produces about 49,357 kWh in year one, before any degradation adjustment.
Value of self-consumption versus exports
Self-used energy offsets the full retail tariff, while exports earn the export credit. With the same 30 kW example, 65% self-consumption gives roughly 32,082 kWh self-used and 17,275 kWh exported in year one. At a tariff of 0.22 and export of 0.08, gross value is about 8,438 per year.
Lifecycle economics and risk controls
Use escalation to reflect rising utility rates and degradation to reflect declining output over time. Net present value discounts future savings to compare alternatives on a consistent basis. Include annual O&M for cleaning, inspection, and minor component replacement so the payback estimate is not overstated.
Construction project integration
For planning, test multiple scenarios: conservative, expected, and upside. Validate roof area, structural capacity, shading, and interconnection limits. Treat incentives as reductions to net installed cost, and align the analysis period with asset life and warranty terms for decision-ready budgeting.
FAQs
1) What should I use for performance ratio?
Many projects fall between 75% and 85%. Use lower values for hot climates, dusty sites, or older inverters. Use higher values for well-maintained systems with minimal shading and quality equipment.
2) How do I estimate self-consumption?
Compare daytime load to expected solar output. Sites with steady daytime operations often achieve 60%–90%. If loads peak at night, self-consumption can be much lower unless storage or load shifting is added.
3) Why does the calculator include degradation?
Modules typically produce slightly less each year. Degradation reduces annual kWh over time, which lowers long-term savings. Including it helps avoid optimistic payback and NPV results.
4) What discount rate should I apply?
Use your organization’s hurdle rate or weighted cost of capital. If uncertain, test 4%–10% to see sensitivity. A higher discount rate reduces NPV because future savings are valued less.
5) Are O&M costs really necessary?
Yes. Even simple systems need periodic cleaning, inspections, and occasional parts replacement. Adding O&M creates a more credible net savings estimate and supports better maintenance planning.
6) How should I treat incentives and rebates?
Enter incentives as a direct reduction to installed cost, matching the project’s out-of-pocket investment. If incentives are paid later, consider a more detailed cashflow model and use conservative timing assumptions.
7) Why might payback differ from the cash payback year?
Simple payback uses year-one net savings and ignores changes over time. The cash payback year uses annual cashflows with escalation, degradation, and O&M, which can shift the breakeven point.