Calculator Inputs
Example Data Table
| Scenario | System (kW) | Sun hours/day | PR | Self-use | FiT rate | Retail rate | Period |
|---|---|---|---|---|---|---|---|
| Small home | 3.5 | 4.2 | 0.78 | 40% | 0.10 | 0.22 | 30 days |
| Typical rooftop | 5.0 | 4.5 | 0.80 | 30% | 0.12 | 0.20 | 30 days |
| Small commercial | 15.0 | 5.0 | 0.82 | 65% | 0.08 | 0.18 | 30 days |
Formula Used
- Energy generated:
kWh = kW × sun_hours_per_day × period_days × performance_ratio - Self-consumed:
self_kWh = generated_kWh × (self_use_% ÷ 100) - Exported:
export_kWh = generated_kWh − self_kWh - Export income:
income = export_kWh × feed_in_tariff_rate - Savings:
savings = self_kWh × retail_rate - Net benefit:
net = (income + savings − fixed_fees) − tax% - Projection: net is adjusted by
degradationandescalationannually.
How to Use This Calculator
- Enter your solar system size and average sun hours.
- Set performance ratio to reflect local losses.
- Estimate self-consumption based on your load profile.
- Input your retail electricity rate and tariff rate.
- Add optional fees and tax if they apply.
- Choose projection settings for long-term planning.
- Press Calculate to see results above the form.
- Use the download buttons to save CSV or PDF.
Feed-in tariff revenue drivers
Feed-in revenue depends on exported kilowatt-hours and the tariff rate. Exported energy rises when daytime site demand is low or storage is absent. Higher inverter clipping, shading, or downtime lowers exports. Confirm the settlement unit, whether the tariff applies to gross generation or net export, and any caps or annual true-ups.
Self-consumption savings versus export
Self-consumed solar offsets retail purchases at the retail rate, often exceeding the tariff. Increasing daytime loads, shifting pumps or HVAC schedules, or adding batteries can raise self-consumption. Demand charges may also change outcomes for commercial sites, so align inputs with your tariff structure. However, higher self-use reduces exported energy, so evaluate both streams together using consistent time periods and the same performance ratio.
Performance ratio and energy estimation
The calculator uses system size, average sun hours, and performance ratio to estimate kilowatt-hours. Performance ratio aggregates losses from temperature, soiling, mismatch, wiring, and inverter efficiency. Use measured production where available, or calibrate the ratio so modeled monthly energy matches recent bills, monitoring, and site conditions. Update sun hours seasonally if your climate varies strongly.
Fees, taxes, and cash-flow realism
Many programs include metering charges, administrative fees, or minimum bills that reduce benefits. Taxes may apply to credits or payments, and VAT can change the effective value. Enter fixed fees per period and apply a tax percentage only to benefits if that matches your accounting method and local regulations. Keep records of contract start dates and indexation rules.
Multi-year projection and sensitivity
Long-term value is influenced by panel degradation and tariff escalation. Degradation gradually reduces annual energy, while escalation increases unit value. Run sensitivities by adjusting escalation and degradation ranges, then compare projected totals to financing costs and maintenance budgets. Consider inverter replacement cycles, insurance, and cleaning schedules when interpreting net figures. Document assumptions, because policy changes can materially shift future cash flows.
FAQs
What does the performance ratio represent?
It combines typical system losses such as temperature, wiring, inverter efficiency, soiling, and downtime. If you have monitoring data, adjust the ratio until the model matches recent measured production for similar months.
Should I use gross generation or net export for tariff programs?
Some programs pay on total generation, while others pay only on exported energy. Use the structure stated in your contract. If paid on gross generation, set self-consumption to 0% so all modeled energy is treated as compensated.
Why can higher self-consumption reduce my feed-in earnings?
Self-consumed energy is not exported, so fewer kilowatt-hours receive the feed-in payment. The trade-off may still be favorable if your retail rate is higher than the tariff rate.
How do I choose the period length?
Use 30 days for a monthly estimate aligned with bills, or 365 days for an annual view. Keep your rates and fees consistent with the same period to avoid overstating or understating benefits.
What do escalation and degradation change in the projection?
Degradation lowers future energy output each year, while escalation raises the value per kilowatt-hour. Together they create a more realistic long-term estimate, but policy changes can override these assumptions.
Can I include fixed charges and taxes?
Yes. Enter fixed fees that apply within the period, such as metering or service charges. If taxes apply to credited value or payments, enter a percentage so the net benefit reflects your expected cash flow.