In short
Income value = sum of the discounted annual surpluses until decommissioning, minus removal. The biggest levers are actual yield, the remaining tariff term and the discount rate.
Formula
Inputs and typical mistakes
| Input | Correct | Common mistake |
|---|---|---|
| Yield | meter readings of recent years, weather-adjusted | design forecast |
| Degradation | derived from measured data, 0.5 % per year in the model | ignored or set too high |
| Tariff | rate per statement until the end of the 20th year after commissioning | wrong end date |
| Post-tariff revenue | market value or direct marketing, self-consumption | old tariff carried forward |
| Costs | complete, with price escalation | insurance, metering or lease forgotten |
| Investments | inverter replacement in year 15 or by condition | not included |
| Decommissioning | economically optimal year | fixed 25 or 30 years |
| Discount rate | risk-adjusted, disclosed | chosen without justification |
Which discount rate is right?
The rate reflects cost of capital and risk. A well-documented system with a secured tariff justifies a lower rate than a system with defects, a short lease or post-tariff revenue. We disclose the rate in the report and show in the sensitivity analysis what one percentage point more or less means. In the 250 kWp example it is about 3 %.
Example
The full worked example of a commercial system and a single-family home with all annual figures is on the page remaining value of a PV system. Use the calculator to enter your own figures.
Modelling revenues after the tariff ends
The most common error in the income value lies in the years after the tariff. Systems up to 100 kW whose tariff has expired receive the annual solar market value minus a marketing fee. For 2025 that was 4.51 ct/kWh minus 0.72 ct/kWh. From 2026 the fee drops to 0.23 ct/kWh, halved with a smart metering system. The rule applies until the end of 2032 (Verbraucherzentrale, German). Carrying the old tariff forward overstates the value.
There is no statutory commitment for the period after that. We then model direct marketing or self-consumption and show in the sensitivity analysis what one cent per kilowatt hour changes.
Negative prices for new systems
For systems commissioned from 25 February 2025, no tariff is paid in any quarter-hour with a negative exchange price. The support period is extended by those periods (sections 51 and 51a EEG, Clearingstelle EEG|KWKG, German). In the income value, revenues shift into later years and the present value falls. Systems commissioned before 2016 are not cut back at negative prices.
Decommissioning as the final item
PV modules have been covered by the German Electrical and Electronic Equipment Act since 24 October 2015. Take-back through registered manufacturers or their take-back schemes is free of charge. Private households can hand in up to 20 modules at municipal collection points (Solarenergie-Förderverein, German). Dismantling, scaffolding and transport remain with the operator. We include these costs in the decommissioning year.
Self-consumption in the income value
Power used on site is valued at the avoided energy price, net and with price escalation. We take the share from meter data or the load profile. For tenant electricity, the price agreed with tenants counts. An estimate of 30 % self-consumption without measured data can quickly shift the income value by five-figure amounts.
What the discount rate should not do
Missing documents or a known defect should not be hidden in a higher discount rate. We model such risks as specific costs or yield deductions. The rate then stays traceable and every item can be checked individually.
Frequently asked questions
What is the income value of a PV system?
The present value of all surpluses the system will still generate from the valuation date until decommissioning, minus investments and removal.
Which discount rate is used for income value?
A risk-adjusted rate reflecting cost of capital and the uncertainty of revenue. It is justified in the report and varied in a sensitivity analysis.