In short
Remaining useful life is the time during which the system can still be operated sensibly, technically and economically. It ends at the first hard limit: roof renovation, end of lease, mounting structure or the year from which costs exceed revenue.
What limits remaining useful life
| Component or factor | What we look at |
|---|---|
| Modules | degradation from measured data, delamination, cell cracks, PID, hotspots |
| Inverters | age, fault history, spare parts; replacement usually after 10 to 15 years |
| Mounting structure | corrosion, clamps, statics, ballast |
| Roof | condition of covering, upcoming renovation, sealing at penetrations |
| Cabling, connectors | insulation values, connectors, UV damage, fire safety |
| Contracts | term of roof use or lease, removal obligations |
| Economics | year from which costs exceed revenue, usually after the tariff ends with full feed-in |
Technical and economic useful life
Many modules run for 30 years and longer. Economically, operation often ends earlier: after the tariff ends, 4 to 5 ct/kWh with full feed-in barely covers maintenance, insurance and metering. In our worked example a 250 kWp system only just covers its costs after 2031. With self-consumption the picture changes: the 10 kWp system from 2012 remains economic until the end of its technical life.
The 20-year depreciation period under German tax law has nothing to do with either. It only determines depreciation.
Effect on value
Five years less useful life cost considerably more value for systems with high self-consumption than for full-feed-in systems after the tariff ends. The evaluation from the calculator shows the effect of a useful life five years shorter in its sensitivity analysis.
Degradation: measured data, not the datasheet
Manufacturers usually guarantee a minimum output after 25 years. What matters for the remaining useful life, however, is how fast this particular system ages. The median module loses about 0.5 % output per year, according to NREL's analysis of almost 2,000 measured rates (NREL). Individual systems deviate considerably, for example due to PID, cell cracks after hail or delamination.
We therefore derive degradation from the meter readings of recent years, corrected for irradiation. If it is well above the median, we investigate the cause with electroluminescence or IV curve measurement.
When profitability ends
The end of the tariff is fixed: 20 years plus the year of commissioning. After that, market value, self-consumption and running costs determine how long the system pays its way. The rule for systems up to 100 kW whose tariff has expired applies until the end of 2032 following Solar Package I (Verbraucherzentrale, German). We calculate each year separately and set decommissioning in the year from which surpluses turn negative. How revenues are modelled is explained under the income approach.
Contracts often set the limit before technology does
On third-party roofs, operation ends at the latest when the roof lease ends. If the lease runs for 20 years with no extension option, a module with a 30-year life does not help. We check term, options, removal obligations and whether an easement is entered in the land register.
A pending roof renovation has a similar effect. If the system has to be dismantled and reinstalled, that costs installation work and yield during the works.
When spare parts run out
If the inverter manufacturer has left the market, spare parts are no longer available. The unit then has to be replaced by a current model with a unit certificate under VDE-AR-N 4105 and notification to the grid operator. We include these costs in the year in which failure becomes likely.
How it appears in the report
We state the technical, economic and contractual remaining useful life separately. The shortest one enters the valuation. That way every reader sees which factor sets the end and what a lease extension or an inverter replacement would change in the value.
Frequently asked questions
How long does a PV system last?
Modules often 30 years and more, inverters usually 10 to 15 years. The system as a whole runs as long as roof, mounting, contracts and economics allow.
How is remaining useful life determined?
By inspecting the individual components on site, analysing yield data, checking roof and contracts and an economic calculation that shows the year from which operation no longer pays.