Two solar systems can look identical on a roof and be worth very different amounts. The difference is usually age: how many years the panels have been producing, how much output they've lost to degradation, and how much manufacturer warranty is left. Whether you own a system and want to understand its value, or you're buying a home with one, this guide shows how to read a system's age and translate it into production, warranty, and resale terms.
How to tell a system's age
Four reliable sources, in order of ease:
- Permit or interconnection date. The utility's Permission to Operate (PTO) date is effectively the system's birthday. It's on the interconnection paperwork and often in county permit records.
- Inverter manufacture date. Inverters carry a date code on their label.
- Panel datasheet + install contract. The contract lists the panel model; the model tells you roughly when it was in production.
- Monitoring history. The monitoring platform shows the first date data appeared.
What degradation actually does
Modern panels lose roughly 0.4–0.5% of their output per year. That's slow and predictable. Here's what it looks like over time for a system that started at 100%:
| System age | Approx. output vs. new |
|---|---|
| 5 years | ~97-98% |
| 10 years | ~95-96% |
| 15 years | ~92-93% |
| 20 years | ~90-91% |
| 25 years | ~87-88% |
The takeaway: an older system is not a "worn out" system. A 15-year-old array still produces roughly 92% of its original output. Sudden drops — a system at 80% when it should be at 95% — point to a fault (a failed microinverter, shading, or soiling), not normal aging.
Remaining warranty is where age really matters
Panels typically carry 25-year product and performance warranties; inverters run 10–25 years depending on type; batteries 10–15 years. So the practical questions for an aging system are: how many warranty years remain on each component, and which component's coverage expires first? A system with premium 25-year panels but a 10-year string inverter has an inverter-replacement cost looming in year 11 — worth knowing before you buy or sell.
A SolarDisclosure™ lookup maps your panels, inverter, and battery to their manufacturers and estimates the remaining coverage years on each — so age becomes a number, not a guess.
Run my warranty lookup →What age means for resale value
Age affects value through three levers, not just "old = worth less":
- Remaining production years. A buyer is really buying the electricity the system will still generate. More remaining life at high output = more value.
- Remaining warranty. Coverage that transfers reduces a buyer's risk and supports value; expired coverage introduces replacement-cost risk.
- Equipment brand. Premium panels from manufacturers with strong balance sheets carry more confidence — you're buying a warranty counterparty as much as hardware.
An older owned system with strong remaining warranty and a healthy production record can still add meaningful value. An older system with expiring coverage, an unknown installer, and no production history is where a buyer should discount. If aging changes your economics, a battery retrofit sometimes helps — see adding a battery to an existing system.
Sources & further reading
General information, not an appraisal. Degradation and warranty terms vary by manufacturer and model — verify for the specific equipment.