Solar Module Performance Warranties vs. Energy Yield Guarantees: What Is Actually Covered

A solar module performance warranty covers the module’s minimum power output under specified test conditions. An energy yield guarantee covers the electricity a system produces over an agreed period. A module can meet its power warranty while the system falls short on annual generation. Removal costs, lost income and weather-related shortfalls are covered only where the applicable terms provide for them.

What Each Agreement Covers

An installation warranty is separate again. A roof leak caused by mounting work may belong under the installer’s workmanship terms rather than the module manufacturer’s warranty.

How a Module Performance Warranty Works

Suppose a 550W module has a 30-year linear power warranty allowing 1% degradation in the first year and a further 0.4 percentage points of original rated power each year after that. Its minimum warranted output would be:

• Year 1: 550 × 99% = 544.5W.
• Year 10: 550 × [99% − (9 × 0.4%)] = 524.7W.
• Year 30: 550 × [99% − (29 × 0.4%)] = 480.7W.

These are contractual minimums, not a forecast of what the module will actually produce. A verified result below the applicable minimum may support a claim, subject to the warranty’s testing rules and exclusions.

When comparing Tongwei module models, match the exact model designation to its datasheet and applicable warranty. A degradation figure from one product should not be applied to every N-type or double-glass module.

Lab Conditions Matter

Standard test conditions use irradiance of 1,000W/m², a cell temperature of 25°C and the AM1.5 reference spectrum. An inverter reading taken on a hot afternoon is not directly comparable with a module’s rated DC power.

For example, a module with a power temperature coefficient of −0.30%/°C would have an estimated temperature-related power reduction of 10.5% at a cell temperature of 60°C relative to 25°C, with other conditions unchanged. That temporary reduction does not by itself demonstrate permanent degradation.

IEC 61215 qualification also does not prove a module will last a specified number of years. The standard explicitly states that its test results are not a quantitative prediction of module lifetime.^[1]

Linear and Stepped Warranties

A linear warranty sets an age-dependent minimum along a declining curve. A stepped warranty sets minimums for defined periods. If a stepped warranty requires at least 90% during the first 10 years, an eligible result of 85% in year eight would fall below that threshold. The wording of the periods matters more than the label alone.

How an Energy Yield Guarantee Works

A generation estimate in a quotation is not automatically a guarantee. The agreement must establish an obligation to deliver a stated output or performance level and explain what happens if it is missed.

For an illustrative 10kW system, assume a reference annual yield of 11,000kWh, a guarantee of 90% of that reference, and compensation of CNY 0.60 per eligible missing kWh.

Without a weather adjustment: the guaranteed minimum is 9,900kWh. If eligible measured generation is 9,600kWh, the shortfall is 300kWh. Compensation would be CNY 180 before any other contractual limits.

Now assume the contract instead requires a weather-adjusted model, and that model produces a reference of 10,000kWh for the same year. At 90%, the adjusted minimum becomes 9,000kWh. The same 9,600kWh result would exceed the guarantee.

The contract should identify the weather data, calculation method and meter used for settlement. A generation meter and an export meter measure different things: electricity consumed on site appears in generation but not in exports.

Performance Ratio Is a Different Test

Some projects use a performance ratio, or PR, guarantee rather than a fixed kWh target. PR relates output to installed capacity and available solar irradiation. IEA PVPS guidance explains that data quality and processing choices affect the resulting performance indicators.^[2]

If a contract requires PR of 82% and the accepted result is 79%, the gap is three percentage points. The payment still depends on the agreed remedy; that gap does not automatically translate into a particular cash amount. There is no universal PR pass mark or compensation threshold for every project.

Which Losses May Fall Outside Coverage?

• Inverter downtime: it does not establish a module defect, but it may count against a system-level guarantee if the provider carries that risk.
• Shade, dirt and snow: responsibility depends on the design assumptions, maintenance duties and exclusions.
• Grid outages and curtailment: the agreement should state whether affected periods are excluded or credited with calculated generation.
• Storm damage: coverage depends on the damage, installation requirements and applicable warranty or insurance. A mechanical load rating alone does not settle a storm claim.
• Removal and lost revenue: a replacement module does not automatically include scaffolding, freight, reinstallation or compensation for missed generation.

What to Do When Generation Falls

Start with the meter and fault history before arranging module testing. System-level performance losses can include outages, soiling and other operating problems, not just module degradation. NREL’s fleet analysis distinguishes these system effects from module-level degradation.^[3]

1. Confirm the comparison. Use the same measurement period and metering point. Check whether the guarantee is annual, multi-year or weather-adjusted.
2. Save the evidence. Export interval generation data, inverter alarms, outage records and maintenance logs. Keep invoices, serial numbers, commissioning records and the applicable warranty version.
3. Identify the likely cause. Ask a qualified technician to distinguish a system outage, shading problem or wiring fault from suspected module underperformance.
4. Notify the responsible provider. Obtain written instructions on testing, return authorization, deadlines and who pays each cost before removing or shipping modules.

For Tongwei equipment, the module authenticity query provides a verification step alongside the serial-number and purchase records. It does not replace proof of warranty eligibility.

Terms Worth Confirming Before You Sign

Ask for the start date, claim deadline, transfer conditions and remedy in writing. Warranty periods may begin before commissioning. Replacement equipment may carry only the remaining original term, and a refund may follow a depreciation formula rather than the original purchase price.

For an energy guarantee, also confirm whether surplus generation in one period can offset a later shortfall, which electricity rate applies, and whether compensation has an annual or total cap. A kWh payment at a fixed rate may not cover your actual lost revenue.

Finally, identify the legal entity responsible for payment. Manufacturer size alone does not establish protection if that entity stops trading. Any insurance, parent guarantee or other financial support needs its own scope and claim conditions.