How Do I Know My Solar Performance Guarantee Is Being Met?

You signed a solar contract with a performance guarantee — some number of kWh your installer promised your system would deliver every year. Months or years later, your electric bill is higher than you expected, and you're left wondering: is my solar performance guarantee actually being met? This guide walks through exactly how to answer that question, using the guarantee number already sitting in your contract and the production data your monitoring system is already collecting.

What Is a Solar Performance Guarantee?

A solar performance guarantee (also called a production guarantee) is a contractual promise from your installer that your system will produce a specific amount of electricity — usually expressed as an annual kWh figure, sometimes as a percentage of the original sales estimate, or as a degradation schedule that steps down a small amount each year. It's different from a sales estimate, which is a projection, not a promise: only language written as a guarantee is enforceable. If you're not sure which one you have, that's the first thing to confirm — see what a production guarantee report actually documents for the specific clauses that determine whether a shortfall is collectible at all.

The guarantee clause matters because it's the one line in your paperwork that turns "my system feels like it's underproducing" into a specific, measurable comparison. Everything below is about how to make that comparison correctly.

The Three Steps to Checking Your Guarantee

Why You Can't Just Compare the Two Numbers Directly

Say your contract guarantees 12,500 kWh and your Enphase portal shows 11,400 kWh for the year — a shortfall of roughly 9%. Before you conclude the guarantee isn't being met, two adjustments matter:

• Weather adjustment. Guarantee numbers are typically modeled against a "typical meteorological year" — average sun for your location. If your area had an unusually cloudy year, actual sunlight (irradiance) was below that baseline, and a well-drafted guarantee should account for that. NREL's PVWatts model, driven by real irradiance data for your location and dates, tells you what your system should have produced given the weather you actually got — not the average weather the original estimate assumed.
• Degradation. Panels lose roughly 0.5% of output per year as a normal part of aging. A system guaranteed at 12,500 kWh in Year 1 isn't held to that same number in Year 5 — most guarantees step the figure down slightly each year, and a fair comparison should too.

Skip either adjustment and you'll get the wrong answer in both directions: a bad-weather year can make a perfectly healthy system look like it's falling short, and a good-weather year can mask a real fault developing underneath it. The weather-adjusted, degradation-adjusted expected number is what your actual output should be measured against — not the flat guarantee figure printed in your contract. Even a system that's performing exactly as designed will show a different raw percentage every year simply because the sun was different, so a single year's raw comparison is never the whole story on its own.

The DIY Way vs. What OwlWatt Automates

You can do all three steps by hand: read your contract, log into your monitoring portal every month, download NREL PVWatts data for your ZIP code, pull the actual irradiance for the year, and run the weather-adjustment calculation yourself. It's not impossible — but it's genuinely tedious to do accurately and consistently, and it's the kind of check that gets skipped until a bill spike forces the question. Most homeowners either never check, or check once, years after installation, by which point a real shortfall has compounded and the claim window in the contract may already be closed.

OwlWatt automates the whole loop: it connects directly to your Enphase system, pulls your actual production continuously, runs the same NREL PVWatts model against your location's real weather, and compares the result to your contract's guarantee automatically, every day — not once a year when you happen to remember. If the comparison shows a shortfall, OwlWatt flags it on your dashboard, and when you're ready to act, it generates a claim-ready shortfall report you can send to your installer.

Want a Fast, Rough Check First?

If you don't have your Enphase login handy, or just want a quick gut-check before digging into your contract, our free underproduction estimator compares your system's output to statewide averages for your size and age — no login required, four fields, an instant answer. It's a screening tool, not a precise or contract-grade figure — but it will tell you whether a closer, weather-adjusted look is worth your time.

What If Your Guarantee Isn't Being Met?

If your weather-adjusted comparison shows a real, sustained shortfall — not just a single cloudy month — you have a documented basis for a claim. How to file a production shortfall claim walks through the process: what to include in your written notice, your contract's claim window (often 90 days after the measurement year ends — don't miss it), and the remedies typically available, which can include a cash payment, a system repair, or additional panels, depending on your contract's specific language.

Before you file, re-check the clauses that determine whether a shortfall is actually collectible — the guarantee number itself, how production is measured, what's excluded, and the claim deadline — all covered in what a production guarantee report explains. A documented, weather-adjusted shortfall is far harder for an installer to dismiss than "my bill feels high."

Related Guides

• How OwlWatt Measures Your System
• Solar Expected vs. Actual Production Explained
• What's a Solar Production Guarantee Report?
• Is My Solar Underproducing? Free Estimator
• How to File a Production Shortfall Claim