Solar energy audit: the home audit, the output audit, and which one you need

"Solar energy audit" is one phrase doing two jobs, and the two have almost nothing in common. If you type it into a search box you get a mix of utility programs offering to inspect your attic insulation and, further down, a few pages about checking whether an installed solar system is producing what it should. Depending on which one you meant, the useful answer is either "call your utility, it is probably free" or "pull twelve months of data and compare it to a model."

This page separates the two, tells you where to go for the first, and explains the second in full, because the second is the one nobody offers you and the one that decides whether you are owed money.

Meaning one: the home energy audit

A home energy audit is about the house, not the panels. An auditor walks through with a blower door, a thermal camera and a clipboard, and comes back with a list: attic insulation at R-19 where it should be R-49, a leaky rim joist, a 20-year-old heat pump, incandescent bulbs in the basement. The goal is to lower how much energy the house uses. Solar enters the picture only as sizing: after the cheap fixes, how big a system does the remaining load justify?

It is a good thing to do, and it is usually the wrong order to do it in after the panels are already up. Where to get one:

• Your utility. Most investor-owned utilities offer a free or heavily subsidized audit through their efficiency program. In Massachusetts it is Mass Save; California utilities run their own; New York has NYSERDA's program. Search your utility's name plus "home energy assessment."
• State programs and weatherization. Income-qualified households can get the audit and much of the work done free through federal Weatherization Assistance, administered by each state.
• Independent auditors. BPI- or RESNET-certified auditors charge $100 to $500 for a full audit with a written report. Worth it if you want a number you own rather than a utility upsell list.

If that is what you came for, you have your answer and the rest of this page is not for you. If your panels are already installed and your real question is are they producing what I was promised, keep reading.

Meaning two: the solar output audit

This one is about the panels, and specifically about a number. When you signed, the installer put an annual production figure in writing, usually in kilowatt-hours per year, and often wrapped it in a production guarantee clause: if the system makes less than some percentage of that figure over an assessment period, they owe you the difference. A solar output audit, also called a solar performance audit, checks whether that promise was kept.

It is done entirely in data. Nobody visits. The audit needs three numbers and one comparison:

Each input has a right source and a wrong one, and most homeowner attempts at this fail on the inputs rather than the arithmetic.

Actual production

Your monitoring account. For an Enphase system that is the Enlighten app or web portal, which stores daily production since commissioning. Use the vendor's number, not a meter reading or the bill: the vendor's record is the one your installer also relies on, so it is the one they cannot dispute. If you have never pulled a year of data out of the app, our guide to reading and exporting Enphase Enlighten data walks through it.

Expected production

This is where people go wrong. The installer's proposal number is not an expectation for the year you actually had; it is a long-run-average estimate, built with whatever assumptions helped the sale. Last year's production is not an expectation either, because last year had different weather. The right expected number is a model of your array (panel count, wattage, tilt, orientation, shading, install date) run against the sunlight and temperature your address actually received in the period. The public model for this is PVWatts, published by the National Renewable Energy Laboratory. We describe how OwlWatt runs it for your roof, and every input is listed so your installer can check the work.

The promised number and the clause

Your contract or proposal. Find the annual kWh figure, then find the guarantee language around it: the percentage (often 90% or 95% of the estimate), the assessment period (usually per contract year, sometimes cumulative), the cure terms, and what the installer owes if it falls short. These four items decide whether a shortfall is a claim. Our page on what a solar production guarantee promises goes clause by clause.

A worked example

A 9.6 kW system in Massachusetts, contract promises 11,000 kWh a year with a 90% production guarantee, electric rate $0.30 per kWh. The app shows 9,100 kWh for contract year two.

Illustrative figures. The last row is the one only an audit produces, and it changes the conversation.

The bottom row matters more than it looks. If the independent model says this roof should have made 10,400 kWh in that weather and it made 9,100, the system is 12.5% under an honest expectation, which points to a physical problem worth an inspection. If instead the model had said 9,200 kWh, because the roof faces east-west and the proposal modeled it as south, then the system is fine and the proposal was oversold. Both are claims under most guarantee clauses. They are different claims, they need different letters, and only the audit tells you which one you have.

What an audit is not

Three things get confused with it, and the confusion costs people months.

• It is not an inspection. An inspection finds broken hardware. An audit finds a gap in output and how big it is. Audit first, because it is cheaper and tells the inspector where to look. The full comparison is in our solar panel inspection guide.
• It is not the installer's "system check." When you call to say production looks low, many installers offer to "run a check." It is worth accepting, and it is not independent. The person running it works for the party that would pay the claim, and "within expected range" is the usual answer, with no model behind it that you can see.
• It is not a monitoring app. The app tells you what the system produced. It has no opinion about what it should have produced. A green dot means the equipment phoned home, not that the number it sent was right. See how to tell if your solar panels are actually working.

Doing a rough audit yourself

You can get a first answer in about twenty minutes, and it is worth doing before paying anyone.

1. Pull the last twelve full months of production from your monitoring app and add them up.
2. Run the free production estimator: six dropdowns and a ZIP code give you a modeled annual range for a system like yours.
3. Compare. Inside the range, or within about 10% of the middle of it: the system is probably fine, and the rest of the gap, if any, is weather and normal losses. More than 15% under: get a full audit with your real roof geometry and real weather, because that is likely a claim.

The rough version uses state-average sunlight and ignores your tilt, orientation and shade. That is why it is a screen and not a verdict. A claim-grade audit models your exact array against the actual weather for each contract year and compares against the clause in your contract, and that is what you attach to a letter.

What to do with the result

If the audit shows no meaningful gap, keep the report. It answers the question, and it is a useful document if you sell the house. If it shows a shortfall, the order is: read the guarantee clause for how to make a claim, send the report with a written demand, and give the installer the cure period the clause allows. If they do not respond, the escalation path runs through your state's consumer-protection law; we have a state-by-state guide to escalating a solar production claim. OwlWatt's part ends at the report. We do not recover money, we never contact your installer, and we make no promise about the outcome; the report is yours and what happens next depends on your contract.