Solar producing less than promised in Las Vegas: how to check it yourself

Pull out your contract and find one number: the estimated production in the first year of operation, in kWh. Compare it with twelve months of what your system actually made, counted from the day NV Energy let you switch on. If you are within a few percent, that is weather. If you are far short, or short every year by more than the degradation written into the same contract, something is wrong, and Nevada law gave your contract a clause about it.

This page walks through that comparison, what is normal for a Las Vegas roof, and what is a fault.

First, make sure you are comparing the right numbers

Most "my system is underperforming" worries start with a unit mix up. Two units, two different things.

• kW is size. It is the most the system can deliver at one instant under test conditions. A "10 kW system" is a size, like the engine size of a car.
• kWh is energy. It is what the system actually makes over time. Your contract's production estimate is in kWh. Your NV Energy bill is in kWh. Your shortfall is in kWh.

A 10 kW system does not make 10 kWh every hour of daylight. It makes its full rating, at most, for a few hours around noon on a cool, clear day, and much less the rest of the time. If someone showed you an hourly figure in kW and you are comparing it with a yearly figure in kWh, the "gap" may be nothing but arithmetic.

The second trap is the meter. NV Energy's meter only sees what you exported and what you imported. Everything your house used the moment the panels made it never crosses the meter, so your bill always shows less than your roof produced. Your app says one number and NV Energy says another explains which is which. For this check, use your inverter's own yearly or lifetime total wherever you can.

What your contract has to say

If you bought your system, Nevada is specific. Under NRS 598.9813 the cover page of a purchase agreement shows the estimated production in the first year of operation. Under NRS 598.9814 the agreement itself has to give the size of the system, that first year estimate, and the estimated annual degradation. It has to describe any guarantee of performance. And it has to set out the installer's duties if the system fails to meet the first year estimate.

Those duties, as the current statute reads, run from the date the utility gave permission to connect. If the system has not met the estimate one year after that date, the installer must either take whatever action is needed to make it meet the estimate the following year, including adding equipment or repairing what is there, or send you written notice that it missed, and then take that action if it misses again a year after the notice.

Leases carry the same duty under NRS 598.9811. A power purchase agreement must describe any performance guarantee under NRS 598.9817. Older agreements may predate some of this, so read what yours actually says. Your rights in a Nevada solar contract goes through the rest of the paperwork.

Why a Las Vegas roof makes less than the brochure

A healthy system in this valley still misses its nameplate every single day. The question is whether it misses by the amount the estimate already allowed for. Five things account for most of it.

Heat

Panels are rated at a cell temperature of 25 C. They lose output as they get hotter. NREL's PVWatts model, the free public estimator from the National Renewable Energy Laboratory, uses a loss of 0.47 percent per degree Celsius for a standard module and 0.35 percent for a premium one. For every 10 C a panel runs above its test temperature, that is roughly 3.5 to 4.7 percent less output.

A dark panel on a Las Vegas roof in July is far hotter than 25 C. That is why a clear July day can produce less than a clear May day. It is not a fault, and a competent estimate already priced it in. Your panel's own coefficient is on its datasheet, under temperature characteristics.

Degradation

Panels lose a little output each year. NREL's review of published field measurements found a median of 0.5 percent a year, with 78 percent of reported rates below 1 percent. A later NREL study of a large US fleet put median system decline at roughly 0.5 to 0.75 percent a year.

What binds your installer is the degradation figure in your own agreement, not a national median. Year five should be compared with the first year estimate less five years of that stated rate.

Dust and droppings

Dust is a film. Droppings are opaque patches that pull down a whole string. PVWatts assumes 2 percent a year for soiling as a default, and in a place where NOAA's climate normals put annual rain at 4.19 inches, much of it never rinses off on its own. Does cleaning your panels in Las Vegas pay? covers what the studies actually measured. Birds under the array are a separate problem, set out in pigeons under your solar panels.

Shade that was not there on day one

A tree that grew, a neighbour's second storey, a new satellite dish. PVWatts assumes about 3 percent shading loss even for systems described as unshaded. New shade on one panel can pull down others wired with it. Walk around the house at 9 in the morning and 3 in the afternoon and look at the array.

Clipping

Most systems are built with more panel capacity than inverter capacity. PVWatts uses a default DC to AC ratio of 1.1. On the best hours of the best days, the panels could make more than the inverter can pass, so the inverter caps it. In your monitoring this shows as a flat top on the daily curve on clear spring afternoons.

Clipping is a design choice, and a sensible estimate models it. It becomes your problem only if the estimate ignored it.

What a fault looks like instead

Everything above is gradual. Faults are not. Look for these patterns in a year of data.

A step down is the one that hides longest, because the system still looks alive from the street. How to find out if your solar system still works walks through reading the inverter, and solar inverter replacement cost covers the most common failed part. NREL's fleet study found inverter availability of 0.99 at the median system, which is a reminder that downtime is normal in small amounts and a problem in large ones.

How to run the check

1. Find the estimate. First year production in kWh, and the annual degradation, from your agreement.
2. Get twelve months of real production. Use the inverter's yearly total. If monitoring is gone, download your NV Energy usage and use exports as a floor, knowing they understate what the roof made.
3. Count from the right day. Year one starts at permission to operate, not at installation.
4. Adjust for age. Year three is the estimate less three years of the stated degradation.
5. Look at the shape. Summer dip, slow slide, sudden step, flat tops. The table above tells you which.
6. Rule out the cheap causes. Dust, droppings, new shade, a light on the inverter.

If you land within a few percent in a normal year, stop worrying and check again next year.

When the numbers really are short

Write it down before you call anyone: the estimate, the actual, the dates you used, and any pattern you saw. Then send the installer a dated letter asking what they will do under the production clause of your agreement, and keep a copy. A phone call leaves no record. A letter starts the clock the statute describes.

If the installer does not answer, the Nevada State Contractors Board takes complaints against licensed contractors. Its investigator decides whether a jobsite visit is needed and can send the contractor a Notice to Correct, which the Board says usually allows 20 to 30 days to comply.

If the company that installed the system has closed, the production duty may have gone with it, but the manufacturer warranties on panels and inverters usually have not. Your solar company went out of business covers what survives.

And if production turns out to be fine but the bill still went up, the cause is somewhere else: your bill went up after solar.

We inspect systems whoever installed them. If you would like someone to read the numbers with you, we will tell you what we find whether or not there is work in it for us.

We recheck this page quarterly. Last checked 2026-09-28. If a number here has moved, tell us and we will fix it the same week.