There is a persistent assumption that the hottest, sunniest places produce the most solar power. Sun, yes. Heat, no. Photovoltaic panels actually get less efficient as their temperature rises, and a rooftop in Phoenix in July is one of the hotter operating environments a solar module will ever face.
This matters practically, because if you do not know about the heat effect you will look at a flat July and blame dust that is not there, or you will shrug off real soiling as just summer.
Why Heat Costs You Output
Silicon cells generate voltage from the energy gap between charge states in the material. As the cell warms, that gap narrows slightly and more electrons get thermally excited on their own, which reduces the voltage the cell can hold under load. Current creeps up marginally. Voltage falls more. Power, being the product of the two, comes out lower.
Manufacturers publish this as the temperature coefficient of maximum power, expressed as a percentage loss per degree Celsius above the rating temperature. Modules are rated at 25 degrees Celsius cell temperature, which is 77 Fahrenheit. That is a laboratory condition, not a roof condition.
The Numbers on a Real Roof
Two things stack up on an Arizona afternoon. Ambient air is hot. On top of that, a dark panel absorbing full sun runs well above ambient, and a flush mounted array with limited airflow beneath it runs hotter still because the roof below is also radiating.
The practical result is that your panels spend summer afternoons operating far above their rating temperature, and the further above, the larger the deduction. Every module's spec sheet gives you its own coefficient. Look yours up if you want the exact figure for your equipment.
Ask any Valley solar owner who watches their app and they will tell you the best single days land in March or April, not June. Long days, high sun, clear air, and modules still running relatively cool.
Telling Heat Losses From Dirt Losses
This is the useful skill. Both reduce output, but they behave differently and you can tell them apart from your monitoring app.
Heat losses look like this:
- Affect the whole array uniformly, since every module is in the same conditions
- Track the daily temperature curve, so the shortfall grows through the afternoon and eases toward evening
- Recover completely on a cooler day with no intervention
- Are worst in the mid afternoon rather than at solar noon
- Are entirely reversible and require nothing from you
Soiling losses look like this:
- Build gradually over weeks and never recover on their own
- Scale the entire day's curve down proportionally rather than sagging in the afternoon
- Persist on cool mornings and mild days
- Are often uneven across the array, worse at low edges and where droppings collect
- Disappear the day the panels get washed
A quick test: compare a cool clear morning this week against a cool clear morning two months ago, at the same hour. Heat is not a factor at 8am in either case. If the recent morning is meaningfully lower, that is dirt, not temperature.
What You Can and Cannot Control
You cannot change physics, and you should be skeptical of anything sold as a fix for it. Spraying water on hot panels to cool them is a genuinely bad idea in Arizona: thermal shock on hot glass is a risk, and our hard tap water leaves behind a mineral film that permanently reduces transmission.
What you can control:
- Airflow. Standoff height was set at installation, but keeping the space under the array clear of nesting material and debris preserves whatever convection the design allows. A packed midden turns your array into an insulated blanket.
- Surface cleanliness. This is the lever that is fully yours. Heat costs are fixed by the season. Soiling costs are optional.
- Timing. A cleaning heading into summer means you are not stacking an avoidable dust loss on top of an unavoidable heat loss during your highest consumption months.
- Realistic expectations. Knowing that summer runs below spring keeps you from chasing a problem that is not there.
Where That Leaves You
Accept the heat loss. It is the cost of living somewhere with this much sun, and the sun more than pays for it. Do not accept the dust loss on top of it, because that one is entirely within your control and it lands in the same months your air conditioning is running hardest.
Two Bits Solar Cleaning uses deionized water so panels dry with no mineral residue, and we clear debris from beneath the array so airflow and drainage stay open. If your summer numbers look off and you want a second opinion, ask us for a quote.