Why heat lowers a panel's power
A solar cell makes a voltage across a junction of two kinds of silicon. Heat gives the electrons in that silicon more energy of their own, which narrows the gap the sunlight has to push them across — and the voltage each cell can hold falls. A panel is a string of cells in series (66 in many SunPower panels, 96 in the X-Series), so a small drop per cell adds up.
Current barely changes with heat. It is set by how much light arrives. So when the panel warms up, voltage falls, current holds, and power — voltage times current — falls with the voltage.
How much to expect
From 142 panels over 30 days, readings taken in good sun:
- About 1.5–2% less power for every 10 °F the panel runs hotter.
- A typical 66-cell panel reads about 38 volts at 80 °F and 36 volts at 120 °F.
- SunPower's A-Series and M-Series datasheets list −0.29% per °C — 1.6% per 10 °F — which matches.
What matters is the panel's temperature, not the air's. In full sun a panel runs 30–45 °F above the outdoor temperature, so a mild, bright spring day can out-produce a hot summer afternoon with the same sunshine.
Normal, or worth a look?
Three different things lower a panel's output, and they leave different fingerprints:
- Heat — every panel a little lower, by the amount its temperature predicts. Normal. Nothing to do.
- Shade — one panel lower for part of the day, back to normal when the shadow moves. Full shade mostly cuts the panel's current; a shadow across part of it can switch a section of cells out, dropping its voltage by about a third, but only while the shadow is there.
- A failed section of cells — voltage about a third low at every hour, in sun or shade, day after day. That one is worth a look.
The simplest test: if a panel reaches its normal voltage at any point on a sunny day, every one of its cells is working. Across all 142 panels we checked, every one did, every sunny day.
For the shapes shade leaves in a day's production, see how to tell if a panel is shaded or broken. For what volts and amps mean, see how a panel and a microinverter work together.
What WattHacker shows you
Each panel gets one line: every cell working, or worth a look. Behind it is a daily check of the panel's voltage against a healthy panel of the same model at the same WattHacker panel temperature, and its current against its neighbors on the same part of the roof.
When shade is why a panel makes less, the app says when it happens — “until about 11:30 AM” — and, because the cells check out, that the loss is the shade and not the panel.
Where WattHacker fits
The WattHacker monitor is a small box that plugs into your existing SunPower PVS5 or PVS6 gateway on your own home network and reads it directly. It doesn't rely on SunPower's or SunStrong's servers, because for owners like you those are either gone or behind a paywall. No technician, no electrician, no rewiring — you plug it in yourself.
- The monitor is $50 once, with a 30-day money-back guarantee.
- Free to see your panel data, forever — every panel, live, no subscription, and no card on file.
- An optional premium plan at $6/mo (billed annually) adds the memory: full history kept for good, year-over-year comparison, and alerts told to you in dollars.
- Built by an electrification superfan who wants to help others use their solar smarter, lower their utility bills, and clean up the planet. Who's behind it.
Still have questions about your own system? I'll go through it with you — your panels, your bill, and what's worth doing about it.
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Common questions
Do solar panels make less power when it's hot?
Yes. A silicon solar panel's voltage falls as it heats up, and power is voltage times current, so power falls with it. Across 142 panels we measured over 30 days, the drop was about 1.5 to 2% for every 10 °F the panel ran hotter. SunPower's own datasheets for its A-Series and M-Series panels list −0.29% per °C, which is 1.6% per 10 °F.
Is it bad for my panels to get hot?
No. This is temperature influence, not damage. The panel loses a little output while it is hot and gets it all back as it cools. It is not degradation, which is the slow, permanent aging every panel goes through over decades.
How hot do solar panels get?
Much hotter than the air. In full sun a panel commonly runs 30 to 45 °F above the outdoor temperature, because it is a dark surface absorbing sunlight. So a 75 °F afternoon can mean a panel at 110 to 120 °F, and that is the temperature that sets its voltage, not the air.
How can I tell if a panel is just hot, shaded, or actually has a problem?
Heat lowers every panel together, by a predictable amount for its temperature. Shade lowers one panel for part of the day and gives it back as the sun moves. A panel with a failed section of cells reads about a third low at every hour, sun or shade, day after day. If a panel reaches its normal voltage at any point on a sunny day, all of its cells are working.
Does WattHacker check this for me?
Yes, on systems where each panel reports its DC voltage and current (SunPower systems do). Each panel is compared every day with what a healthy panel of the same model reads at the same WattHacker panel temperature, measured by sensors at each panel, and with its neighbors on the same part of the roof.
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