Why Your Solar Panels Produce More in Spring Than Summer

If you’ve been watching your solar production data, you might have noticed something counterintuitive: some of your best production days happen in April or May — not in the blazing heat of July or August.

This isn’t a glitch. It’s physics.

The Paradox: More Sun, Less Power

Summer brings longer days and higher solar elevation angles. More sunlight hits your panels for more hours. Logic says summer should dominate your production charts.

And on a total monthly basis, it usually does — July and August typically deliver the most kilowatt-hours simply because days are so long. But look at peak instantaneous power or energy per hour of sunshine, and spring often wins.

The culprit is temperature.

How Heat Steals Your Watts

A solar cell is a semiconductor device, and semiconductors are sensitive to temperature. As a silicon cell heats up, three things happen that reduce its output.

First, the bandgap narrows. The energy threshold that photons must exceed to free electrons decreases slightly. This sounds like it should help — more photons qualify — but the effect is small and is overwhelmed by what happens next.

Second, the open-circuit voltage drops. This is the big one. Higher temperature means more thermal energy in the crystal lattice, which increases the rate of electron-hole recombination. Freed electrons are more likely to fall back into holes before reaching the circuit. The voltage decrease is roughly linear with temperature: about -0.3% per degree Celsius for typical silicon panels.

Third, the current increases slightly with temperature — but this gain (around +0.05% per °C) is far too small to compensate for the voltage loss.

The net result is the temperature coefficient of power, typically around -0.35% to -0.45% per °C above the Standard Test Conditions (STC) reference of 25°C.

Putting Numbers to It

Let’s make this concrete. Your panel is rated at 400W under STC: 25°C cell temperature, 1000 W/m² irradiance.

On a hot July day in Poland, ambient temperature might be 35°C. With direct sun heating the panel, cell temperature easily reaches 55-65°C. That’s 30-40°C above the STC reference.

At a temperature coefficient of -0.4% per °C and a 35°C rise, you lose 14% of rated power. Your 400W panel delivers about 344W at peak, even under perfect sun.

Now take a crisp April day: ambient 12°C, bright sunshine, light breeze. Cell temperature might only reach 30-35°C — just 5-10°C above STC. Power loss: 2-4%. Your panel delivers 384-392W.

That’s a difference of 40-48 watts per panel — purely from temperature.

Multiply by 20 panels and you’re looking at 800-960W of peak production difference between a cool spring day and a hot summer day with identical sunshine.

Why Cell Temperature ≠ Air Temperature

Your panels run significantly hotter than the air around them. Under 1000 W/m² irradiance, cell temperature typically exceeds ambient by 20-30°C, depending on mounting and wind.

Roof-mounted panels (with limited airflow underneath) run hottest. Ground-mounted or tilted rack systems with good ventilation stay cooler. Wind speed matters enormously — a steady breeze can reduce cell temperature by 10°C or more compared to still air.

This is why the industry uses a metric called NOCT (Nominal Operating Cell Temperature), typically 42-46°C, which represents cell temperature under specific realistic conditions (800 W/m², 20°C ambient, 1 m/s wind). It’s always well above ambient.

The Spring Sweet Spot

Spring — particularly March through May in Central Europe — offers a unique combination:

Strong irradiance. The sun is already high enough for good direct beam radiation. April and May solar elevation angles in Poland reach 45-55°, producing solid irradiance values.

Cool temperatures. Ambient temps of 8-18°C keep cell temperatures moderate, minimizing thermal losses.

Clear skies. Spring often brings stable high-pressure systems with excellent atmospheric clarity.

Long-ish days. While not as long as June, April and May days are already 13-16 hours in Poland.

The result: high irradiance hitting panels that are running cool and efficient. It’s the PV sweet spot.

What This Means for Forecasting

Temperature isn’t just a footnote in PV modeling — it’s a primary variable. Any forecast that ignores cell temperature will systematically overpredict summer output and underpredict spring and autumn output.

Volcast models this explicitly. It doesn’t just ask “how much sun?” — it calculates the expected cell temperature based on irradiance, ambient temperature, and wind conditions, then applies the temperature coefficient to predict actual power output.

This is one of the concrete advantages of physics-based forecasting: the temperature effect is built into the physical model, not learned from historical data that might not capture unusual conditions.

Next time you see a glorious spring day outperforming a sweltering summer afternoon, you’ll know exactly why.