STC vs Real-World Solar Output

If you’ve ever looked at your solar controller and thought: “Why is my 200W / 400W / 500W panel not making its full wattage?” — you’re not alone.

Most solar panels are rated at STC (Standard Test Conditions). STC is a lab test used to compare panels fairly, but it’s not what you’ll see on a caravan roof or at camp. In the real world, heat, sun angle, wiring losses, shading, and controller limits all reduce output.

This page explains the difference in plain English and shows what to expect from your solar setup.

What does “STC” mean?

STC = Standard Test Conditions, which is a laboratory standard used by manufacturers to measure panel output under controlled conditions:

  • Solar irradiance: 1000 W/m² (very strong sun)

  • Cell temperature: 25°C (cool panel cells)

  • Air mass: 1.5 (a specific sun angle through the atmosphere)

✅ STC is great for comparing panels
❌ STC is not a promise of daily output

Real-World Output (What you’ll actually see)

In real life, your panel is usually:

  • Hotter than 25°C (especially on roofs)

  • Seeing less than 1000 W/m² (clouds, haze, time of day)

  • Mounted flat (sun not perfectly aimed)

  • Losing power via cables, connectors, and the controller

  • Affected by partial shading (even a small shadow can drop output)

So instead of hitting the label wattage, most systems see real-world results like:

Typical real-world output guide (rule of thumb)

  • Cool, clear winter sun: ~70–95% of rated wattage (best case)

  • Clear sky, warm panel: ~55–85% of rated wattage (very common)

  • Hot summer roof panel: ~45–75% of rated wattage (common on caravans)

  • Hazy / partial cloud / bad sun angle: ~20–60% of rated wattage

These numbers are general guides — but they’re realistic expectations for camping and off-grid use.

The biggest reason you “lose watts”: HEAT

Solar panels lose efficiency as they heat up.

A common spec is a power temperature coefficient (often around -0.35% to -0.45% per °C above 25°C). That means when the panel cells get hot, output drops.

Example:
If your panel cells reach ~65°C on a hot day (which is common on a roof), that’s 40°C above 25°C.

If the coefficient is -0.4% per °C:
40 × 0.4% = ~16% power drop (before you even include other losses)

Your solar controller can also “cap” output

MPPT controllers

MPPT controllers convert voltage into current efficiently and usually provide the best real-world harvest.
✅ Best for roof panels and higher-voltage panels
✅ Handles mixed conditions better

PWM controllers

PWM controllers are simpler and cheaper, but they often “pull down” panel voltage to match the battery, which can reduce panel wattage significantly.
❌ Real-world power can be much lower, especially with higher-voltage panels

If you’re running a larger panel on PWM, your controller may be limiting what you can actually use.

Wiring and connection losses (they add up)

Common real-world losses include:

  • Long extension runs (voltage drop)

  • Thin cable (too small for current)

  • Loose or corroded connectors

  • MC4 adaptors and branch connectors

  • High resistance joins / plugs

Rule of thumb: If cable is undersized or long, you can lose a noticeable percentage of your power before it even reaches the controller.

✅ Tip: For longer runs, consider thicker cable and/or higher panel voltage with MPPT.

Shading: the silent killer

Even a small amount of shading can drop output dramatically — for example:

  • a roof rack bar

  • an aerial

  • a tree branch shadow

  • a partially shaded corner

Panels have bypass diodes to help, but shading is still one of the biggest causes of “low wattage” complaints.

✅ Tip: If you can’t avoid shade, try adjusting your parking position and re-check the controller readings.

“So what should my panel produce?”

Here’s a practical, easy expectation check:

If you have a 200W panel:

  • Typical real-world: 110W–170W

  • Best-case (cool + perfect sun): 170W–200W

If you have a 400W panel:

  • Typical real-world: 220W–340W

  • Best-case: 340W–400W

If you have a 500W panel:

  • Typical real-world: 280W–420W

  • Best-case: 420W–500W

If you’re seeing much lower than this in clear sun around midday, something may be wrong — or your system may be limiting output (controller, battery state, wiring, shade, etc.).

Quick troubleshooting checklist (clear day, around midday)

If output looks low, check:

  1. Battery charge state

    • If the battery is nearly full, charging power can reduce (normal behaviour)

  2. Controller display

    • Compare PV voltage, PV current, and PV watts

    • Also check battery voltage and charging stage

  3. Shading & angle

    • Remove any shadow and re-test

  4. Connections

    • Inspect MC4s and adaptors for looseness or water ingress

  5. Cable size & length

    • Long thin cable = big losses

  6. Panel surface

    • Dust, salt spray, and grime reduce output (especially for marine use)

Want us to help you confirm performance?

We’re happy to help you validate whether your panel is operating correctly.

Send our support team:

  • Photos of your panel install (full panel + connections)

  • Your controller model

  • A photo of the controller screen showing PV volts + amps + watts

  • Time of day + location + weather conditions

Helpful links

FAQ

Does “not hitting rated watts” mean the panel is faulty?

Not usually. Most of the time it’s normal real-world conditions (heat, sun angle, controller limits, wiring losses, shading). Faults are more likely if output is very low in clear midday sun and you’ve ruled out the common causes.

Why does it look better in winter sometimes?

Panels can perform better in cool conditions because heat reduces efficiency. Clear winter skies can also be excellent for solar.

Can my battery being full reduce wattage?

Yes. As the battery approaches full, the controller reduces charging current, so PV watts often drop.

Why is my portable blanket making different power to my roof panel?

Portable panels can be aimed at the sun and kept cooler, while roof panels often run hotter and stay flat.