Why Your 300 W / 400 W / 500 W Solar Panel Doesn’t Always Produce Its Rated Watts (STC vs NOCT vs Real-World)

It’s completely normal for a 300 W, 400 W or 500 W solar panel to produce less than its “rated” wattage in everyday conditions.
This isn’t a fault — it’s simply how solar panels are tested, certified and affected by real Australian weather.

This guide explains why peak wattage (W) on the label is not what you’ll consistently see on your MPPT or panel meter, and what “good” output looks like in real conditions.

1. The Number on the Panel (300 W / 400 W / 500 W) = STC Rating, Not Real-World Output

Every solar panel worldwide is tested using the same lab standard:

? STC — Standard Test Conditions

• 1,000 W/m² artificial sunlight

• 25°C cell temperature (not air temperature)

• 0° angle with perfect light

• Zero wind, zero dust, zero shade

• Factory-new, perfectly clean panel

This environment does not exist outdoors.

Your panel is rated at STC so all brands can be compared fairly — but it doesn’t represent typical outdoor performance.

2. Real-World Output Is Based on NOCT, Not STC

? NOCT — Nominal Operating Cell Temperature

NOCT simulates more realistic outdoor conditions:

• 800 W/m² sunlight

• 45°C cell temperature

• 20°C air temperature

• Light wind

• Natural mounting angle

This is much closer to what your panel experiences on a caravan roof, RV, 4WD canopy or ground mount.

Most solar panels produce 60–75% of their STC rating in NOCT conditions.

Examples:

This is normal and expected performance across all brands in Australia.

3. Heat Has the Biggest Impact (Especially on Caravan Roofs)

Panels lose power as they heat up.
Your panel might be rated at 25°C, but black caravan roofs easily climb to 60–75°C in the sun.

Solar panels typically lose:

• 0.32% – 0.45% output per °C above 25°C

Example:

• If cells reach 70°C, that’s ~45°C above STC

• Power loss = 45 × 0.35% ≈ 15–18% less power

This is why panels often produce more in cool weather than in summer.

4. Angle, Orientation & Time of Day Matter

Your panel will produce the highest output when:

• Sun is directly overhead

• Panel angle is 90° to the sun’s rays

• No shading is present

• Ambient temps are mild (morning or late afternoon)

Common real-world reduction factors:

• Panel lying flat on the roof → 10–25% loss

• Slight haze, cloud or humidity → 5–20% loss

• Non-optimal angle in winter → 15–35% loss

• Late morning/early afternoon → 10–40% lower than solar noon

This is normal and unavoidable for any fixed installation.

5. Shading, Even Small Amounts, Can Reduce Output by 50–90%

Shaded cells drop voltage dramatically — even a:

• TV antenna

• Air-con shroud

• Vent

• Roof rail

• Awning shadow

• Tree branch

…can cause heavy power loss.

Shingled panels (like the ones you sell) handle partial shade better than traditional panels, but no panel is shade-proof.

6. Dust, Salt, Haze & Panel Cleanliness Affect Watts

Dirty, salty or dusty panels typically lose:

• 5–20% output on caravans & 4WDs

• More in coastal or outback environments

Regular cleaning restores lost performance.

7. Your MPPT Reported Watts Will Always Be Lower Than STC

Even on a perfect day, an MPPT is measuring:

• Real sunlight

• Real temperature

• Real voltage drop

• Wiring losses

• Charge limits

• Battery acceptance rate

• MPPT efficiency (~97–99%)

You will not see a sustained 300 W / 400 W / 500 W output unless conditions are better than STC — which almost never happens outdoors.

Occasional short spikes above STC are normal on:

• Cold mornings

• High-altitude locations

• Strong sun with cool wind

But these are momentary, not constant.

8. What Is Considered Good Performance?

In real-world Australian conditions, typical peak outputs:

These are healthy results and match industry averages for fixed panels on caravans and campers.

9. How to Maximise Your Panel’s Output

• Keep the panel clean

• Ensure MC4 connections are fully clicked and dry

• Avoid shading from roof objects

• Install panels away from A/C units & vents where possible

• Use appropriately thick solar cable to reduce voltage drop

• Use a quality MPPT (your customers often use Victron, SolarEpic, DC Mont, etc.)

• If testing, measure Voc, Vmp & Isc with the panel disconnected, not when the battery is full

10. When to Contact Support

You can contact us if:

• One panel is producing much less than the others

• Voltage (Vmp or Voc) readings are incorrect

• You suspect a damaged cell, hotspot or bypass diode issue

• Your MPPT shows 0 W on a sunny day

We’ll guide you through quick diagnostic tests.

Summary

Your panel’s STC rating (300 W / 400 W / 500 W) is a lab-standard figure, not what you should expect on the roof of a caravan or 4WD.
Real-world output is affected by:

• Heat

• Angle

• Weather

• Shading

• Dirt

• Battery charging behaviour

Most panels in Australia produce 60–75% of their STC rating under normal conditions — and this is completely normal across all brands.