Panel production explained

Many things impact the production of a solar panel, like location, pitch, orientation, season and any shading issues. Here we take a look at all of these to give you a better idea what you can expect out of your solar panels depending on your installation conditions.

NOTE: The graphs you see below are simulated, not real life systems. For more information about the simulated data, including a link to the source of the data, NREL's PVWatts® Calculator, see the notes at the bottom of this blog.

Panels don't achieve max output

The first thing you'll notice with your panels is that they don't perform to the wattage on the datasheet. In fact, your system will rarely achieve more than 85% of your panels combined peak power output, even with a premium panel. Note we’re not talking about your system yield in kWh, we’re talking about peak power output in kW. The reason for this is that the standard test conditions used to rate the panel output aren’t achieved in the real world. Here’s some reasons why:

This shows the daily production of a 5 kW north facing array on clear days in different seasons. Notice the production never achieves the maximum output.

  • Panels are flashed with 1000wm2 of simulated light, which is high even by Australian standards.

  • They are tested in a temperature controlled environment of 25 degrees, so heat losses aren’t factored in.

  • Panels are perfectly perpendicular to the simulated light.

  • They are brand new with no degradation, perfectly clean, dust free and no cable losses.

  • Output on your inverter is shown in AC, so losses for inverter tracking and conversion are factored into the production figure you're likely to be looking at.

Seasonal Impacts

Solar Seasons are a Month Early

The season has a big impact on panel production, particularly on higher pitched east and west arrays. We actually view the solar producing seasons a little differently to the standard seasons, as we think they're a month early! What we mean here is that we view summer in terms of solar production as November, December and January. This is because production depends more on the suns position in the sky, which peaks on December 21 and bottoms out June 21. Hence the best months for production are December and the months either side, and the worst is June and the months either side. We call these, Solar Seasons!

Solar Summer

Solar Summer production - We can see that east and west panels produce really well, even compared to north. Note this graph is showing 5 kW of panels.

A great time for solar production. There is more light intensity, more clear days and the sun rises earlier and sets later. In many Australian cities popular for solar, the sun rises almost directly above us in the height of summer. As the panels prefer the suns rays to be perpendicular, north panels mounted around 25 degrees don't actually get this during summer. East and west panels do in the morning and the evening, which is why their yield is around the same with north panels for this period of the year.

Early starts and late finishes are a feature of our solar summer. A west system is still producing reasonable amounts of power at 6pm and the bulk of the production is in the afternoon. This is a big bonus for families with higher afternoon usage. East can also be good if you use some electrical appliances early in the morning.

Solar Autumn and spring

First of all, yes these are two seasons, but they're similar so we've grouped them together. Remember as well that the solar seasons are a month early, so we're talking about February, March and April then August, September and October.

Solar Autumn and Spring production - East and west drop off a bit during this time of year. Peak production is still high for north panels, but yield is lower.

The production of east and west panels drops in Autumn and Spring. The peak production of north panels remains high because of the cooler temperatures and the lower sun angle. This makes up for the lower light intensity. The lower sun angle means the panel is closer to perpendicular to the suns rays in the middle of the day, provided they're mounted on a roof at around 25-30 degrees. So this isn't the case for lower pitched panels. Shorter days mean less production and there's normally (we stress, normally) less clearer days in these seasons compared to summer.

Solar Winter

Solar Winter production - Very low for east and west. Even north panels experience reduced output on peak production. Note this is a 5 kW array.

As you'd expect, the worst time of year for production. The less light intensity, more overcast days, lower sun angle and shorter days take its toll on production, and it's more than most people think. This is even more so if your panels are mounted east and west. The loss for east and west is worse in winter due to the lower sun angle. Their later production benefit for west and earlier for east are nullified.

The shorter days and more clouds mean that a perfect simulated production curve like the ones above rarely occur. In the other seasons, you'll see it regularly depending on where you are. This means your overall yield is low, more so than is represented on graph above.

Pitch

the Best Pitch

The pitch of the panel should ideally be perpendicular to the suns rays as often as possible. This is not possible on a standard roof, as the sun angle is changing all year. The optimum pitch is usually just under your angle of latitude, as you want to favour the summer months slightly where there's more clearer days. Provided you have a roof pitch above 10 degrees and less than about 35 degrees, we would recommend flush mounting the panels on your roof. The cost of changing your pitch or tilt framing probably won't justify the performance gain in these cases. You'd be better off adding more panels. Lower pitches will favour higher sun angles, like those in summer, and higher pitches are more suited to lower sun angles at other times during the year.

Flat Panels

These are not recommended, not because of their production, but because of their inability to clean themselves in the rain. If the panels are lower than 10 degrees, dirt that would normally wash off the panels in the rain can remain on the panel. So unless you plan on having someone clean them, we recommend investing in a tilt kit if your roof is less than 10 degrees.

Lower the Pitch, Lower The Orientation Impact

We spent a lot of time discussing the impacts of orientation across the seasons. The pitch of the panels impacts the effect of panel orientation. Lower the pitch, lower the impact of orientation. If you have a 10 degree pitch install in Perth or Brisbane, a west or east panel would only produce 4-7% less than a north over the year. If you compare this to a 30 degree pitch, you're looking at 14-16%.

Heat

Temperature coefficient shown on the panel datasheet of a Trina panel.

Panels don't like heat. The hotter it gets, the less a panel outputs. You can compare how panels will fare in the warmer climates by looking at the temperature coeffcient of power on the datasheet. This figure shows you the drop in performance for each degree over 25. So if you had a panel that had a -0.4% temperature co-efficient and the panel was operating at 50 degrees, that’s a 10% drop in output. Remember, the temperature outside is almost always less than the temperature of the panel, so panel temperatures in excess of 50 degrees are not uncommon.

Shade

Shade is more detrimental than people think. Shading on one panel will impact the others on the string, as it creates a bottleneck. This bottleneck effect is something that micro inverters or optimisers can negate, but the one panel being shaded will still have a reduced output. This is not the only issue with a shaded panel.

 This image from a SolarEdge technical paper (available below) shows cells on a standard panel and the position of the bypass diodes.

This image from a SolarEdge technical paper (available below) shows cells on a standard panel and the position of the bypass diodes.

Shading can be classed into two categories, hard and soft. The shading caused by a cloud is unavoidable, and what we call a soft shadow. Soft shadows are caused from objects far away and usually impact all of the panels. They are not going to reduce the lifespan of the panel, but hard shadows can.

Hard shadows are from objects close by, like flues, chimneys, trees or buildings. Even when this type of shading impacts one cell on a standard panel, it's usually enough to activate a bypass diode. There's three bypass diodes on most panels, but they have a limited lifespan, so using them should be avoided. If you have hard shadows, consider some preventative measures like relocating the panel or moving the shading. Installing micro inverters or optimisers will also reduce how often a bypass diode operates when a panel is shaded.

You may find that you get hard shadows only through the winter months, this is OK. In terms of your overall production, winter output is low anyway, so impact of the shading from a yearly perspective is usually small. While a micro inverters or optimisers would be better, the bypass diodes shouldn't fail if they're only being used for a few months of the year.

The full technical paper from SolarEdge entitled Bypass Diode Effects in Shaded Conditions can be found by clicking here.

Summary

As you can see, a lot of things can impact the production of a solar panel. If you have tricky conditions on your site, it's imperative that you speak to someone who understands the concepts above and can design your system to perform at it's optimum.

Production Graphs

The simulated production graphs were generated by using the NREL's PVWatts calculator. We generated hourly production data for 5 kW roof mounted systems in Perth at a 25 degree pitch for north, east and west. We then graphed the results of peak days in each season and compared it to live systems around Perth to ensure reasonable accuracy. As they are a simulation, they may not be 100% accurate, but we believe they are fit for this purpose. We highly recommend NREL's PVWatts calculator if you would like to do the same - http://pvwatts.nrel.gov/index.php