A common question comes up over and over on expedition forums: why is my solar output so vastly different from the reported manufacturer specs? Whether it’s way below the panels’ rating or even sometimes above, overlanders often struggle to scope and size their solar setup. So why is that? Is it due to manufacturers misleading consumers with unattainable efficiencies?

 

Short answer is NO.

 

Provided the panels are from a reputable manufacturer, the specs are very accurate, in fact they are usually conservative. Panasonic, for example, allow a -0%, +10% tolerance in their specs. Panels that do not meet the spec are downgraded to a lower wattage panel. The issue is not the manufacturers padding the specs, it's that the conditions they are tested under are rarely achieved in real life.

When using manufacturer specs the two most relevant factors are:
• Operating cell temperature. STC is too low in most cases. Typical temp coefficients are between -0.25% and -0.5% per °C so at a realistic operating temperature of 45°C, you will be down to 90-95% output. Some panels provide a PTC rating or CEC PTC rating which is attempts to take cell temperature into account (mandatory in California for example).
• Panel age. They lose between 0.25% and 1% of their output per year on top of other losses.

Then you have to consider that you can only harvest energy that hits the panel.

• One thing in our favor is that tropical surface irradiation is actually greater than STC. 1050W/m² vs 1000W/m². STC uses an air mass 1.5 model (close to 1000W/m²) but in the tropics this is conservative.
• Of course, the above assumes no clouds or haze. On a very cloudy day, irradiance might be as low as 10% of that. Real insolation data should be used (and is used commercially).
• If you are mounting panels flat, then you can assume roughly the cosine of your latitude is lost. i.e. at 60° latitude you'll miss out on 50% of the available irradiance. Add to this the fact that the panels are not tracking throughout the day so there are more losses there.
• Then the panels must be dust/dirt free.
• Finally, the MPPT efficiency should be taken into account.

 

Here is an example walk through to help someone considering working it out for themselves:

1000W installed
(as per STC manufacturer figures)

=> 900W when you account for normal operating temps (at 45°C)
=> 855W after 5 years of panel ageing or slightly unclean panels (roughly equivalent)
=> 837W after factoring in MPPT efficiency (98%)

But in this case Watts are only a measure of generation potential, not expected output. For that the location and weather are important. Really what is of interest to us, is how many Watt-hours (or equivalent Amp-hours at 12V) we should expect to generate in a day.

It comes down to available insolation (how many hours of sunlight) and inclination (flat or inclined). This requires actual measured data and is location specific so it's impossible to generalize. For this example, let's look at sunny Norfolk in the UK:

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This is how many equivalent hours of sunlight is expected to fall on a flat square meter of ground in that location, each day. This is an average of the last seven years. It's clear that the season is important!

Using our potential 837W from earlier we can get:

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Which demonstrates that in summer you'll probably generate more than you can store but in December you'll hardly get anything. Linear rules of thumb aren't feasible outside the tropics so really a model like this should be used.

So to answer our initial questions: YES, manufacturers specs are reliable and accurate but need to be adapted to real-life conditions. For those of you currently sizing their solar array, feel free to use this calculator I created to get a sense of how much energy you can expect to generate.