Shading on solar panels and how to defeat it without breaking the bank

'Watts Law' says that Power (W) = Voltage (V) x Current (A)

Therefore, the higher the voltage and current, the more power is produced.


When a solar panel is shaded the voltage may drop a little, but the current drops a LOT.

Imagine a garden hose with water rushing through it. The tap with the mains water pressure is the 'voltage' and the flow of water is the 'current'. Now stand on that hose. The pressure from the tap is still the same, but the flow of water is significantly less. Same with solar panels. Shading causes very little change to the panel voltage, but the current is severely reduced. Let's go back to Mr Watt and his law.


Unshaded panel... 32 Volts x 8 Amps = 256 Watts of power
Shaded panel...32 Volts x 4 Amps = 128 Watts of power.


This solar panel is connected in a daisy chain fashion (in series) to perhaps nine other panels in what is called a 'string'. Without shade the panel voltages are combined to make 320 Volts and the current is a constant flow of 8 Amps making 2560 Watts of Power (2.56kW).

However with shade, the combined voltage is still 320 Volts but the current for ALL of the panels drops to the lowest common denominator, the 4 Amps of the shaded panel, so now the total production is down to 1280 Watts (1.28kW).



That's why shade is bad for solar, so let's discuss what can be done about it.

The good news though is that what is written above is more a 'worst case' scenario because there are plenty of things that can, and do, reduce power losses caused by shade and many of them are totally free. Here are six typical options.


1. Avoid it. Put panels in places where shade isn't.

2. Let the 3 to 6 diodes built into every solar panel do their job of shutting down sections of the panel when shaded so that the full current from the rest of the panel is maintained.

3. Let the inverter do it's job of maximising the output using advanced MPPT algorithims.


4. Do nothing, let the shade reduce the output a bit as the solar will still make far more power than needed anyway, shaded or not.


All of items 1-4 above are FREE.


5. Install optimisers on the shaded panels only


6. Install optimisers or micro inverters on EVERY panel, shaded or not.


Optimisers cost between $80 and $120 each depending on brand, quality and capability, micro inverters are about $250 each.


How much power is really lost by shade?

The diodes in solar panels do a pretty decent job and the inverter algorithims probably an even better job, especially with modern inverters with the best efficiency (e.g. Huawei with 98.4%  or SMA with 97%). In most cases we've investigated shading drops output by less than 10%.


Which is another way of saying that spending money on optimisers is unlikely to achieve much more than a 6% gain because (a) optimisers use up some power and (b) even an optimised panel is going to produce less power when shaded because the optimiser isn't there to increase the power of the shaded panel, it's there to protect the rest of the panels.


Therefore, you might see an extra 2kWh of power a day if you deploy some optimisers on a typical 6.6kW panel system. Whilst an extra 730kWh a year may sound pretty good, you need to also determine whether those extra 2kWh are going to be used in the house during the day, stored in a battery, or exported as surplus. If exported then they are only worth 14 cents a day or $50 a year.


What is the 'Shade Fix' feature built into SMA inverters?
It is point 3 above. MPPT algorithms that maximise the output of the panels by adjusting the string voltage, current, resistance. SMA do it well, as do Fronius and Growatt, and Huawei being a major IT company, do it even better, but really, all modern inverters do the same thing. SMA choose to give this now standard feature a fancy name (Shade Fix) but that's just their marketing department getting over-excited and citing a dubious 'case study' that has had some cold water thrown on it.


What about really BAD shading?
Think back to the garden hose analogy. The current for each shaded panel is cut in half, so if you have heavy shading on most or all of your panels simultaneously then the output is going to be substantially reduced whether you have optimisers fitted or not. However if the shade sweeps across your panels so that only a few are exposed to shade at a time, then yes, optimisers in heavy shade really can work very well. You might recover 25-30% of your lost power.


What we recommend
We typically take an approach of points 1 - 4 above, with a few optimisers on shaded panels if your budget allows.

We also use optimisers when we are putting panels on a third or fourth roof orientation. Some people want to see the individual output of their panels inverter reporting software, a feature that requires optimisers. Some want added safety on their roof with arc fault detection and instant and automatic shutdown, and that requires optimisers. We note as well that the new 5kW Huawei inverters can be expanded to 10kW of panels as long as full optimisation is carried out, and further, if the Huawei battery is installed, the STCs (Government discount) should be paid on the extra panels.


So, more reasons for optimisers than just shade.



Pictured below, Huawei DC optimiser.