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

The picture below is a good example of how shade affects a solar installation.
The shade from the palm tree is just touching part of a single solar panel.

 

Before the shade hit that one panel, the string of 12 panels was making 2kW of power.

Now it's only making 1kW.


So 50% of the entire system output is lost because just one panel is partly shaded for a while.

 

In this case it's a palm tree, but on most houses there are flues, and A/C units, TV antennas, all of which have a similar effect if they shade solar panels.

 

When we design a system we do our best to avoid shade by positioning the panels away from it, and the picture above clearly shows that the installer did their best to do exactly that...but trees grow so shade that wasn't there at install time may appear later.

When we compare the outputs of installations with a great deal of shade, typically in leafy suburbs, unless the roof is blanket covered by shade most of the time, the typical losses over the course of a year are 30% compared to an unshaded system.

Whilst that seems a lot (10kWh a day lost from the usual unshaded 30kWh daily average), its usually not really such a disaster, because almost everyone is installing a solar installation that is two to three times larger than they actually need to cover their daytime power use. Very few people use more than 10kWh of power between the 'solar production hours' of 8am to 3pm. An unshaded 5kW inverter and the usual 6.6kW of panels make around 30kWh average a day.

Did you know?
Many people ask us to install panels on a West facing roof so that they can run their AC in the afternoon from solar power when the kids get back from school.
A 6.6kW install, all installed on a West facing roof, will make just 2kWh from 3pm to 7pm of the average 38kWh Summer daily output. Not a lot of people know that !!

So...before you start spending $2,000 extra on power optimisers or micro inverters everywhere, in order to squeeze more power out of your system because of shading, the reality is that if you have shade, those optimisers usually won't recover more than five units (kWh) of the lost ten units, and that as the extra power is 'surplus' to requirements then its only worth 7.135 cents per unit, or about $130 a year.

Optimisers are little black boxes (picture below) that connect to your solar panels and cost a bit under $100 each. Their job is to isolate the panel they are connected to from the others they are 'strung together' with. If shade hits the 'optimised' panel then the optimiser will make some voltage adjustments so that the flow of current going through the rest of the string is unchanged. Tigo optimisers fit to any inverter except SolarEdge and Huawei who have their own. Tigo and Huawei optimisers can be 'selectively deployed' which is a fancy way of saying that you can put them on as many or few panels as you (or your solar installer) see fit. SolarEdge designed their inverter so that an optimisers needs to be on every single panel, shaded or not. Enphase have micro inverters that do exactly the same job as optimisers, and again, like SolarEdge, one needs to be on every panel.

In our experience, if the roof is heavily shaded for a lot of the year, then it's usually not worth fitting optimisers, but if the shading is sparse but unavoidable over a few panels at certain times day of year, then fitting a few Tigo or Huawei optimisers where needed works a treat and is cost effective.

A very nice side benefit of optimisers...
If you have panels installed on, for example, your East facing roof, and then install some others running from the same inverter MPPT on the North or West facing roof then the sun is going to be hitting them with different strengths of irradiation through the day, which means voltage mis-matches all day long, and therefore a loss of power because the current will reduce to whatever the lowest level is in the string. This is exactly the same as what happens when a panel is shaded, and another benefit of optimisers. Fit the panels on the 'other' roof with optimisers and they will adjust their voltages up and down so that the current and power is maximised.

25 year warranties on optimisers?
We really don't understand why a complex box of electronics stuck up on a roof under a panel, would be reliable enough to justify a 25 year manufacturer warranty, but Huawei and SolarEdge both offer this remarkable warranty.

Panel level monitoring
As you might expect, those who invest in optimisers often want some additional evidence that they are actually working, and seeing the individual outputs for each panel is interesting, at least for a while.
SolarEdge include this reporting for free, Huawei charge $400 extra for their 'Smart Safety Box' that does panel level reporting amongst other things, and the equivalent from Tigo is closer to $800. Huawei and SolarEdge integrate the panel level reporting into their software, Tigo being a 'third party product' have their own reporting.

 

Pictured below, Huawei optimiser.

 

 

 

Pictured below, 8 panel string with 4 on North East and 4, optimised, on North West.

 

 

 

 

 

 

 


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