Shade



A typical system of 18 x 370W (6.66kW) solar panels make an average of 30kWh per day
on an unshaded normally sloped roof in Perth

That's an average contribution of 1.67kWh per panel per day.
That 'day' is typically 8am to 4pm. Nothing much gets produced outside those hours.
So, like us, solar panels work an 8 hour day.

Most solar installs run two 'strings' of panels back to the inverter.
Generally speaking each 'string' can have a minimum of 4 and a maximum of 13 panels connected.
In this example we'll use two strings of 9 panels.

One of those panels gets shaded by a flue, tree, etc, between 9am and 11am.
If the shade is bad enough, the panel's built-in diodes will activate and that panel is taken out.
Now you have 8 panels working on the string, and 1 panel bypassed.
It's not dragging the other panels down, it's simply not contributing.
At the end of the day, you lost 2 hours out of 8, so 25% of 1.67kWh or 0.4kWh.
If that happened every day then you would have lost 146kWh by the end of the year
worth between $4 and $42 depending on how much of your solar you 'self-consume' or export.

There's no need to spend any money 'fixing' this particular problem. It's already fixed by the diodes.
If there was nowhere else for that panel to go then you've got a 75% efficient panel. Better than none.

The REAL problem is when shade isn't strong enough to activate the bypass diodes.

Do you have a heavy or light shading scenario on the panel?
Solution, buy an optimiser anyway. That's what almost everyone else we consult with does.

A bit of partial shade from a TV antenna for example. Those things are quite tall and can reach several panels.

Partial shading of a panel may not activate any of the bypass diodes.
The panel isn't able to produce as much electrical current.
Let's say that the shading makes it produce 25% less current.
That makes the other 8 panels on the string produce 25% less current as well.

This is a perfect case to buy an optimiser for about $90.
The optimiser plugs into the shaded panel and continuously measures the current flowing through the string.
If the optimiser detects that the rest of the string is making more current than the shaded panel it is attached to,
it adjusts the voltage of the panel down, which makes the current go up, to match the rest of the string.

So, the slightly shaded panel makes 25% less power, but the rest of the string carries on as normal.

The BIG dilemma. The one you were thinking about when you read this.

Is the shading bad enough to activate the bypass diode?
Usually very hard to say which is why the advice is...
If in doubt install a $90 optimiser.

I often get asked what to do about 'heavy shading'.
Trees all around shading large sections of panels for large amounts of the day.
I have to check each case individually, but often the answer is 'suck it up'.
OK, not quite as impolite as that, but the meaning is the same. Do nothing.

If the shade is so strong and universal, then optimisers, micro inverters will achieve next to nothing.
Those people should either not get solar at all, or accept that their 6.66kW system will make the same power as a 3kW system.

That last line is actually very important.
"Make the same power as a 3kW system'
Why do we all get such huge solar installs, making 30kWh per day?

It made sense when we were paid 47 cents for surplus power, it even made sense when the payment was 7 cents, but now it's just 3 cents?

If you took a normal house, where they average 10kWh per day of solar 'self-consumption' they are using 10kWh and exporting 20 kWh.
20kWh @3 cents * 365 days a year = $219. I don't know many people who get too excited about that amount of money.

So, the person with a heavily shaded roof who only gets half the output, let's say 15kWh is really only losing out on 15kWh of 3 cent exports per day,
which comes to just $164 a year. A disastrous loss a few years ago of 47 cent feed in tariff is almost nothing now.

Now, it's a different story when battery economics line up. they are getting there, and if we end up with TOU (time of use)
pricing and peak rate prices are 60 cents as forecast, then batteries will definitely make economic sense, and the person
with all that shade will not have enough surplus to charge up a battery.

If you have a Huawei inverter then you should buy a Huawei optimiser. The new L1 and M1 Huawei inverters have in-built panel level
reporting so you can see in the reports what each optimised panel is doing..even if you only have one.

If you have any other type of inverter apart from SolarEdge and Enphase, you can get a Tigo optimiser.
Tigo also have a Gateway that costs a lot extra but allows individual reporting of optimised panels too.
If you have a SolarEdge, then all your panels have optimisers fitted already because that's how that system works.
If you are considering micro inverters instead of the usual 'string' inverter, then this is effectively an optimiser
on every panel too.


If you want a quote for solar, with or without optimisers, then call us on...

(08) 6102 2527


or email



This review was written by Andrew MacKeith, Solar4Ever service manager since 2011.
Most recent update April 2021