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HOW DO YOU KNOW IF YOUR ROOF IS GOOD FOR SOLAR?

We love solar power, but not everyone’s house is suitable for solar power and not everyone’s roof is suitable.

So how do you know if your roof is good for solar power?

Well, there are 3 things you've got to think about, one is the angle of your roof.

The optimum angle of a roof to get the maximum amount of solar power averaged throughout the year is the same as the latitude of the location of that roof. So for example, latitude in Darwin is 12°, move south to Sydney – is about 34° and then if you get as far south as Hobart, it climbs up to 47°. So for example, in Sydney the perfect angle for solar panels is 34° from horizontal. Now you probably don’t even know what the roof angle is, why should you? If you have a house in Australia it’s probably between 18° and 22°.
“Oh no its not 34°!”, what does that mean? Well, it means almost nothing. As a rule of thumb if there is a difference of 10° that’s going to translate to a less than 5% hit to your power output. So in other words, it’s really not worth worrying about, in fact I’d goes far to say that if your roof has got any angle at all, don’t worry about it.

A good solar installer will tell you what the power hit is because you haven’t got the perfect angle.
Even in the worst case it will be a lot less than 10%. Let them work that out for you.  But as a rule of thumb, if your roof has got any angle at all just don’t worry about racking, you’ll take a small power hit if it’s not exactly the latitude but it will be very small compared to other things.

Now, the one exception to that the rule is if you’ve got flat roof.  The problem is that if it rains and you’ve got dust on the panels, it’s not going to self clean because the water is not going to slide off.  So if you’ve got a flat roof, then you probably want to consider racking, which is quite expensive – probably $60 per solar panel and if you’re going to get racking then you might as well put the panels on a perfect angle which is the latitude of wherever the roof’s located.

The next thing you should consider is the direction that your roof’s facing in the southern hemisphere, the perfect direction is north. If you haven’t got a north facing roof area or have a very small north facing roof area, then…five years ago when solar panels were 4 times the price, I would not have recommended that you put them on a east or west facing roof because they would turn what was then a kind of marginal investment into a “nuts” investment, that 15% would have really hurt you.

Five years later, solar panels are a quarter of the price, you will get about a 15% power hit compared to a north facing roof, if you go east or west, which in the scheme of things with the price of solar panels now, is not a really a big deal for most people. So if you have only got an east or west facing roof, it’s not a show stopper.

The last one, the biggie… is shading. If any shade at all falls on your roof, then the only way to quantify whether that is going to be a problem or how big of a problem it is going to be, because there will be a problem – shading is a problem, is to get a proper shade analysis done.
Now unfortunately, not many solar installers do this, but if you have shade on your roof, it really is worth finding a solar installer that will get an instrument called a “Sun Eye” that looks like this.
They go up, they stick it on the roof, they press the button, it takes a 360 degree picture of the tree line or anything that casts a shadow on your roof. It’s got a GPS in it so it knows exactly where in the world it is, it calculates any time of day, any time of year, for the whole year where the sun is going to be.

It produces a sun profile, a shading profile and it quantifies almost exactly how much shade will fall on the solar panels throughout the year. And it’ll give you a number – you’ll get a 40% hit, you get a 20% hit etc, but that is the only way to quantify it.

I’ve seen some advice on websites, terrible advice saying, “oh look on nearmaps for shade” which is a mapping website, you can look in winter, you can look in summer, you can see where the shade is, that’s bullshit! Nearmaps will give you one slice of time, you know maybe 3 o’ clock on a June afternoon, you know… move along 3 hours and the shade will be somewhere completely different. So there’s no other solution that I know of that will quantify whether shade is a problem on your roof.

So the three things you’e got to worry about when you’re considering if your roof, has potential for solar is – the angle, if it’s not the same as the latitude of where the roof’s located, it’s not perfect, but it’s probably not an issue unless you’ve got a flat roof. The next thing is the direction, ideally it will be north facing but like I said, west and east is can be pretty good these days because solar panels are a lot cheaper than they used to be. And then the shading, if you’ve got any shading on your roof at all, try and find a solar installer that will do the proper shade analysis. They’re pretty hard to find, it drives me up the wall that more of them don’t do it but you know it’s a big investment and shade will kill the output of a solar power system, so get a shading analysis system if you have shade issues.

A good solar salesman, a good solar installer will tell you what’s imperfect about your roof because it’s probably imperfect and they’ll give you a number, a percentage away from perfect that your solar power system will be. So you know, they’ll tell you that you are going to take 10% hit, a 20% hit based on your angle, your direction and your shade. You want to know that number and then you can make up your own mind as to whether it’s worth proceeding.
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