Solar: Installer visits

When we bought this house, it had a 2kW solar inverter and 8 panels on the roof - I guessed that they were 250W and thus, it was a 2kW system all up. This isn’t really big enough for us, and the system is quite old, so one of the first things I started looking into was upsizing it.

After doing a pile of research and asking around, I ended up emailing Natimuk Solar, and after a bit of back and forth via email they did a preliminary quote for me: he didn’t think he could squeeze more than 6.6kW of panels on the roof, so they did the quote for that and a 5kW inverter. Oh, and arranged pre-approval to connect to the grid with a 5kVA feed-in (effectively 5kW, for our purposes).

I was disappointed in the array size, but stoked on the feed-in (I had assumed that our notoriously-stingy power network would not enlarge the approval and I would be limited to 2kVA export). Feed-in is not something we’re optimizing for (because it’s roughly half the price of electricity we buy), and I would like to optimize for “area under the curve” to minimize the amount of time of the year we’re buying electricity, but still, I’ll take it.

Anyway, after dealing with workers out sick and a bunch of other stuff going on, he was able to make a house visit today and we actually yakked for ages (I warned him from the get-go that working from home I don’t get much opportunity to socialize, and he could leave any time and I wouldn’t be offended!). We climbed up on the roof and took some measurements, and he’s going to come up with a couple plans for me based on those.

But just spitballing, the facts are: the “old” system seems to be operating fairly well, and it’s likely not cost effective to replace it, so we will instead augment it and only remove it when it ceases operating. The outdoor unit for the split-system air conditioner in the kitchen makes things really awkward, because not only does it take up space on the best facing roof area, but the shadow it casts also takes up even more area around it, probably 6 or 8 panels worth!

So the current “plan” (informal) is to put panels on the front of the house, so facing east, on one string, have the existing system facing west, and then we’re actually thinking about putting some on the south side also, because that section of the roof is huge. In winter time, the south-facing panels will contribute almost nothing, but in the summer (when the vast majority of our current electricity use happens) that’ll add a ton of capacity to the system. In fact, just running mental math standing on the roof with our measurements, it might be a total of 10kW.

I believe the east-facing string will fall off in the mid afternoon, and the south-facing string will fall off dramatically in the winter, but for probably at least a fair portion of the year when our power usage is at it’s highest, more panels will win out.

Then later on if the kitchen AC dies or we replace it and the swamp cooler with a central system, we can clean up that side of the roof, and put more panels on there, probably doubling the size of the system, but going well above that if you’re counting “useful hours” over the entire year.

The alternative is I can tear out the kitchen AC now, and possibly relocate it, and have the better roof already. We’ll have to think on that one.

But for the south facing panels, it’s important to note that the “south” roof doesn’t entirely face south - our house is oriented slightly to north-north-west. But pretending the house faces due-south, and the roof pitch is 10 degrees (measured entirely unscientifically with Android’s camera), and using a sun position calculator with our latitude and longitude I figure a peak (best) angle of 66.6 degrees and a worst angle of 19.82 degrees (or nearly 70 degrees off the optimal 90 degree angle).

Everything that I’ve looked at (and it must be stressed that I am not an expert) shows that there’s negligible effect on efficiency up to angles of around 60 degrees off the optimal angle, after which efficiency sharply drops off. Is that true? I’ve no idea, but for the sake of not spending two months trying to validate it with my shockingly poor understanding of it, let’s pretend it’s accurate.

That’d mean that the sun’s elevation below 40 degrees in the sky is going to ruin it, but how many days of the year is that? By my calculations, about 107 days, or about 30% of the year. So by putting panels on the south roof, I figure I’m paying roughly 30% more for the same annual production as if they were on the north roof? That doesn’t seem so bad, and probably why the solar guy suggested it (he did stress it wouldn’t be optimal, but during the summer it’d be a huge help).

Will wait and see what his actual numbers suggest.

Horsham, VIC, Australia fwaggle



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Horsham, VIC, Australia

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