Solar Panels on roof's do they actually bother checking before Installing? 2

[Alistair_Heaton]

I have already 39 panels on my roof and they come in at 110kg per m2.

Its a new roof and I did some quick and ruff calc's with a wind load of 25kg per m2, snow 85kg per m2, suction of 45kg per m2.

600mm spacing between trusses so pretty standard.

My roof comes in at 1.6 safety factor. So I didn't bother saying anything.

Now just went round to a house with a roof with 1000mm spacing...… because they are thinking about getting them. Quiet why they have that spacing I have zero clue.

The solar installers are saying its good to go and no problems hand the cash over..…

To me they all come across as more Electrical inclined than Structural in the Solar game. And to be honest more fuse box whallah's than Electrical Engineers.

I will admit I have never done house roofs so have no real feel for what's a normal safety factor. But if mine comes in at 1.6 with 600mm spacing its a bit dodgy in my book with 1000mm.

Just wondering if anyone else has seen this throw them up without checking for solar panels.

Also wondering if the slot up the back of the rack and panels can turn the whole roof into a slated aerofoil.

 

[LittleInch]

Might be better to post this in the structural engineering forum. They have many similar posts about panels in florida where they get rather strong winds on a regular basis...

This isn't really an engineering disaster or failure just yet.

But 39 panels - WOW. That's one heck of a roof area.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Alistair_Heaton]

oops meant 31 panels or 9.61 kWp onto a 8.5kW inverter. Could have got 36 on though if it wasn't for the roof window.

produced 9500 kWh last year and we sell to the grid. Its giving about 7% capital return after home consumption has been taken off.

Its on my escape from aircraft country retreat. Basically 3 hours drive from any airport and no airways overhead so I can do my astrophotography and drink beer in peace without dispatch appealing to my wallet offering overtime.

 

[VEBill]

If the panels are installed parallel to the roof surface, then some of those load cases would be, very roughly, essentially unchanged (with or without panels).

 

[Alistair_Heaton]

Its a pitch roof at 42 deg.

But even a flat roof having an additional 110kg/m2 stuck on top of the 85kg/m2 snow load is not insignificant.

They don't do flat roofs round here. Most roofs are between 38 and 45 deg pitch.

The older roofs are cleaned tree's trunks for the diagonal members. Then smaller trunks for the lateral ties between trusses, then branches laterally to attach bark shingles to. Then usually they have a load of asbestos plates screwed ontop of that 20 years ago to make it more water proof.

They were put on in Soviet times. The whole area is awash with pretty horrible disintegrating soviet asbestos panels. I have 5 tons of them to get rid of. As it costs 1100 euro a ton to get shot of them most of the locals hire a digger for 200 euro a day and dig a hole and bury them on the property.

New roofs are normal building trusses in shape but quiet often DIY'd on site, membrane fibre board, batons and then metal sheets of what ever profile takes your fancy or these none asbestos fibre sheets. Its just there doesn't seem to be any standard spacing and the sensible option of doing them the same width as roof insulation doesn't carry. My roof comes from Finland thankfully.

 

[VEBill]

"...additional 110kg/m2..."

Why do your solar panels weigh as much as completely solid cast iron panels 1.5cm thick?

 

[Alistair_Heaton]

There is a big panel of tempered glass in them plus rack under them

Your right though I moved a decimal point when I looked at my calcs using 0.11 kNn/m2 its 11 kg/m2.

Still 13% increase in the dead load case. I am presuming they usually use 1.8 safety factor to get me down to 1.6.

My only excuse is pretending to be an electrician and chatting on forums when ever I walk past the laptop.

 

[VEBill]

For some reason, my brain picks out suspicious numbers.

Circa 1989, I was looking over the shoulder of the Structural Engineer that was using a CAD terminal (vector graphics CRT) to design the rather large (36 foot clear span, 28 inch deep) parallel chord roof trusses for my house.

I asked about the "0.99" being displayed in one field (I already suspected what it was, because of its value). He told me it was the "100 Year" snow load as compared to the design strength, presumably with some margin.

Thinking quickly to get the direction correct, I asked how much to make it 0.7 instead. "$238.". Per truss? "No, for the entire house." Yes please.

Roofers have actually commented how the roof is very stable, and "...feels like a floor."

---

I've been ordering 100w Cdn$100 solar panels. Seven (7) so far. Generating zero energy as they're still in the basement.

Eventually I'll have a kilowatt or two, wired up for bulk heat.

 

[Alistair_Heaton]

Well that's the sort of drift I get over this "bollocks"

If you tackle it now its a months beer tokens to get a roof that's bullet proof for the next 40 years.....

And to be honest I don't trust architects to do structural stuff. Foundations yes....

I am well up to speed these days with my inverter and battery and wiring. Its a complete mind screw though for the fuse box whallahs in my house, the concept that internally we can have 30.5 Kw through a 32 amp mains fuse confuses the hell out of them.

And I have fulfilled a life dream and will get a proper decent 3 ph TIG welding unit which ain't shy on the amps even with 400V 3ph.

 

[Tomfh]

We use a general allowance of 20kg/m2 for panels. When they supply us with the data sheet for the panels they always seem to come in somewhere between 10-20.

It can make a difference to a roof design. For existing roof sometimes people cheat and include it as live load, which in Australia is usually 25 kg/m2. It’s fair enough I guess, as you’re not going to put other things on top of the panels.

No doubt many roofs aren’t checked at all.




Wind is often critical too, in which case a bit of ballast helps.

 

[Alistair_Heaton]

This is the current roof that's on the building I want to mount them on.

It has asbestos plates on top of the shingles. And then snow on top. Its got quiet a steep pitch so it falls off with out much build up currently. Still don't fancy its chances of surviving much longer.

 

[waross]

I was considering buying a new modular home.
I talked to several manufacturers about the possibility of adding solar panels to the roof.
All of the manufacturers had a solar option.
Stronger roof? Certified for solar? Instructions as to how best to locate and fasten the panels?
NOPE.
For about $100 they will preinstall a plastic conduit through the roof.
That's it. One piece of plastic pipe.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[VEBill]

My house has about 24m[sup]2[/sup]of glass facing essentially south (for passive solar gain), shaded by the roof overhang in summer (when the Sun is higher in the sky); so it comes with a roof that is naturally facing essentially south.

The south facing roof has 24-inch deep parallel chord roof trusses spanning only 16-feet, so you could probably park a car on that section of roof.

But I'm not too sure that I want panels on the roof; I'd rather have them on the ground where I can walk over and do maintenance on them.

Have you considered just putting the solar array on the ground? If you have an open area, then the shading might not be an issue.

Seems simpler overall.

 

[Alistair_Heaton]

Yes I did look at it, but there are issues with that with site security so none of the local clueless can electrocute themselves. You also have to carry third party liability insurance.

If you stick them on the roof then your house insurance covers it all with no hit to your policy. You can put them on stands up in the air so the bottom edge is 2 meters off the ground but you still need third party liability.

You would also need to bury the AC cable which would have to be armoured under ground cable to the inverter.

They do have on ground in country but they tend to be large commercial installations. You can do up to 22kW grid feed in relatively easily above that getting the grid connection gets highly expensive.

I have roof space for 2 x 8.5kW and a 4.2 on the sauna topping out at 21.2 kw. If I was to go over that it would be 6000 euro to go up to a 40 amp fuse on the supply from 32amp.

Plus it would be a PIA to cut the grass.

 

[IRstuff]

21.2 kW? Yikes, you running a grow house? I thought getting 6 kW was doing pretty good, and that was only to cover a $250/month elex bill during the summer, when I was hitting 900 kWh.

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[Alistair_Heaton]

We have guaranteed grid feed in money for 15 years. Works out the pay back time is 9 years on the installation including a inverter replacements after 5 years. After 9 years its 11% return on capital.

The electricity price has gone up 13% in the last 3 years and the feed money is index linked to that.

 

[phamENG]

As I understand it, our local power company got the state to nix that requirement. Now you can't get paid for producing extra power. Otherwise I would be putting them on my roof. I don't have enough room to produce all our needs at peak demand (or the sun, this time of year), and without the offset while the house is empty all day the economy disappears pretty quickly for me.

 

[Tomfh]

As I understand it, our local power company got the state to nix that requirement.

Yeah, it'll happen more and more as renewable energy penetrates further.

Here in Australia the state of South Australia has a lot of solar and wind power. They're a relatively small state that's hooked into the main grid (which is primarily coal powered), and thus they can dump their excess solar and wind power fairly easily, and draw from the grids coal power when the renewables aren't delivering. But last month the interconnector to the main grid was damaged in a storm, and they had to go it alone with their renewables and their backup gas generation, which was a very precarious situation. The energy market operators are now looking at ways to reign in the solar and wind generation during times of renewable overgeneration, both by commercial controls over wind and solar generators, and also controls over rooftop residential solar.

 

[Alistair_Heaton]

I have a Kostal penticore hybrid inverter which is made and designed in Germany.

It is already setup and can take all the German control boxes. You can limit your input to a % of production or fit a thing called a ripple controller which the grid can then control. The battery stuff can also be used to store your over production so when the ripple controller kicks in then it starts loading up the battery.

There is a also a software defined switch which then turns on or off a 2A capacity switch which you can use to control relays to activate car charging or hot water production etc. Not sure if that can be controlled by the ripple controller which gets its information via the internet or ip over powerline. But you have a energy meter just after your main cb and it monitors that and does various things depending on what the grid tells the box.

Currently there is a contract for injection with index linked price per kWh above the market rate which lasts 15 years which is in national law. I shall see what happens. The ability to run aircon 12H during the summer at a really comfy level and have the hot water on all day. Leave a 3kW pool heater on etc. And when we get an electric car which will happen at some point in the next 15 years we can charge that as well.

But I can see restrictions for new sites coming in, in the not so distant future. But hopefully us end of line domestric producers with few other producers in the area will get left alone even after the 15 years.

 

[Alistair_Heaton]

As a side note the problem I am having is to many large inductors in the house. Induction hob and 8.5kw heatpump plus all the led drivers. The power factor goes to poo from the grid when the heatpump compressor starts up and I am producing at the same time. The grid has to cover all the reactive power and the inverter covers all the real...

I have spoken to Kostal and the energy meter does record power factor. So they are thinking about adding a feature that balances the power factor ie if the compressor kicks in and the power factor goes to shit it will cut the inverter output until it goes back to something sensible and then start producing again.

I haven't triggered the automatic cut off yet for dodgy power factor but its been close a couple of times apparently, enough to warrant a visit from the grid bloke. Who to be fair was very pleasant about it all. Apparently modern heat pump and hob is an acceptable reason. Some dodgy old large motors while running a grey market machine shop in a domestic property are not.

During the winter heating season its not a problem because we don't produce much anyway Summer when the cooling turns on it is a different story... Just have to leave it turned on