×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Are you an
Engineering professional?
Join Eng-Tips Forums!
• Talk With Other Members
• Be Notified Of Responses
• Keyword Search
Favorite Forums
• Automated Signatures
• Best Of All, It's Free!

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

#### Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

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

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

(OP)
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.

### RE: Solar Panels on roof's do they actually bother checking before Installing?

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.

### RE: Solar Panels on roof's do they actually bother checking before Installing?

(OP)
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.

### RE: Solar Panels on roof's do they actually bother checking before Installing?

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).

### RE: Solar Panels on roof's do they actually bother checking before Installing?

(OP)
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.

### RE: Solar Panels on roof's do they actually bother checking before Installing?

2

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

### RE: Solar Panels on roof's do they actually bother checking before Installing?

(OP)
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.

### RE: Solar Panels on roof's do they actually bother checking before Installing?

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.

### RE: Solar Panels on roof's do they actually bother checking before Installing?

(OP)
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.

### RE: Solar Panels on roof's do they actually bother checking before Installing?

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.

### RE: Solar Panels on roof's do they actually bother checking before Installing?

(OP)

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.

### RE: Solar Panels on roof's do they actually bother checking before Installing?

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 ### RE: Solar Panels on roof's do they actually bother checking before Installing? My house has about 24m2of 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. ### RE: Solar Panels on roof's do they actually bother checking before Installing? (OP) 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. ### RE: Solar Panels on roof's do they actually bother checking before Installing? 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.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg
FAQ731-376: Eng-Tips.com Forum Policies forum1529: Translation Assistance for Engineers Entire Forum list http://www.eng-tips.com/forumlist.cfm

### RE: Solar Panels on roof's do they actually bother checking before Installing?

(OP)
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.

### RE: Solar Panels on roof's do they actually bother checking before Installing?

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.

### RE: Solar Panels on roof's do they actually bother checking before Installing?

#### Quote (phamENG)

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.

### RE: Solar Panels on roof's do they actually bother checking before Installing?

(OP)
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.

### RE: Solar Panels on roof's do they actually bother checking before Installing?

(OP)
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 :D

### RE: Solar Panels on roof's do they actually bother checking before Installing?

Alistair. Do they worry about the starting PF or the running PF.
1. Capacitors on the AC motor.
2. Boosting the voltage on the inverter when the PF drops.
That's how we pick up VARs on a paralleled generator.
We use a cross compensation system that monitors the reactive current from both paralleled generators and tweaks the voltage set points so that the generators will share VARs properly.
Rather than a lot of gear to monitor the VARs you may be able to use a simple circuit that adds a predetermined bias to the voltage set point when the AC is running.

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

### RE: Solar Panels on roof's do they actually bother checking before Installing?

Inverters these days (well, at least the commercial size ones, not sure about residential) have the ability to maintain power factor at a specific location.
Indeed, there are now requirements for sites over a certain size here to specifically set inverters up to alter VAr output depending on system voltage, and they're also capable of power factor management in addition to real power output, although they're obviously still VA output devices.

### RE: Solar Panels on roof's do they actually bother checking before Installing?

(OP)
The inverter can be set for between 0.9 either side. Mine is set at 1 and I can't change it as an owner. But it's a fixed output.

The grid contract has various levels of allowable power factor fluctuations. And the grid smart meter will cut you off if you exceed those limits.

Per say the pump is fine if it's the only thing coming online. But with the addition of an induction hob with 7.5 kW output, and the local female belief that pans heat quicker if you select max to start with. Education does not stop them. I can pull the PF quiet low. Luckily the heating controller has a locked out thermostat with a timed boost feature so they can't screw up the under floor heating. If you combine the two at the same time it gets near the 0.5s limit. Once the compressor is online it's fine. I stuck an extra fan coil on the cooling circuit so at 25 Deg house temp with a 20 degree target there is enough output that it's more than the heat pumps min cooling output. Which has stopped the short cycling. The heat pump companies solution was to stick a 500ltr buffer tank which wasn't an acceptable solution for me.

The grid guy wasn't bothered after he found out it wasn't due to some old Soviet community workshop gear being used. I suspect he changed the settings on the meter and increased the window up to 1 second. But would prefer to get it fixed properly.

All it needs to fix it is for the inverter to come off line in side 0.5 seconds and let the grid take the full load. Then come back online when it's back to running. The book of is 0.7 during start up I think. Then it's above 0.9 while running.

It was fine until I put the induction hob in.

### RE: Solar Panels on roof's do they actually bother checking before Installing?

#### Quote (Alistair)

It was fine until I put the induction hob in.
Interpose a timer between the thermostat and the A/C.
When the thermostat demands cooling, drop out the induction hob for about 10 seconds while the A/C starts.
You may have to add a contactor to the hob.
Wiring:
The hob is energized by normally closed contacts.
When the thermostat energizes the relay, the contacts open, dropping out the hob.
A set of instantaneous contacts starts the A/C.
A set of normally open timed contacts re-energizes the hob about 10 seconds later.

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

### RE: Solar Panels on roof's do they actually bother checking before Installing?

(OP)

Its not traditional on/off thermostat system either.

Its one of these ground loop heatpump outside temperature compensated controllers. The idea is it works out the required heat input or extraction to maintain the target level and to be at it when you have specified the time to be at it.

In cooling mode it alters the loop water temp to maintain the temp when you have enough output so the evaporator doesn't freeze up. So the min is 6 deg C but if you have the temp at target it will raise the feed temp until the temp stays constant. It has a delta E value that controls when it switches on and turns off the compressor. IF your within that delta E it leaves the compressor running all the time and only alters the expansion valve to vary the power output.

Your temp can be reading 2 degs C below your target temp for the area but the pump will fire up because the outside temp has increased enough that your delta E is over threshold. The fancoils will crank up but it won't blow freezing cold. half to an hour later the temp will be at the target temp and the temp of air coming from the fanicoils will be markably cooler but still not freezing. if you have the cooling turned off and then come in and turn it on then it goes into max extraction mode and belts out air at sub 10 degs until it gets to inside its delta E then it ramps the feed temp up again so you just have a flow of air at just the right temp to maintain the temp required.

Persay the fancoil controllers are set at min temp in summer and max temp in winter so they run all the time the heat pump needs them to be on. They get powered off a relay which is switched by the circulation pump coming on. When the heatpump controller shuts that zone down they power off. Which it does when the outside temp lowers and it works out you don't need any more extraction.

After I realised what was going on with the short cycling in August and fitted another fancoil It went from restarting every 45 mins down to one start a day.

I will look at the manuals for the heat pump it may have a compressor on 220V switched output. If it has that I can feed that to a timed relay which kills the inverter grid connection for 10 seconds or the Inverter DC strings.

Plus also it does the hot water as well. So will fire up to heat that as well completely independently of the heating zones. But I suppose I could have a mechanical timer for that as well.

### RE: Solar Panels on roof's do they actually bother checking before Installing?

Hi Alistair;
Insert the circuit that I described into the circuit to the contactor that starts the A/C compressor.
It may delay the start of the A/C by some tens of milli-seconds.
If the inspector and the smart meter are happy, let it go.
If it becomes a problem, there is a solution.
There is a possibility of a circuit race.
If you have to make a change, best to post a circuit diagram.

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

### RE: Solar Panels on roof's do they actually bother checking before Installing?

(OP)
There is no contactor for the compressor. The whole thing runs on a 220V ebus data link. There is a heat exchanger ventilation system in the mix as well that also has a CO2 sensor inputting into the mix. If it ups the airflow it compensates by adding/subtracting more energy.

If the solar injection at PF 1 goes up ie it is vastly more than the compressor pulls does that help or make the PF worse?

I am not going to make changes to the current setup. Now its not short cycling because the evaporator freezes 14 times a day its not such a worry.

### RE: Solar Panels on roof's do they actually bother checking before Installing?

Power factor.
In an inductive circuit we have Watts doing useful work.
We have Volt-Amps-Reactive or VARs at 90 electrical degrees to the Watts and which do no useful work.
And we also have the product of the Volts and the Amps or VAs which is the result of the combination of the Watts and the VARs.
The power factor is the ration of the Watts over the Volt Amps, often expressed as a percentage.
When your A/C is drawing VARs from the grid the PF is the Vars divided by the Volt-Amps.
As you produce more Watts from your solar system, the Watts and the Volt-Amps drawn from the grid both reduce. The VARs stays the same.
As the Watts drop, the Volt-Amps drop and the PF percentage drops.
On a sunny day you may drop the PF all the way to zero.
In the past, the PF was determined by the monthly ration of kilo-Watt-hours to kilo-Volt-Amp-Reactive-hours and short excursions were averaged out.
With a smart meter responding to the instantaneous or near instantaneous PF you have a problem.

#### Quote:

I suspect he changed the settings on the meter and increased the window up to 1 second.
One second/one month = almost instantaneous.

#### Quote:

If the solar injection at PF 1 goes up ie it is vastly more than the compressor pulls does that help or make the PF worse?
Short answer: More solar contribution will make the PF at the grid meter worse.

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

### RE: Solar Panels on roof's do they actually bother checking before Installing?

You could just install some power factor correction capacitors.

Just remember they are there if you kill the main to do work you need to isolate the capacitors too or you could get a shock.

### RE: Solar Panels on roof's do they actually bother checking before Installing?

The inverter simply needs to hold PF at the meter rather than holding PF at the inverter terminals. Don't know that specific inverter, but within the current limits of the power electronics they can provide what ever power factor is desired.

### RE: Solar Panels on roof's do they actually bother checking before Installing?

(OP)
"Short answer: More solar contribution will make the PF at the grid meter worse. "

That was my thought as well thinking back to imaginary numbers and vector diagrams from 30 years ago now.....

I am thinking about going from a 8.5kw farm up to 21.5kw. With another 2 inverters in two more buildings.

"The inverter simply needs to hold PF at the meter rather than holding PF at the inverter terminals. Don't know that specific inverter, but within the current limits of the power electronics they can provide what ever power factor is desired."

That's what I want. There is an installer only menu with these options which came out with the last firmware update... maybe my question had an effect. The last update you had the option on putting a set value in only.

I am 100% certain though that the installation company will have zero clue about it. Just like they had zero clue about putting a battery in. They didn't want me to have a energy meter in the first place.

### RE: Solar Panels on roof's do they actually bother checking before Installing?

#### Quote (davidbeach)

The inverter simply needs to hold PF at the meter rather than holding PF at the inverter terminals. Don't know that specific inverter, but within the current limits of the power electronics they can provide what ever power factor is desired.

#### Quote (Alistair_Heaton)

That's what I want. There is an installer only menu with these options which came out with the last firmware update... maybe my question had an effect. The last update you had the option on putting a set value in only.

In order for the inverter to do this it needs to firstly know what the power factor is at the meter, it can obviously work out what it is at the inverter. Inverter OEMs will generally have their own product that suits their inverter for this purpose (as well as for zero export limiting).

### RE: Solar Panels on roof's do they actually bother checking before Installing?

(OP)
Its got one of these as its energy meter. Which is the OEM approved and supported meter.

https://www.tq-group.com/en/products/tq-automation...

Its fitted just after the main CB and everything goes through it both ways.

### RE: Solar Panels on roof's do they actually bother checking before Installing?

AH,
This topic seems to have moved past the mass per panel subject you started with, and I'm late to the party. I just want to pipe up and say that I have a couple of panels that are a bit bigger than 1m^2 which I use for various projects/experiments. I carry them around by hand comfortably. They certainly don't weigh anything close to 100kg each... more like 20kg.

### RE: Solar Panels on roof's do they actually bother checking before Installing?

(OP)
We sorted that out I screwed up my decimal points when moving from kN/m2 loading to mass. Its 11kg/m2 dead load without the frame and wires. Same with the other values. Sitting waiting for the Estonian log cabin expert to turn up to survey the barn to see what can be recycled and what has to be burned. A new roof is a certainty but I have zero clue about slotted logs pinned together with wooden pegs for load bear/shear bearing. The brother in law reckons quarter of a ton of moss from the forest is going to be involved.

### RE: Solar Panels on roof's do they actually bother checking before Installing?

(OP)
As expected 50% of the logs are condemned to fire wood...…

#### Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

#### Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

#### Resources

eBook - Manufacturing the Cars of Tomorrow
In this ebook, we'll explore how additive manufacturing is going to transform the way cars are made. This includes commentary from thought leaders such as Ford's CTO, Ken Washington, Customer case studies of ways 3D printing is being used today, and a variety of part examples where 3D printing is already impacting how automobiles are made. Download Now
White Paper - Smart Manufacturing for Semiconductor
New technologies and approaches present great opportunities for semiconductor manufacturers to achieve high levels of innovation, yield and improvement. This white paper explores some of these cutting-edge technologies and how they can be applied effectively in the semiconductor industry. Read about how Smart Manufacturing is transforming the semiconductor industry. Download Now
White Paper - Analysis and Simulation in Aircraft Structure Certification
Organizations using simulation and analysis tools effectively see the benefits in their ability to achieve certification faster and with drastically less total cost than those who do not maximize these tools. Read this White Paper to learn about how digital tools such as analysis and simulation help in aircraft structure certification. Download Now

Close Box

# Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

• Talk To Other Members
• Notification Of Responses To Questions
• Favorite Forums One Click Access
• Keyword Search Of All Posts, And More...

Register now while it's still free!