Posted by: @old_scientist

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The first law of thermodynamics states that energy can't be created or destroyed, so where does the excess clipped energy go when a solar array is generating 4kW on a sunny day but the inverter clips it's output to 3.6kW? Is the 400W of clipped energy lost as heat from the inverter? My inverter is in the loft so I've no idea how hot it gets on a sunny day.

Einstein!!  I think we lose it in heat in the inverter, don’t need to worry about that in the winter, solar is useless then.  I’m thinking wind might be a better option if someone comes up with a decent device 

Posted by: @old_scientist

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The first law of thermodynamics states that energy can't be created or destroyed, so where does the excess clipped energy go when a solar array is generating 4kW on a sunny day but the inverter clips it's output to 3.6kW? Is the 400W of clipped energy lost as heat from the inverter? My inverter is in the loft so I've no idea how hot it gets on a sunny day.

It's a good question and one which I have asked myself.  We really need a solar panel physicist to answer, I am a lapsed physicist but never really studied this particular specialty.

Firstly I doubt the panel does generate 4kW when clipped, because it's not correctly loaded, but for sure the solar energy does still hit the panels, so the question remains valid albeit slightly modified.

I think  that when it is clipping the inverter presents less than a matched load to the panels, so I suspect some of the energy is absorbed by the panels resulting in them heating up more than would otherwise be the case.  I could easily believe that, in addition, there is an effect on the reflectivity, but definitely don't know this to be the case. 

As you say it has to go somewhere, the above seems plausible at least .

It's worth bearing in mind that solar panel efficiency has remained stubbornly around 20% for a decade more.  This being the case some additional excess energy lost to heat is unlikely to cause anything to melt.

I understand that it gets dissipated as heat, mostly in the panels but some in the inverter. Since most solar power generates heat in the panels (80%) because they are only 20% efficient the extra heat isn’t a lot. We don’t need spurious heat though!

In the solar panels.  

On a nice sunny day, a square metre of panels would get 1kW from the sun.  Of this we might expect 200W of electical power if a suitabel MPPT inverter is connected, and then 800W as thermal power warming up the panel itself.  If you disconnect the panel, then the whole of the available 1000W will heat up the panel. 

When the inverter MPPT backs off, it just leaves some of the power in the panel.  If could choose the best MPPT point and to dissipate the excess power in itself instead - but no inverter will do this, as it would cause overheating and unreliability in a physically small inverter, while the panels are designed to be able to dissipate the thermal power.

Not that it's a good idea to leave panels in the sun unconnected - I think eventually they do get damaged this way for some reason.

Why do panels get damaged if not connected?

When they are normally in the sun, they get 800W/m^2 evenly distributed, and they warm up everywhere.  Silicon doesn't like being super hot for too long, it will eventually degrade, but it's a long term problem.

I think if you leave a panel disconnected, you can think of the extra 200W/m^2 as actually electrically generated and then dissipated by the panel itself.  The graph above shows that as temperature increases, the open circuit voltage of a PV panel drops - a positive feedback effect.  So I would expect an unconnected panel to have hotspots where the current flows, and these will degrade it faster.

@robl thank you, makes perfect sense, and my wife is relieved to know the inverter isn't sat in the loft smouldering due to excess heat because of an oversized array.

Very enlightening discussion. 

I know we will be a pain to the PV installers we will hire, but..

We are wondering what to look for panels considering temperatures etc - to install in SE England. 

Quick googling showed me this: 

The way things look, we may have G99 for the inverter with 5kw before the project execution starts. 

Since the combination of fitting plus scaffolding makes a big proportion of the price it doesn’t look as if it matters much. I’m surprised that mono-crystalline doesn’t have a price premium since the difference used to be larger. Your actual quotes might not align with the marketing presentation!

Absolutely right! 🙂 

And not all will quote once they see a hybrid inverter ready/approved. We rarely see installers actually praise what was already there.. 

I think I must query the sources on which Greenmatch have based their prices @batpred 
Did they provide source references?

One of the major UK Wholesalers will also sell to the general public.
That's Midsummer Wholesale, near Cambridge.

Look at today's price for a typical Longhi 435W mono-crystalline panel

You'd need to add VAT and carriage to that retail price.
Typical pallet-rate carriage would be around £100-£150 depending on the number of panels on it

The datasheet is on that same page.

It contains graphs of tail-off of generation over time, temperature ratings etc.

Can I suggest that you avoid buying Trina solar panels.
They have been blacklisted by MOD (for example) due to the use of Uyghur slave labour.

You'd then select a rooftop rail-mounting system, such as Fastensol.
The rails themselves are light, but come in 3.55m lengths.

The Fastensol Manual (installation guide) is available on the pages for the Roof Hooks

However, @judith  is quite correct to point out that the two main costs are:

• scaffolding
• labour

and those are completely dependent on your roof design & access.

Posted by: @batpred

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not all will quote once they see a hybrid inverter ready/approved.

It all rather depends on whether you need to end up with an MCS installation number.
You'd need that if you ever intend applying for the Smart Export Guarantee (SEG) to earn income from exporting to the grid.

If the SEG is irrelevant because your inverter isn't going to export, then you don't need an "MCS approved" installer.

You either do it yourself or employ a local roofer.

A solar-panel installation company will also want to have a say in what goes on the end of the wires.

Some companies will insist on an enclosure (a "string combiner") which contains isolators, fuses and anti-lightning suppression.
Others will check what's already offered within the inverter, and be happy to terminate wires with MC4 connectors for direct connection.

Whoever does the work, there needs to be care taken over EARTHING

At some point in the lifetime of the panels your house will be in the path of a thunderstorm.
Even if there is no direct lightning strike, an earth-path will move electrons (or "positive holes") up to the panels in order to counter the electromagnetic charge in the clouds.

Most installers do not earth the electrical connections to the panels.
Some might insist that the roofing rails are separately earthed.

Each location is different.

Earthing needs to take into account the mains supply to the property (PME supplies have earth tied to neutral by the DNO)
and what earths have been installed for any EV chargers.

Ignoring earthing is not an option!