Solar newbie...

[Crofter]

OK I'm not a total newbie, I've designed and built a couple of off grid systems. But this is different.

 

I've got a small all-electric newly built property which needs essentially no heating at all through the summer months. There's a 200l direct UVC in there which currently heats up from an E7 tariff.

I'm considering installing a PV system with a diverter, primarily aimed at covering that DHW demand.

I'm going to assume that I need about 8kwh/day to heat the tank (incoming 10°, outgoing 60°, two seven minute showers at 10l/min).

As it's a holiday let, it's primarily occupied from April-September inclusive. So I'm looking to design a system that can deliver close to 8kwh/day over that period.

 

What I've come up with so far is:

6x 500w panels (£450)

3kw inverter (£300)

Diverter (£300)

Plus obviously mounting hardware, cabling, isolators, fuses, etc. So I should be looking at not too much north of £1k, plus any labour and fees.

 

I'm considering getting it signed off by a qualified but non MCS sparky. I believe I still need DNO approval though?

 

If I go down this route, would there be anything wrong with using secondhand gear? 

 

The panels would probably be either ground mounted, or on a south facing gable (or possibly a combination of both). Playing around with PVGIS, I've learned that my optimal angle is 39° for total annual output, and 74° for the best winter production. Somewhere in-between would give me the greatest number of months which hit my target of 240kwh. All academic if I end up putting panels on the gable, of course.

I could think about roof mount as well, but the roof is E-W. It's corrugated steel over timber battens.

 

Anyway like I say, I'm new to this, I might be overlooking something, or overthinking things. Feedback and suggestions would be much appreciated.

[Dillsue]

With a 3kw G98 certified inverter you don't need to ask permission from your DNO, just tell them afterwards, but......

 

Maybe I'm thinking of someone else but is this Buildings supply piggy backed off another buildings supply?? That might impact DNO permissions.

 

Nothing wrong with secondhand gear but you cant legitimately use an old inverter in a new installation without the inverter complying with current standards.

[ProDave]

For your DNO sign off, with a G98 inverter you will need to fill in a form and provide some drawings including location of equipment and a wiring diagram.  as well as an Electrical Installation Certificate from the sparky.  Make sure the sparky can provide the wiring diagram as well as the EIC.

 

for G98 you notify the DNO afterwards.

 

The G98 application is linked to your electricity meter's MPAN number.  If as hinted both properties share a single supply and meter, you will only be allowed one G98 application.  If you want solar PV for both, you might have to get a second supply installed so each building has their own supply and meter.

 

If buying secong hand inverters, make SURE they are not so old that they pre date the present G98/G99/G100 types.

[Crofter]

Thanks both.

The property has a completely separate supply with an E7 meter.

 

Random example of a secondhand inverter (hope this isn't in breach of any forum rules, I have no idea who the seller is)

https://www.ebay.co.uk/itm/306596178881?_skw=grid+tied+inverter&itmmeta=01KABNM23RJZP4ZYC7R6XJWNQR&hash=item47628e6bc1%3Ag%3Alq0AAeSwWNRpG1Si&itmprp=enc%3AAQAKAAAA8FkggFvd1GGDu0w3yXCmi1eDtXN4tIIR6UuOxc%2FS0lIttlUJ0Z7JI8%2BWN6xaqUGAYWWoFtcYpBGviBiu9ELNmvH7YU6aU%2BaBBT3OZ%2FmvQ9XFTDopJQNPu%2FXvJL4716s1gHyn4Jyz6PFu8rGn20ohVw77ZkKLTwxBZt5kHTHuWr7Qh%2FcgOvxjp1dsXSnEoQCVFTtCUgMFnngx0DlpPVO%2FicepK6nrX7HcO6uE76fYuBhxxJttvCDWjrPDtAPGSw%2FcIsMhSKztfxfv0y4X0m4sgwtWgEPTaAC1c37eP83GP%2B0D4lF3%2BWqs%2FbFCZ9nR67Etlw%3D%3D|tkp%3ABk9SR5ai0PXSZg&LH_BIN=1

 

Likewise the diverters seem to go for about half price secondhand.

[JohnMo]

2 hours ago, Crofter said:

would there be anything wrong with using secondhand gear

Nothing be careful with price, current adverts look more expensive than new for panels.

 

I have a fully second hand system and bought a inverter the same as the the one above (just slightly bigger) second hand, no issues.

 

cool energy do a diverter, it will drive two immersions one after the other. Other brand tags for the same item, selling lots cheaper, so do an image search - may have to wait for it come from China.

[ProDave]

36 minutes ago, Crofter said:

Thanks both.

The property has a completely separate supply with an E7 meter.

 

Random example of a secondhand inverter (hope this isn't in breach of any forum rules, I have no idea who the seller is)

https://www.ebay.co.uk/itm/306596178881?_skw=grid+tied+inverter&itmmeta=01KABNM23RJZP4ZYC7R6XJWNQR&hash=item47628e6bc1%3Ag%3Alq0AAeSwWNRpG1Si&itmprp=enc%3AAQAKAAAA8FkggFvd1GGDu0w3yXCmi1eDtXN4tIIR6UuOxc%2FS0lIttlUJ0Z7JI8%2BWN6xaqUGAYWWoFtcYpBGviBiu9ELNmvH7YU6aU%2BaBBT3OZ%2FmvQ9XFTDopJQNPu%2FXvJL4716s1gHyn4Jyz6PFu8rGn20ohVw77ZkKLTwxBZt5kHTHuWr7Qh%2FcgOvxjp1dsXSnEoQCVFTtCUgMFnngx0DlpPVO%2FicepK6nrX7HcO6uE76fYuBhxxJttvCDWjrPDtAPGSw%2FcIsMhSKztfxfv0y4X0m4sgwtWgEPTaAC1c37eP83GP%2B0D4lF3%2BWqs%2FbFCZ9nR67Etlw%3D%3D|tkp%3ABk9SR5ai0PXSZg&LH_BIN=1

 

Likewise the diverters seem to go for about half price secondhand.

Check with the seller it is G98 approved,  It should be at only 2 years old.  Susprised they don't state that,

 

Only 1 input so all panels need to be on one string.  2 inputs is better if you want panels on different facing roof elevations etc.

[Crofter]

25 minutes ago, JohnMo said:

Nothing be careful with price, current adverts look more expensive than new for panels.

 

I have a fully second hand system and bought a inverter the same as the the one above (just slightly bigger) second hand, no issues.

 

cool energy do a diverter, it will drive two immersions one after the other. Other brand tags for the same item, selling lots cheaper, so do an image search - may have to wait for it come from China.

I doubt I'd bother with secondhand panels, they're the cheap bit after all.

[Crofter]

7 minutes ago, ProDave said:

Check with the seller it is G98 approved,  It should be at only 2 years old.  Susprised they don't state that,

 

Only 1 input so all panels need to be on one string.  2 inputs is better if you want panels on different facing roof elevations etc.

Good point. I'm playing with PVGIS and wondering about putting two panels on each aspect of the roof. I hadn't considered that this could complicate things.

I've heard of micro inverters, would those be useful in this situation?

Or, can I just run panels in parallel? On my boat I have always done that, but that's a smaller system.

[ProDave]

You wire the panels in series.  I have 8 panels on each string and 2 strings, one facing east the other facing west.  With only 4 panels that may not work as each string may not achieve the minimum input voltage.

 

What is limiting you to 3kW?  the default domestic install is 3.68kW (the max without prior approval under G98)  That is probably where there is most choice.

[Crofter]

15 minutes ago, ProDave said:

You wire the panels in series.  I have 8 panels on each string and 2 strings, one facing east the other facing west.  With only 4 panels that may not work as each string may not achieve the minimum input voltage.

 

What is limiting you to 3kW?  the default domestic install is 3.68kW (the max without prior approval under G98)  That is probably where there is most choice.

So I looked up that Solis and it's got a 60v startup. I'm used to boat stuff which will kick in at much lower voltages. So 60v means I need to put two panels in series to get above startup.

Presumably that means I could have a pair of panels on each elevation, but I'd ideally want separate inverters for them?

Nothing is limiting me to 3kw, I'm just trying to keep this system small, simple, and low cost. Anything above what's needed to heat the UVC is likely just going to go to waste. As a holiday house, it is unoccupied throughout much of the day, and it has low heating demand. And as a non MCS install I wouldn't be getting anything for export. I'm a bit sceptical that the current export rates are sustainable tbh and I don't want to build a system based on them.

[Crofter]

What prevents you from combining the inputs in parallel? Sorry if I'm missing something obvious.

E.g. on my old boat I had two 260w panels in parallel, I just ran them to the same input terminals on the MPPT. I had another MPPT which had four 100w panels all in parallel. On a boat parallel is the way to go because of variable shading, the panels can end up with quite different levels of output.

[ProDave]

Domestic grid tie inverters generate a high voltage output so need a high voltage dc input to do that, hence it is normal to connect the panels in series.

 

forget all you know about boat inverters.

[Crofter]

OK I get it now, I looked at the spec of that Solis and it's limited to an input current of 14A. The panels I'm looking at have Imp of 13.5, which seems awfully close. So any attempt to wire in parallel would go way above the max current.

[JohnMo]

59 minutes ago, Crofter said:

What prevents you from combining the inputs in parallel? Sorry if I'm missing something obvious.

If doing that on two roofs, the amps produced follows the lowest output panel, so if you had a side in shade and the other in the sun, output of whole system would be equal to the shade side. You can get around it with optimisers, but not fully. Each side of roof needs it's own MTTP. Plenty of cheap string inverters with 2x MTTPs. City Plumbing is my go to, for solar.

[-rick-]

4 minutes ago, JohnMo said:

If doing that on two roofs, the amps produced follows the lowest output panel, so if you had a side in shade and the other in the sun, output of whole system would be equal to the shade side. You can get around it with optimisers, but not fully. Each side of roof needs it's own MTTP. Plenty of cheap string inverters with 2x MTTPs. City Plumbing is my go to, for solar.

 

I'm pretty sure this is wrong. It is an manufacturer approved installation setup to install east + west facing panels in parallel so that the bulk of generation comes from the east in the morning and west in the afternoon. Current in parallel sums.

 

https://www.fronius.com/~/downloads/Solar Energy/Technical Articles/SE_TA_Efficient_East_West_orientated_PV_systems_with_one_MPP_Tracker_EN.pdf

[JohnMo]

That sounds good until you read the rules 

"Shading must be avoided" how do you avoid shade on the west panels in the morning?

[-rick-]

6 minutes ago, JohnMo said:

That sounds good until you read the rules 

"Shading must be avoided" how do you avoid shade on the west panels in the morning?

 

They are talking within a string, not between strings in parallel.

 

So they want to maintain each string at a similar voltage. Their document explains that the output voltage of panels doesn't vary that much by the amount of light, it's the current that moves more and that doesn't really matter. If the voltage of the two strings is mismatched then you will experience more loss. Electrically, the string with the higher voltage will dominate and less power will be drawn from the string with lower voltage (reducing yield). Significant shading of a panel will reduce voltage significantly, but the difference between direct and indirect light mostly appears in current generated.

[Crofter]

As I've said, I'm familiar with off grid systems, but I think the principles are the same.

 

Panels in series will run at the current of the lowest output panel on that string, and at the voltage of all the panels added together.

Panels in parallel will run at the voltage of the lowest performing panel and the current of all panels added together.

 

Panels in series give a higher system voltage, allowing smaller cable sizes and generally better efficiency. But shade on any one panel will drastically cut output of the others.

Panels in parallel don't get the benefits of higher voltage, but they function independently and shade on one panel doesn't drag down the others.

 

Panels put together in a string will act like one big panel, subject to the above rules.

 

There's nothing to stop you from making up all sorts of combinations, so long as you stay within the specifications of the receiving charge controller.

[ProDave]

6 minutes ago, Crofter said:

There's nothing to stop you from making up all sorts of combinations, so long as you stay within the specifications of the receiving charge controller.

That is the critical statement.  I doubt a single or even 2 panels in series would achieve the minimum voltage to start most grid tie inverters.