Solar panels and Inverters - AC or DC?

[sasquatch]

Hi all,

I'm dipping my toe into the waters of PV generation and trying to spec out my first few panels/system (off-grid), and am a bit stuck on where to go...

I have a garage roof that's south facing and gets mostly full sun coverage during the day, but there's a few large trees at one end that can cause a little bit of shading on the end panel. I should be able to just about fit 5 x 405W panels on the roof - looking at theJA Solar 405W Mono MBB Percium Half-Cell panel.

In an ideal world I'd like to wire these using micro-inverters so I get 240-ish V out the end to do with as I please (into a Victron MultiPlus for storage to start with I think). The reason for Micro-Inverters would be that should I decide to move to putting panels on the main house, which is east-west facing I think it would be easier to split the panels without needing to mess around with multiple inverters/keeping strings apart to avoid shading issues/etc.

For some reason in my mind micro-inverters just seem a simpler/neater approach - no need to run/handle chunky DC cables, each panel is essentially 'stand alone' so shading of one won't affect any other the others - am I missing something (other than the cost being higher, I guess...)?

Does anyone have any experience of using Micro-Inverters? I'm specifically looking at the Hoymiles manufacturer - as they do 1/2/4 port versions and seem to be rated for UK use should I wish to grid-tie in future. They're also cheaper than the Enphase variants but seem to have broadly speaking the same feature set. It looks like i'll also need their 'control stick' for programming but it's not clear how much access you get to the underlying configuraton without being a 'registered installer' - seems to be the case for both Hoymiles and Enphase?

Cheers

Jon

 

[Redpacket]

It'd probably help to read up on AC coupled vs DC couples systems.
You might want to check the micro inverters will operate & make 240VAC without being grid tied, eg they might need to see a grid reference frequency to run (I don't know either way).
The Victron unit might not be the best charger (limited charge rate + not sure if it uses switchmode or "MOSFETS as rectifiers"), but it's a great inverter (love mine). A good switchmode charger would likely be better.
Short version:
AC coupled is simpler & modular like you mention but has high losses (10-20%) round trip to/from storage. They need to be connected by an electrician.
DC coupled is more efficient, can be done DIY if system voltages <120VDC (I have multiple strings of 2 panels, yes lots of wiring!)

 

[hermitdave]

I am doing something similar to what you are planning. I picked up 14 panels and am planning to install 4 on the extension roof as stage 1.
I have decided on AC couple system much like yourself.
I came across Ecoworthy 1200 and Growatt 750 / 1000s and cant decide which one to start with first.
Ecoworthy has 4x inputs and Growatt has 1x input so you need to string 4 panels.

I just came to the forum to ask if anyone has used either

 

[sasquatch]

Hey Dave

The eco-worthy looks like an interesting option as well - don't suppose you happen to know if/how they can be individually monitored?
AC coupled with micro-inverters seems the best approach given the random shading and potential east-west panel placement plus being modular - I know there's losses involved in AC-DC-AC conversion for storage but to be honest storage is sort of secondary to my needs anyway (it's more a 'because I can' than 'because I need').

I've dropped a note to a UK supplier of Hoymiles inverters asking the grid-tie question - I think if I'm going to be running DC cables from panels anyway then I'll probably end up going for a DC coupled system and bump storage up the priority list a bit.... so many choices!

 

[hermitdave]

Hey Dave

The eco-worthy looks like an interesting option as well - don't suppose you happen to know if/how they can be individually monitored?
AC coupled with micro-inverters seems the best approach given the random shading and potential east-west panel placement plus being modular - I know there's losses involved in AC-DC-AC conversion for storage but to be honest storage is sort of secondary to my needs anyway (it's more a 'because I can' than 'because I need').

I've dropped a note to a UK supplier of Hoymiles inverters asking the grid-tie question - I think if I'm going to be running DC cables from panels anyway then I'll probably end up going for a DC coupled system and bump storage up the priority list a bit.... so many choices!

Micro converters often come with hub. Each inverter logs with the hub and uploads to their cloud services. Ecoworthy has series connectivity/ Wifi module. Same with Growatt

 

[OffGridInTheCity]

You started off saying off-grid (as apposed to grid-tie). I don't have any experience with micro-inverters - butI do have 45 'traditional' panels running to combiner boxes - and I don't find it that hard to arrange them for sun/shade. They are in 3 sets of 3s5p (or 3s15p overall). They are 3s because of the charge controllers I chose.

For shade considerations - you want to wire them so the series group (the 3s in my case) areshaded together - e.g. you want the panels in the series to get ALL the shade rather than each series getting a little bit of shade. When series (of 3 panels) are shaded - the voltage remains pretty much the same as the ones (series of 3 panels) in the sun, but the power (amps) drop significantly - so its perfectly find to combine these sets in parallel. For example, 1 set might be 110v@3a then 4a then 6a ... and the ones in the sun are 115v@30a then 35a then 40a ... and they combine very well because the 110v vs 115v is very little inefficiency - so things settle at (I don't know exactly) at 114v @ Xa where X is the addition of the amps.

Here's a pic showing the shade moving across one of myarrays from left to right- and as an example, the 2 x right hand panels (on array) + 3rd on (on the far gazebo) are wired as a 3s set because of shading pattern - e.g. they don't get full sun till mid morning (9:30-11:30am depending on time of year).



When I first started - I obsessed over wire distance / loss of power but 10awg wire is cheap and then 6awg when combined and I'm running 150ft lengths (from array to controller) and its just doesn't seem to be a big issue as the charge controllers report full/expected power for the panel ratings.

Its a fundamental decision to go DC or AC from roof - and will affect all equipment downstream. I don't see evidence that either is 'the single best way' - so it might just be whatever you are more interested in ?

 

[sasquatch]

Part of the reason for me wanting to avoid DC coupling on the roof was because I can't easily arrange the panels in mixed parallel/series - there will be essentially 5 panels in a line and it's the end two that are likely to get hit by shading.

I contacted one of the UK distributors of the Hoymiles inverters, and learnt two interesting things: 1. They're no longer selling them, and switching to Enphase, and 2. Both Hoymiles and Enphase micro-inverters only operate in grid-tie. I've seen talk of building a 'fake grid' (generator, loopback from inverter, etc.) to keep them happy but to be honest that sounds like a recipe for disaster!

Back to the drawing board for me I think...

 

[OffGridInTheCity]

Part of the reason for me wanting to avoid DC coupling on the roof was because I can't easily arrange the panels in mixed parallel/series - there will be essentially 5 panels in a line and it's the end two that are likely to get hit by shading.

I contacted one of the UK distributors of the Hoymiles inverters, and learnt two interesting things: 1. They're no longer selling them, and switching to Enphase, and 2. Both Hoymiles and Enphase micro-inverters only operate in grid-tie. I've seen talk of building a 'fake grid' (generator, loopback from inverter, etc.) to keep them happy but to be honest that sounds like a recipe for disaster!

Back to the drawing board for me I think...

>2. Both Hoymiles and Enphase micro-inverters only operate in grid-tie.
yea, that's why I started off talking about off-grid as I don't have deep knowledge but all the micro-inverter solutions I've seem 'seem' to be oriented to grid-tie.

>essentially 5 panels in a line
Perhaps a 2s2p (optimized for shade)and then a separate 1s1p (in the shade). Its OK to have more than1 charge controller (I have 3 of them) as the output can all go to the same battery bank- and the 1s1p is likely to be a 400w cheaper one.

Not pushing DC or particular design - just trying to be encouraging

 

[hermitdave]

Part of the reason for me wanting to avoid DC coupling on the roof was because I can't easily arrange the panels in mixed parallel/series - there will be essentially 5 panels in a line and it's the end two that are likely to get hit by shading.

I contacted one of the UK distributors of the Hoymiles inverters, and learnt two interesting things: 1. They're no longer selling them, and switching to Enphase, and 2. Both Hoymiles and Enphase micro-inverters only operate in grid-tie. I've seen talk of building a 'fake grid' (generator, loopback from inverter, etc.) to keep them happy but to be honest that sounds like a recipe for disaster!

Back to the drawing board for me I think...

In UK off grid is a very limited duration scenario. I wouldn't worry too much about.
I think I'm warming up to growatt for extension. It's also IP65 rated so will stick it outside.
Just realised that I might be able to mount a few more panels sideways.
Started mounting brackets today only to realise that the extension roof doesn't have any insulation so going to check how much it will cost to insulate before I proceed.

Managed to mount 4 brackets rather quickly

 

[sasquatch]

Sounds like you're making decent progress there Keen to see pictures when you're done, if you're happy posting them.

Another part of the reason for not wanting grid-tie right now is that there's already grid power to the garage, but I'm not sure what the current handling capacity is. It's on a 16A breaker on the MCU but I'm not sure if that's by choice or design, so need to do some digging. I'd be worried about excessive current draw over the grid connection back to the main house, especially as I have a 32A EV charger as well (not fed via the garage, thankfully!)

 

[hermitdave]

Sounds like you're making decent progress there Keen to see pictures when you're done, if you're happy posting them.

Another part of the reason for not wanting grid-tie right now is that there's already grid power to the garage, but I'm not sure what the current handling capacity is. It's on a 16A breaker on the MCU but I'm not sure if that's by choice or design, so need to do some digging. I'd be worried about excessive current draw over the grid connection back to the main house, especially as I have a 32A EV charger as well (not fed via the garage, thankfully!)

That all depends upon how many panels you are putting up. I was contemplating 4x 250 and i might consider a few more horizontally mounted. With 7 panels, totally max wattage would be 1750. They are used panels with 10% degradation, max is likely below 1500 watts.

In amps, that would be under 10A. Leaf EVSE is 10A continuous and that draws 2400 W. I think you should be fine but start small ? connect a few and then expand ?

 

[sasquatch]

That all depends upon how many panels you are putting up. I was contemplating 4x 250 and i might consider a few more horizontally mounted. With 7 panels, totally max wattage would be 1750. They are used panels with 10% degradation, max is likely below 1500 watts.

In amps, that would be under 10A. Leaf EVSE is 10A continuous and that draws 2400 W. I think you should be fine but start small ? connect a few and then expand ?

Good point - I was thinking that batteries + inverter could cause problems but given I can only get an absoloute max of 2kW on the roof (5 panels max, assuming 405W panels) it would be a bit pointless sticking huge battery storage in there anyway.

 

[hermitdave]

That all depends upon how many panels you are putting up. I was contemplating 4x 250 and i might consider a few more horizontally mounted. With 7 panels, totally max wattage would be 1750. They are used panels with 10% degradation, max is likely below 1500 watts.

In amps, that would be under 10A. Leaf EVSE is 10A continuous and that draws 2400 W. I think you should be fine but start small ? connect a few and then expand ?

Good point - I was thinking that batteries + inverter could cause problems but given I can only get an absoloute max of 2kW on the roof (5 panels max, assuming 405W panels) it would be a bit pointless sticking huge battery storage in there anyway.

405w? Blimey I thought most on market are 300ish