Which type of inverter for 8.5kW East-West PV system 3phase or 1phase ?

[bladerunnerpv]

Hi all, I've been reading this forum with interest for some while now, gradually trying to get my head around various topics from the discussions, and doing a fair bit of web searching to try and learn as much as I can.

I haven't seen a thread on my particular question, and am looking for some advice, so here goes. I have no practical electrics experience so apologies if I'm slower on the uptake. Obviously as a beginner I don't appreciate what's inadvisable or maybe even completely stupid ;-)

Background :
House with basically an East-West aspect, roof ridgeline is 170deg magnetic on a compass, so 10deg East of South, making one roof face 080deg the other 260deg, or w.r.t. South -100, +80 . I'll just refer to them as East-facing and West-facing for simplicity. Roof pitch is about 30deg.

What would I like it to do :
1 Use less grid elec to save me some money with prices being uncertain to say the least !
2 provide some level of EPS/UPS
i. to gas boiler so it doesn't get cut off in the winter
ii. for some medical equipment used overnight all year round
iii. provide enough for a fridge freezer and a freezer during power cuts, the fridge-freezer is more important
iv. to power wi-fi router which is presumably necessary to continue managing the PV system (mobile networks will go dead in a power cut)
3 potentially provide some hot water
4 down the line provide some electric vehicle charging, but that's prob. at least 2yrs away, maybe 5yrs

I've been doing some example builds using a free online system planner, which helps cost a system (at wholesale prices, parts only). It provides whole year graphs of split between PV, battery and grid usage, and can show a day's usage, so I can appreciate winter output will be limited, but would like to get as much out of the system as I can.

This led me to think that (ignoring cost for now) maxing out the panels is the way to go.

So I reckon :-
For a 390W Qcells panel
- the East roof will take at least 10 panels, possibly 11, depending on panel size and any fitting restrictions - there's a velux and a soil stack vent about at about 2/3rds way across the roof width, with a gap of about 1.5m
6 in a block of 2 rows x 3 columns and another block of 4 in 2rows x 2 columns,
- the West roof will take an upper row of 7 panels and a lower row of 4 panels

So for 390W panels that's a peak of:
East side 3900W or 4290W
West side 4290W
East+West 390Watts = 8190 to 8580W

If I go for slightly smaller dimensioned 345W panels I get
East side 10-12 x 11x345 = 3450 to 4140W
West side 11x345 = 3795W
East+West 345Watts = 7245 to 7935W


Light path:
The East side has some wintertime obscuration/shading due to nearby rooflines, first thing in the morning in midwinter there's an unobscured light path to the east, then sun goes behind nearby rooflines then iirc rises over them again later in the morning. My guess is that the top row of panels will get light before the bottom row in deep midwinter when sun rise back over roofline. Summertime is fine.

West side is OK I think

Kit: Ideally I'd prefer to go with british / european / south korean / australian / american
Battery : ~10kW ?

Questions I hope someone can help with please
1) What type of inverter, and what impact does it have on battery storage

1a) For the inverter I'm not sure if I need in 3phase or 1 phase and how does this relate to the PV output to the inverter?
It appears (to me) that for hybrid inverters 1phase inverters go to about 5 or 6 kW, so in summer a potential 8.5kw PV input makes it look underspecified and if it's stuck in hot loft my instinct smells problems, 3 phase inverters go a lot higher.

e.g. GivEnergy HY-5
Input Data
Number of MPPT 2
V range of MPPT 150 - 550V
Max DC input power 6500W [inadequate ??]
Max Input current 11A
Max PV voltage 600V
Nominal voltage 360V
Start up voltage 150V

e.g. Growatt MIN 6000 TL-X Dual MPPT Single Phase
DC attributes
Number of MPPT 2
V range of MPPT 80 - 500V
Max power 8100W [just adequate ??]
Max Isc current 16.9A
Max voltage 550V
Start up voltage 100V

e.g.
Soltaro All-In-One 5kW Hybrid, this claims some EPS backup
DC attributes
Number of MPPT 2
V range of MPPT 125 - 500V
Max power 7500W [just adequate / inadequate ?]
Max current 15A
Max voltage 600V
Start up voltage 125V

Three phase inverters look (to me) to start from about 3kW and go up and up and up
I picked a 10kW Fronius one to look at, they obviously make smaller ones too
DC attributes
Number of MPPT 2
V range of MPPT 270 - 800V
Max power 15000W [Looks way too high?]
Max Isc current 40.5/24.8A
Max voltage 1000V
Start up voltage 200V [Looks much higher - is this a problem for East roof in winter?]

Or this Growatt MOD 6000 TL3-X
DC attributes
Number of MPPT 2
V range of MPPT 140 - 1000V
Max power 9000W [Looks about right ?]
Max Isc current 16A
Max voltage 1100V
Start up voltage 160V


1b) Can a 3phase inverter only be used with an AC coupled system ?
(I'm not clear in my mind about AC and DC coupled systems, I've seen this https://www.deegesolar.co.uk/ac_coupling_vs_dc_coupling_solar_battery_storage/ which explains it at top level but ...)

1c) How is a 3phase inverter linked to a battery storage - don't batteries need DC input ?
Would I need a 3phase hybrid system like this
Giv-3P-Hy6
but appears to be commercial use?
Is this AC coupled only ?
And if I read correctly the backup power output is 3phase AC...

I think my house is single phase supply by looking at this
https://www.ukpowernetworks.co.uk/h...ngle-phase-and-three-phase-electricity-supply

Sorry I'm a bit confused here as you might have guessed by now!

2) Does the startup voltage on the inverter matter during winter - is lower startup volts better when less energy being produced? So is an inverter with a lower startup voltage more useful in winter as it will not waste so much incoming energy supply battery/house more kWh overall?
Or does the Max Power Point Tracker on the panel (inverter?) manage it all by balancing volts and amps in W = V x A equation?

3) Should I be looking at microinverters for the East facing roof ?

4) EV charging
5a) Can the solar system provide enough electric for the house and car, even in summer ?
Estimating tool suggests 6838 kWh Annual solar generation for a Soltaro All-In-One 5kW Hybrid based system
Our annual usage is ~4000-4200 kWh, so 2400kWh spare p.a, ~6.6kWh/day
but presumably the bulk of generation is Apr-Sep ? So would end up relying on grid for EV charging Oct-Mar

5b) is 7kW EV charger tech going to change over the next 10yrs, if I pay out for a PV system now is it going to be obsolete in 10-15yrs time ?

I speed read this https://www.smarthomecharge.co.uk/guides/22kw-three-phase-charging-is-it-worth-it/
which indicates 7kW is the usual way for home chargers and any faster requires 3phase home supply which is expensive to do - it quotes £3k+

5c) Should I be considering changing to a 3phase supply for EV? In which case a 3phase inverter is essential ? Although household will presumably run on 1phase of the 3ph available ?

Thanks very much to anyone who takes a look and responds helpfuly

 

[johnb2713]

I'm not an expert in solar, there are many on here and I'm sure a few will give you some advice.

1) You are better to get a professional to plan and instal the system. You wont get the relevant paperwork for DNO etc without doing this.
2) Single phase / 3 Phase, I would suggest that single phase is more likely to match your house supply, very few houses have a 3 phase supply, a photograph of your meter cabinet will confirm.
3) Lower start up voltage is better.
4) EV charging and can the solar provide enough...... how many miles do you do, what EV? My i3 does 5 miles/kWh and I have run it 90% from the roof since April this year doing around 130 miles per week.
5) EV chargers or more correctly wallboxes, most people tend to use a 7kW box and charge overnight of off peak tariff or during the day with solar etc. To work timings, battery capacity, state of charge and charge rate information needed. My i3 has 30 kWh battery so if it is 25% SOC it needs 22.5kWh, with a 7 kW wallbox that's going to be a little over 3 hours. To work efficiently with solar you need a smart wallbox such as ZAPPI to adjust the charge rate to the surplus energy.
6) Winter time is not good for solar generation, an EV tariff is advantageous, Octopus GO for example I pay 7.5p off peak and 29.2p peak. I also charge my solar battery system and run many appliances during the off peak time.

Solar PV is a real money saver, the battery in my view is crucial to the success of the system. My system had a 7kWh battery which was just about enough in the summer, isnt enough for winter so it's being joined by an additional 14 kWh battery shortly.

 

[bladerunnerpv]

I'm not an expert in solar, there are many on here and I'm sure a few will give you some advice.

1) You are better to get a professional to plan and instal the system. You wont get the relevant paperwork for DNO etc without doing this.
2) Single phase / 3 Phase, I would suggest that single phase is more likely to match your house supply, very few houses have a 3 phase supply, a photograph of your meter cabinet will confirm.
3) Lower start up voltage is better.
4) EV charging and can the solar provide enough...... how many miles do you do, what EV? My i3 does 5 miles/kWh and I have run it 90% from the roof since April this year doing around 130 miles per week.
5) EV chargers or more correctly wallboxes, most people tend to use a 7kW box and charge overnight of off peak tariff or during the day with solar etc. To work timings, battery capacity, state of charge and charge rate information needed. My i3 has 30 kWh battery so if it is 25% SOC it needs 22.5kWh, with a 7 kW wallbox that's going to be a little over 3 hours. To work efficiently with solar you need a smart wallbox such as ZAPPI to adjust the charge rate to the surplus energy.
6) Winter time is not good for solar generation, an EV tariff is advantageous, Octopus GO for example I pay 7.5p off peak and 29.2p peak. I also charge my solar battery system and run many appliances during the off peak time.

Solar PV is a real money saver, the battery in my view is crucial to the success of the system. My system had a 7kWh battery which was just about enough in the summer, isnt enough for winter so it's being joined by an additional 14 kWh battery shortly.

Hello John, thanks for taking the time to help

1) Agreed, I do like to do my research before spending the hard-earned though The software tool I'm using does help show anticipated proportions for solar / battery / grid kWh usage so I can work out nominal savings and it adds up all the electrical hardware and roof mounting costs.
As I understand it I need someone MCS certified for the DNO paperwork

2) The house supply is definitely single phase
Part of what I'm trying to do is work out how 8.5kW of panel output feeds into an inverter and what type of inverter is needed

3) OK, something to keep an eye on then for quotes I receive

4) I don't have an EV yet, but govt will make me one day so wish to 'future-proof'. Thanks for the no.s on your EV.

5) Thanks for the sums John, gives me a ballpark to work in so to speak. I'll need to have a look at Zappi and understand how they relate to AC or DC coupled systems, not got a complete system block diagram in my head yet ! What's SOC ? Presume % car battery charge ?

6) OK, thanks. I'm anticipating about 1-2kWh per day on average over winter. I read somewhere an estimate of 4h/day sunlight in summer and 1h/day in winter on average.

I can see more battery = more kWh storage = more self consumption = save£.
Provided there's enough PV to charge them,
or do you get enough on octopus economy4 rate to recharge "house battery" as well as charge the EV ?

What do I need to look for UPS/EPS/ESS w.r.t battery ? I see the GivEnergy Giv-HY-5 datasheet has max discharging power of 2.5kW,
So that's enough for a 800W microwave for a ready meal, but not enough for an electric oven, and just enough for a 2kW toaster ?
The battery capacity in kWh presumably dictates how long that power can be provided, so a 10kWh battery discharging at 2.5Kw is 4 hours "supply".
And PV system must not feed back to grid during power cut.

Thanks again.

 

[johnb2713]

Hello John, thanks for taking the time to help

1) Agreed, I do like to do my research before spending the hard-earned though The software tool I'm using does help show anticipated proportions for solar / battery / grid kWh usage so I can work out nominal savings and it adds up all the electrical hardware and roof mounting costs.
As I understand it I need someone MCS certified for the DNO paperwork

Correct but youre unlikely to find anyone to sign off the paperwork that hasnt supplied and installed the system.

2) The house supply is definitely single phase
Part of what I'm trying to do is work out how 8.5kW of panel output feeds into an inverter and what type of inverter is needed

With you East \ West roof configuration you will be a way off your rated 8.5kW of panels. My system has 7kW of panels feeding a 6kW inverter.

4) I don't have an EV yet, but govt will make me one day so wish to 'future-proof'. Thanks for the no.s on your EV.

Ive just watch a review of the new MG Z4, £25k, it really is a game changer.

5) Thanks for the sums John, gives me a ballpark to work in so to speak. I'll need to have a look at Zappi and understand how they relate to AC or DC coupled systems, not got a complete system block diagram in my head yet !

A DC coupled system is where energy from solar is passed via the inverter straight into the battery. An AC coupled system, the solar feeds the inverter which converts the energy to AC. Somewhere on your house supply you have a battery with an inbuilt charger which takes the AC power, converts it to DC and charges the battery. The likes of EV wallboxes are all fed from AC.

What's SOC ? Presume % car battery charge ?

Yes - State Of Charge

6) OK, thanks. I'm anticipating about 1-2kWh per day on average over winter. I read somewhere an estimate of 4h/day sunlight in summer and 1h/day in winter on average.

I can imagine that to be about right.

I can see more battery = more kWh storage = more self consumption = save£.
Provided there's enough PV to charge them,
or do you get enough on octopus economy4 rate to recharge "house battery" as well as charge the EV ?

00:30 to 04:30 - Charge battery, EV's, Run dishwasher, washing mc, tumble dryer and immersion heater all on 7.5p kWh

04:30 to panels start - House runs on battery

Panels start to Panels stop - Everything runs on panels / battery. Battery recharges if power is available as does the Immersion heater, excess energy is dumped into the hot water tank via MyEnergi EDDI. Clever device, if there is 1kW of excess energy, EDDI runs the immersion heater at 1 kW even though it's a 3 kW element.

Panels stop to 00:30 - Run on battery

What do I need to look for UPS/EPS/ESS w.r.t battery ? I see the GivEnergy Giv-HY-5 datasheet has max discharging power of 2.5kW,

I suspect if you had 2 batteries then the discharge rate would be doubled.

So that's enough for a 800W microwave for a ready meal, but not enough for an electric oven, and just enough for a 2kW toaster ?

Yes which is why, IMHO the systems advertised with a 3.6kW inverter are nigh on pointless

The battery capacity in kWh presumably dictates how long that power can be provided, so a 10kWh battery discharging at 2.5Kw is 4 hours "supply".

Correct

And PV system must not feed back to grid during power cut.

Correct, grid tie inverters must shutdown within a specific time to protect linesman.

Thanks again.

Youre welcome

 

[binky]

John has covered most points, so I will just add Q cells are my favourite panels, they are tuned for low light performance which suits the UK.

Don't exceed the max DC input wattage. Even with a split array, the panels can have moments when the generate max watts, not often, but can happen. Always max wattage on roof and if necessary use 2 smaller inverters. That would also allow 2 strings per roof, which would help with the shading, or you could go microinverter route. I'm not a great fan of adding things that can fail on the roof, but they should last 20 years. Stack pipe can be cut down in height,

You don't need MCS for the DNO, but you will need that for the flexible tariffs from Octopuss etc.

 

[johnb2713]

Don't exceed the max DC input wattage. Even with a split array, the panels can have moments when the generate max watts,

With an 8kW array split across an East/West roof, is it really likely to exceed the inverter wattage at say (for example 6kW) ?

Why is it so important, surely if the inverter doesnt take the wattage available then it just remains unused capacity? (question not a challenge).

 

[binky]

With an 8kW array split across an East/West roof, is it really likely to exceed the inverter wattage at say (for example 6kW) ?

Why is it so important, surely if the inverter doesnt take the wattage available then it just remains unused capacity? (question not a challenge).

probably fine on a 6kW inverter, but at midday you can hit near peak outputs in summer, depends on roof pitch, location in the UK etc. I am blessed down here in Devon with having the best irradiance figures in the UK, so I tend to think in terms of the panels knocking out full welly! If you live in Scotland it's going to be a different experience to mine. It's a shame I no longer have PV-sol, professional solar design software, it was very good at simulating irradiance, and arrays then offering best size of inverter to max yearly outputs, I could have given the OP a definite answer in about 10 mins.

I'm not sure what happens if you overpower the DC side of an inverter, I've never cared to find out as they cost money

I see a lot of the available inveters will take say 8Kw input on a 6kW system, but if you had a warranty issue, they always ask details about the array, so if you had say 9kw on an inverter that has max DC of 8kW, I suspect they would tell you to foxtrot oscar. They also check returned inveters for 'user damage' and some companies will not supply a warranty replacement until they have done fault finding checks, or will send one out but invoice you for it if they find it's your fault. It's still taught to solar installers that undersizing inverters to 80% is acceptable, something I disagree with as inverter efficiency is so much better these days. One of my customers found that his 2.7kW array actually could peak at just over 3kW - something the panels were not suppossed to be capable of, so I much prefer size for size inverters, rather than 'clip' a system. That might be something to do with choosing better equipment than other installers. The link below is for Califonian state panel ratings, they use a different test to replicate real world conditions, ie what the panels are likely to output on a roof with a breeze. So where as Standard Test Conditions may rate a panel at 250W, their test would rate it at say 230W. I used to use this list to compare available panels and select those manufacturers with least drop in wattage.

https://solarequipment.energy.ca.gov/Home/PVModuleList

 

[johnb2713]

I just considered panels like a generator, capable of 8kW but the inverter (load) only taking 4kW, I cant see an issue, it's not really any different to turning export off on mine and the panels being able to produce more than the inverter can use. I can see a potential issue where a manufacturer rates his inverter at 6kW but if fed with suitable power it can produce 7 or 8kW until something dies.

Thanks for the heads up

 

[Chrisbee]

If half his panels are on each roof then they can't both produce full power at the same time because when the sun is centred on one roof it's oblique to the other.

So isn't the limiting factor the max voltage that each MPPT can take? Assuming one MPPT per roof.

 

[johnb2713]

If half his panels are on each roof then they can't both produce full power at the same time because when the sun is centred on one roof it's oblique to the other.

So isn't the limiting factor the max voltage that each MPPT can take? Assuming one MPPT per roof.

That would seem to be logical.

I suppose one caveat maybe if the array is oversized compared to the inverter, the array may exceed the capacity of the inverter whilst only running at 70%. I personally dont see why there would be an issue as long as max voltages are complied with.

 

[binky]

If half his panels are on each roof then they can't both produce full power at the same time because when the sun is centred on one roof it's oblique to the other.

So isn't the limiting factor the max voltage that each MPPT can take? Assuming one MPPT per roof.

depending on roof pitch, you would be surprised how close they can get to max output - steep pitch, lower combined power, shallower pitch, higher combined power. I wouldn't cherry pick which constraint to comply with, you comply with all restrictions to ensure a reliable working system. The reason for this is you can get some odd weather effects that can create short, but notable spikes, like cloud edge effect whereby the edge of a cloud can very briefly focus the irradiance on the panels. You can also get some really good outputs in Spring and Autumn, the sorts of days when the air is crystal clear, cold, and it's a sunny day. Summer tends to have more atmospheric moisture or dust in the air, so knocks back peak outputs. Mid winter with snow on the ground, you can also get boosted irradiance from reflected light - more of an issue in the Alps than the UK, likewise water front properties get boosted outputs. The panels on the less sunny side will be working off background irradiance, so voltages can be at max level even if the power isn't.

This is where professional design software comes into it's own, as it allows for all factors that can affect the panels, it's stopped me installing a few inverters I thought would be Ok over the years. You can use the 'Sunny Design' portal for free, you just need to create an account, which allows for all the factors above. I think several other inverter manufacturers offer similar tools.

 

[johnb2713]

depending on roof pitch, you would be surprised how close they can get to max output - steep pitch, lower combined power, shallower pitch, higher combined power. I wouldn't cherry pick which constraint to comply with, you comply with all restrictions to ensure a reliable working system. The reason for this is you can get some odd weather effects that can create short, but notable spikes, like cloud edge effect whereby the edge of a cloud can very briefly focus the irradiance on the panels. You can also get some really good outputs in Spring and Autumn, the sorts of days when the air is crystal clear, cold, and it's a sunny day. Summer tends to have more atmospheric moisture or dust in the air, so knocks back peak outputs. Mid winter with snow on the ground, you can also get boosted irradiance from reflected light - more of an issue in the Alps than the UK, likewise water front properties get boosted outputs. The panels on the less sunny side will be working off background irradiance, so voltages can be at max level even if the power isn't.

every day a school day, thank you!

 

[binky]

every day a school day, thank you!

I sometimes forget how much I have learned, and forgotten again - it's age related

 

[bladerunnerpv]

Thanks to all for your comments and assistance, sorry not to get back to you sooner, had a dose of reallifeitis recently, I hope to follow up soon.

Thread locked to prevent being resurrected (again)