Solar battery proposal questions

[Ay8452]

Hey folks

 

Looking for some advice on proposals I've received for my solar setup particularly inverter sizing. Proposal:

 

14 X Aiko Neostar 3S 475W N-Type, Mono Glass All Black, Gen 3 Panels (good panels?) - 4 of the panels are on a separate roof to the other 10 and are heavily shaded in the late afternoon/evening. 

 

Sigenergy battery - thinking either 8kWh to 10kWh for now. Not sure if going bigger makes financial sense based on my estimated usage really?

 

We have 3-phase supply and plan to have it well balanced (new electrics going in as part of renovation). Estimated use is 4-5,000 units (not moved in yet, we are still renovating) for a family of 2 adults and three young kids, not including future EV charging for a car doing 10k miles a year, plan to charge overnight via the Grid.

 

For 3 phase Sig energy inverter sizing; I've had conflicting advice with some installers recommending a 6kw, 8 kw, 10 kw, and even a 15 kw. Based on my basic research 8 kw seems sensible? 

 

Questions: What inverter size would you recommend for this kind of setup considering it's 3 phase? Would 8kW be sufficient, or is it worth stretching to 10kW?

 

For the 4 panels that are shaded, some have recommended optimisers whilst others have said not to as they go "wrong" over time - thoughts? Roof is SE facing pitched at 36 degrees.

 

Appreciate your insights!

[Bramco]

58 minutes ago, Ay8452 said:

For 3 phase Sig energy inverter sizing; I've had conflicting advice with some installers recommending a 6kw, 8 kw, 10 kw, and even a 15 kw. Based on my basic research 8 kw seems sensible? 

 

Sizing the inverter depends on few things - what is the maximum PV output, what is the maximum draw from the house etc.

 

Given you'll almost never get the maximum PV output and the estimated output is 6kW, you'd be OK with a 6kW inverter.  On the house side, the biggest draw from appliances is about 3kW, so you could draw 6kW from the batteries without pulling anything from the grid.  But if you want a fast EV charger, they would pull more but then it would depend on what the batteries can deliver. Don't know what Sigenergy can do but it will be on the spec.  If you plan to do EV charging from off peak, at night, then this might not matter.

 

Personally I'd put more battery capacity in rather than a larger inverter. If you are on O Go Int, then you can dump almost everything in the batteries in the evening at 15p before the cheap rate starts and then refill them at 7p. It's a bit of a no brainer. So the larger your batteries, the more you gain.

 

Having said that, we are single phase, so I don't know how this would work with 3 phase.

[SteamyTea]

4 hours ago, Ay8452 said:

Estimated use is 4-5,000 units (not moved in yet, we are still renovating) for a family of 2 adults and three young kids

Have you monitored and analysed your current usage patterns.

That is a better place to start.

[Ay8452]

1 minute ago, SteamyTea said:

Have you monitored and analysed your current usage patterns.

That is a better place to start.

 

Problem is I am renovating/haven't moved in yet. And I'm currently renting a tiny house which doesn't reflect useable so have to estimate for now 

[SimC]

There may be an issue with the 3-phase Sigenergy inverter combined with the 4 panel string, in that the MPPT startup voltage is stated at 180 Volts.

In comparison the 1-phase inverters have MPPT startup at 50 Volts, but only size up to 6 kW.

 

The Sigenergy datasheets and install guides provide no information on how the 3-phase unit reacts to unbalanced loads. If the inverter only outputs equal power on each phase when there is an unbalanced load, then the net result is that some phases will be importing from the grid and some will be exporting, rather than all 3-phases being grid neutral. It is not clear whether any specific 3-phase grid meter from a supplier operates with net metering calculations or not for billing purposes.

 

With the 10kW 3-phase inverter, the max output current per phase is 16.7 Amps, which equates to just over 3.6 kW on each phase. If your loads draw more than this on a single phase, then the remainder will be imported from the grid.

 

The Aiko NeoStar 3S 475 W panels will probably only produce around 30 V each in lower light conditions, which only produces 120 V when the 4 are connected in series.

This string input is unlikely to start even in the brightest conditions, so is not worth installing with the proposed equipment.

 

These observations may not provide a solution for you, but I hope they are helpful in what to look out for when choosing a new system.

 

 

 

[SteamyTea]

4 minutes ago, Ay8452 said:

Problem is I am renovating/haven't moved in yet.

Do you have the smart meter data from your last house?

 

If you think you only use about 5,000 kWh/year. That is 1,000 kWh/person.year.

70% of that will be in the winter. So 6 kWh/person for 120 days (ish). Or 3,000 kWh for the family.

So to run purely off batteries (not realistic) you would need 25 kWh of usable storage.

Now as you are renovating, you could put in a large (say 400lt) DHW cylinder and heat that up. You could store about 15 kWh/day in that (can be heated directly from PV when excess and a combination of ASHP and PV, with a bit of grid import when no excess).

That will then reduce your battery size to 10 kWh.

Storing energy thermally is probably the cheapest and most reliable method, and you need hot water all year round.

[JohnMo]

4 minutes ago, SteamyTea said:

So to run purely off batteries (not realistic) you would need 25 kWh of usable storage

Really depends on tariff and how many bites you have at charging. When charging your house, it will be using grid energy anyway - if no PV.

 

So on cosy you can easily charge 3x a day and get 7 hrs off peak. Which is a lot of cheap energy even with a modest battery.

 

Trouble with big water storage is modest PV it makes almost no difference to the water temperature, plus with no PV you always have to throw plenty of energy in to it to make useable flow temperatures. What are going to do with the hot water when you have it?

[SteamyTea]

5 minutes ago, JohnMo said:

What are going to do with the hot water when you have it?

Shower and bath.

My biggest usage in bath water, my one luxury in life, and a daily one at that.

This time of year it takes about 2 kWh/day, winter goes up to 3.

 

[S2D2]

I agree with the Cosy point, it's very difficult to justify trebling battery size when you can just use Cosy with +50% on the cheap unit rate compared to the likes of Go.

[SteamyTea]

On the other hand, Octopus may stop the tariff on a whim.

They are a relatively new company, and a technology company, not a power company (they bought established companies). 

Market Disrupters do not have a very good long term track record, very few last.

 

[JohnMo]

12 minutes ago, SteamyTea said:

On the other hand, Octopus may stop the tariff on a whim.

But maybe run long enough that battery technology gets cheaper and allows more to be installed with out breaking the bank

[SteamyTea]

33 minutes ago, JohnMo said:

But maybe run long enough that battery technology gets cheaper and allows more to be installed with out breaking the bank

Yes.

I am not sure where the real, sensible, break even point us going to be.

10p/kWh delivered seems sensible as then it is competing with gas, which most people have.

[JohnMo]

12 hours ago, SteamyTea said:

10p/kWh delivered seems sensible

A bog standard gas heated house, with low standing electricity loads, it's never going to be viable for a battery. Not sure how you make the maths work.

 

On the other hand, an all electric house, with stuff like a treatment plant, with a continuous compressor running, borehole pump, ASHP etc and PV. The argument becomes more compelling, way easier to justify.

 

Really each case will have a different break even cost point. Mine was a no brainer and just part of a series of changes to bring utility bills down substantially. PV maximise, ASHP, battery to utilise generated PV, smart meter to get a decent electric tariff. Back in 2023 with gas and electric we were paying about £220 each month, now down to below half of that.

[SteamyTea]

18 minutes ago, JohnMo said:

A bog standard gas heated house, with low standing electricity loads, it's never going to be viable for a battery. Not sure how you make the maths work.

Don't think I have mentioned space heating, but to clarify.

It is about offsetting loads i.e. heat water by gas or use electricity. 

[Ay8452]

On 30/07/2025 at 19:18, SimC said:

There may be an issue with the 3-phase Sigenergy inverter combined with the 4 panel string, in that the MPPT startup voltage is stated at 180 Volts.

In comparison the 1-phase inverters have MPPT startup at 50 Volts, but only size up to 6 kW.

 

The Sigenergy datasheets and install guides provide no information on how the 3-phase unit reacts to unbalanced loads. If the inverter only outputs equal power on each phase when there is an unbalanced load, then the net result is that some phases will be importing from the grid and some will be exporting, rather than all 3-phases being grid neutral. It is not clear whether any specific 3-phase grid meter from a supplier operates with net metering calculations or not for billing purposes.

 

With the 10kW 3-phase inverter, the max output current per phase is 16.7 Amps, which equates to just over 3.6 kW on each phase. If your loads draw more than this on a single phase, then the remainder will be imported from the grid.

 

The Aiko NeoStar 3S 475 W panels will probably only produce around 30 V each in lower light conditions, which only produces 120 V when the 4 are connected in series.

This string input is unlikely to start even in the brightest conditions, so is not worth installing with the proposed equipment.

 

These observations may not provide a solution for you, but I hope they are helpful in what to look out for when choosing a new system.

 

 

 

 

Thank you for your reply.

 

If ALL the panels were on the same string fitted with optimisers for each panel then it "should" negate the issue with the start up voltage right? This is what my installer is suggesting

[Thorfun]

Out of interest, are you going to run the house on 3-phase? We have a 3p supply but run the whole house on a single phase and so our PV inverter is also single phase and can power the whole house. If splitting over 3-phases you will have to balance what appliances use each phase PV production so, afaik, you’re more likely to “waste” generated electricity if nothing on a phase is running. 
 

It just all complicated the house electrics for me and I’m happy we kept it simple! 
 

I have run a 3p cable from the cabinet to the house so I can install a 3p EV charger at a later date if required. 

[Kelvin]

3 phase just seems to over complicate everything especially when you add in PV. I’d do as above and run everything on a single phase and a future EV on another phase. 

 

Why run all the panels on a single string even with optimisers. Run each roof on its own string and using optimisers for the shaded roof. Although if it’s heavily shaded wouldn’t micro-inverters be better? My west facing roof gets a tiny bit of shading for around 40 minutes very late in day. It’s not enough to impact the output very much so I didn’t bother with optimisers. It will be worse in the winter of course but I plan on adding a third string on the ground in front of the garage as it’s unused and I could get 5/6 panels there. 

[-rick-]

29 minutes ago, Thorfun said:

Out of interest, are you going to run the house on 3-phase? We have a 3p supply but run the whole house on a single phase and so our PV inverter is also single phase and can power the whole house. If splitting over 3-phases you will have to balance what appliances use each phase PV production so, afaik, you’re more likely to “waste” generated electricity if nothing on a phase is running. 

 

This isn't exactly correct. The grid acts as a buffer and your electricity meter counts net generation. So you can generate on 1 phase and consume on another without being charged. Assuming you have a modern smart meter. Obviously you aren't strictly directly using the energy that you generated but I doubt that matters to anyone unless you aim to be off-grid, or expect long grid outages.

 

A reason for three phase is that it's easier to export from larger arrays as you get 3x16A automatic export allowance, and it will generally be easier to get permission to go bigger if equally split among the phases.

 

Agree that inside a house the need for 3 phase is minimal. There are some uses (big cooking appliances) but other than that and it's not much needed. Having the flexibility to install EV chargers or power a workshop are nice though.

[Thorfun]

39 minutes ago, -rick- said:

Assuming you have a modern smart meter.

this is the key point! how many suppliers offer a modern smart 3-phase meter that actually does net metering? when i had my meter installed a couple of years ago getting a 3-phase smart meter was next to impossible.

[-rick-]

I thought that was sorted now. Roll out started about when you were getting one and is now in full swing?

[Thorfun]

52 minutes ago, -rick- said:

I thought that was sorted now. Roll out started about when you were getting one and is now in full swing?

don't know. i've not tried since as i'm happy with single phase for now. if i ever need to start using 3-phases i'll look in to it then!

 

hopefully the OP has considered this though.

[Dillsue]

3 hours ago, -rick- said:

 

This isn't exactly correct. The grid acts as a buffer and your electricity meter counts net generation. So you can generate on 1 phase and consume on another without being charged.

There was a long thread on here a while ago on net metering and I think it was eventually shown that the net metering only works if youre exporting at exactly the same time as you're importing. If there's 1kw load on L1 and at the same time 1kw of export on L2 then no import or export gets recorded. If there's 1kw of load on L1 and no export on L2 then the meter starts recording the import. If the L1 import stops and 5 mins later 1kw of export starts on L2 it doesn't cancel the import already recorded on L1

 

Be nice if it offset total export against total import but that old thread suggested it doesn't. There'd be a massive scramble for 3 phase connections if it worked that way!

[-rick-]

Interesting point. I assume banking on the 30 min billing windows as a benefit to single phase (because if you aren't generating enough solar you are using grid energy anyway?).

[puntloos]

The point about octopus tariffs possibly going away is a fair one, but assuming they stick, a few notes:
 

- The best use of solar is to bridge the octopus 4pm-7pm peak. You can both export previously acquired kwh at a great rate, plus you can avoid paying a premium to buy kwh.

- My 10kwh battery will carry my all-electric house typically until 1am. My base load of the house is typically ~600W. (I have a few servers, 2 ASHP etc). 

- Reminder to factor in battery wear to your calculations. I once debated charging extra battery capacity during the low period, then discharging it at high (basically 1 extra cycle per day!), but when factoring in the cost to replace the battery after X/2 cycles basically it stopped making sense

- My plan is to use my hybrid inverter to also take power from my car battery (V2H). Waiting for an affordable, approved V2H setup. (V2G seems not worth it, much more complex)

- Reminder that most solar setups do not cover you for power outages unless your installer knows what they are doing. You'll probably need a high current contactor or 3 (I have 3 phase). 

 

[Beelbeebub]

I'm just about to finish getting a similar system up. 

 

Large house - about 5,500 kwh a year demand on a single phase. 

 

24x neostar 2 (465w IIRC) on a single array with slight shading issues (big tree and house shade some of the array at various times of day/year). 

 

The large number of panels was my way of conterwcting the less then ideal site. I'm making up for lost generation from shading, pointing SW and having a low pitch by just having more panels and stopping up the inverter size from 6kw to 8kw . The step up in cost was a few hundred £, which was far cheaper than trying to get the panels in a more optimum position. 

 

The panels seem OK, honestly won't know until about 10 years from now. 

 

Dealing with the shading by splitting the array into 3 strings of 12,6 and 6 panels (inverter has 3 inputs) so the shading only knocks out the smaller blocks. 

 

8kw solax IES inverter with 3x 5kwh battery modules. 

 

I did look at the Sig system - it looks very nice (frankly a step up from the Solax) but was about 1.5k more like for like and finding a local installer was hard.

 

The Solax unit seems pretty solid, if a little more utilitarian - think Skoda vs Audi. Went in very easily took about 30 minutes. Just a case of stacking boxes. The main advantages the Sig seems to have are

A) looks

B) can have higher total battery (solax is 20kwh per inverter, though inverters can be paired up) 

C) Sig has multiple battery sizes, solax does 5kwh

D) potential V2H/car charger with Sig (though I believe solax are working on one too) 

 

The solax advantage seems to be cost and potentially availability.  They have also been established quite a while and offer various units from small all the way to industrial ie they appear to be a likely to not go bust (I hope) 

 

The solax also handles power cuts more or less out of the box. You don't need a separate gateway box like other brands (I think Sig need one) but it does require an extra 2 conductors to the unit (5 cores rather than 3) which might be a consideration if you are locating the unit well away from your consumer unit and meter. 

 

As a side note when will these modular systems put the inverter at the bottom of the stack! At the moment you would need to lift of the inverter modules (the BMS is separate module) add the battery, then restack the inverter and adjust the cables. If the inverter was the bottom unit adding modules would be very simple, just stack a new one on. 

 

 

Edited 9 hours ago by Beelbeebub