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How much can an east/west solar solar system be oversized?

Solar Photovoltaic (PV)

Last Post by bobflux 8 hours ago

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Falesh

(@falesh)

New Member Member

Joined: 2 months ago

Posts: 2

Topic starter 14/04/2026 1:16 pm [#2948]

I've got a 3.6 kW system installed on the west side of our house at a 29° angle. I live in York, so middle-ish latitude in the UK. The inverter is a H1-3.7-E-G2 which is also connected to a Fox ESS EP6 5.76 kWh battery. The house is going to be full electric, hob, heat pump, etc, and I've got an export tariff with Octopus.

I would like to add an extra string of solar panels to the east side of the house, but I need to keep the cost right down to be able to afford it. Because of that I was thinking about over sizing and not upgrading the inverter. Because the roof is east/west both strings should peak at different times, which should help reduce the max current. I think my inverter can pass through any excess current to fill the battery too. If I also added an extra Fox ESS EP6 5.76 kWh battery that would mean I could take 11.5 kWh of clipped solar power to use when the peak is done. I would also like to have more capacity during the non peak months of the year.

Does anyone know how much extra capacity would be reasonable to add? Since the panels are east/west they will not be able to reach their peak of 3.6 kW, so there should be some extra play there as well as with the battery.

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colinc

(@colinc)

Eminent Member Member

Joined: 7 months ago

Posts: 22

14/04/2026 3:04 pm

There's no definitive calculation you can make.

Knowing your Inverter Size & Type, along with your typical/ average house load metrics, and DNO export limit, will help in your decision.

You have not mentioned shading. "Gary does solar" Channel has a good Utility to help figure out the 'numbers'.

Scaffolding is a 'fixed cost' to consider, ie. more panels = less cost/panel.

It might be worth thinking maximising no. of panels if you plan to increase battery capacity in the future, or perhaps uprate your diverter.

ROI is also a consideration, but it is not necessarily the primary deal maker or breaker.

This post was modified 10 hours ago 2 times by colinc

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Mars

(@editor)

Illustrious Member Admin

Joined: 5 years ago

Posts: 4474

14/04/2026 4:26 pm

Posted by: @colinc

↑

Scaffolding is a 'fixed cost' to consider, ie. more panels = less cost/panel.

Unless you hire the cowboys our neighbours used, who installed the entire array off ladders, clinging on for dear life at the ridge of the roof for a couple of days! 🤣

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bobflux

(@bobflux)

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Joined: 2 months ago

Posts: 73

14/04/2026 5:22 pm

Best performance occurs with PV string near inverter nominal input voltage (360V). So panels should be selected to not exceed max inverter MPPT short circuit current (20A), and number of panels in series so the MPP voltage matches inverter nomina input voltage.

For example Vertex S+ TrinaSolar 500W:

Isc 15.86A is <20A inverter limit, OK
Impp 15.03A <16A inverter limit, OK

Umpp 33.30V so with 11 panels you're at the inverter nominal 360V

Uoc 40.1V so 11 panels are well below the 600V inverter limit

Cheaper example Vertex S+ Trinasolar 435W:

Isc 10.64A

Impp 9.99A

Umpp 43.6V

If you have limited space, this smaller panel with higher Umpp is a better match as you only need 8 to reach 360V inverter nominal input voltage, and you get 3480Wp. It simply has more cells in series and less cells in parallel, so it has higher voltage but lower current.

It's not mandatory to hit the nominal input voltage, you can go lower or higher, with a small penalty in efficiency. But if you use a low number of panels it is better to select the higher voltage ones so the inverter hits its startup voltage threshold earlier in the morning.

It may seem ridiculous to put so many panels on a "3.7 kW" inverter, but read this: https://diysolarforum.com/resources/hybrid-inverters-internals-and-power-ratings.457/

On this kind of hybrid inverter, the label rating "3.7kW" applies to the AC output only. It is perfectly capable of ingesting its max input power rating of 7.4kW and split that between battery charging and AC output, within the limits imposed by the battery and the max 3.7kW AC output. With only a 5kWh battery it will probably hit the max battery charging current limit, but you can always add more battery later.

Keep in mind that the MPPTs can only boost voltage from panels to the internal DC bus voltage, so the optimum arrangement on String 2 also depends on how String 1 is built. Having a large voltage mismatch between both strings is not optimal.

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