H3 Pro 15kW – Real-world oversizing to 21.12kWp? (48 × 440W panels)

[merango]

Hi all,

This might seem similar to my other post but wanted to put a little more tech stuff in to ask the right questions. I'm looking for some technical guidance/opinions from those with hands-on experience of the Fox ESS H3 Pro 15kW 3-phase hybrid inverter and sensible PV oversizing.

Current setup:

* Fox ESS H3 Pro 15kW 3-phase hybrid
* 34 × Jinko Tiger Neo 440W N-Type All Black Mono panels (14.96kWp)
* Current strings: 12 + 12 + 10
* 7 × ECS2900 batteries
* House currently single phase but fed from a 3-phase supply
* Future addition of an ASHP and larger hot water cylinder

Proposed change:
Add another 14 × 440W panels, taking total system size to:

48 × 440W = 21.12kWp

Questions:

1. Is 21.12kWp considered sensible/acceptable oversizing on the H3 Pro 15kW within Fox best practice and warranty expectations?

The datasheet appears to allow up to 30kWp max array power and approx. 7.5kW recommended PV input per MPPT (3 MPPT inverter), but I’d appreciate real-world installer experience.

2. Would you favour keeping everything on the single H3 Pro 15kW versus adding a second 7kW 3-phase inverter?

My goal is simplicity and self-consumption rather than chasing absolute export.

3. What string configuration would you recommend for 48 panels?

Current is 12 + 12 + 10. Would a restrung configuration make more sense to optimise MPPT usage and different roof aspects?

Roof layout:

* Rear roof currently 24 panels
* Front roof proposed to become 24 panels
* Different sun profiles (front morning / rear afternoon)

4. Real-world clipping expectations?

I appreciate there will be clipping in peak summer, but in the UK is this usually modest/acceptable with a ~21kWp array on a 15kW hybrid, especially when batteries, house load, hot water and future ASHP demand are considered?

The system objective is:
House load → battery charging → hot water / ASHP → export

Rather than purely maximising export income.

Would really appreciate thoughts from anyone running a heavily oversized H3 Pro system or installers who’ve done similar.

Thanks in advance.

[EcoByte]

I'm an end user with similar kit, so I can give you some real world info but can't provide technical help!

I have the H3 Pro 15, 40x 430W panels 17.2kWp (9 East, 25 South, 6 West), 3x EP11, 2x 11kW heat pumps, multiphase Zappi and EV and 300 litre hot water cylinder heated by the heat pumps located in the NW near Preston. I had the 3 phase upgrade as part of a renovation project and a 3 phase consumer unit fitted. The Zappi is wired so as not to drain the house batteries.

The south facing arrays are split 12 on one string and 13 on another. The east and west panels (15) are on the 3rd string with tigo optimisers. This was the installers choice. The system can produce well over the 15kW export limit of the inverter and this month the Fox app has recorded peaks in excess of 19kW as it has been fairly cold but bright. The excess charges the batteries.

I'm on Octopus Intelligent Go and charge the batteries, car and heat the hot water over night. This is a financial decision. From March the system easily provides enough power to run the house and I generate enough to export and have a credit at the end of each month up until October. The accrued credit pays for the winter months.

As your proposed system will be over 21kWp you should have plenty of excess power and I would recommend exporting as much as possible while export rates are still reasonably good. I regularly produce over 100kWh per day in the summer months and the house load is not that high as the heat pumps are not working. Conversely, I can produce 1kWh on a poor winter's day and the heat pumps can drain the batteries before the evening on a very cold dull day.

Hope this is of some help!

[merango]

Thank you for all of that, cements my decision to keep the 15kW inverter and just add the extra as it will cover the darker / overcast days with better total generation. Happy to forfeit the £40-£60 losses over a year due to the clipping as we have a credit of £670 with Octopus since last year and that's after winter and taking £250 out of the account.

Waiting for the a couple of Heat Pump quotes as we are likely to need larger rads in some of the rooms but we are on 15mm pipes not the small microbore rubbish so the flow to the rads will be better, will have to wait and see what they come back with. Can I ask why you have 2 x 11kW ASHP? Must be a pretty large property no?

We're also replacing the loft insulation with 150mm PIR boards then overlaying that with 300mm of Rock wool at right angles with lifted boards so its not compressed. Should give us about 500mm at least of new loft insulation to help with the heat loss in winter. Also lifting all the first floor boards and installing acoustic rock wool blocks as when the house was rewired last year, the electricians removed it all and never replaced it!

It should all help with the thermal calcs for the heat pump and hopefully in July a new grant of £9,000 kicks in as we currently have an oil boiler to rip out. We get the £7,500 at the moment but there is talk of it going up in July to £9K so it makes it worth doing before Winter. I was looking at the Fox Heat pump but can't seem to find it in the UK yet.... it's on the website though.

Thanks again,

[EcoByte]

Glad I could help a little bit! We have a large bungalow, at least 350 sqm, with tons of roof space and a heat loss of approx 20,000W. I had two quotes and both recommended 2x heat pumps. I would recommend getting a good installer who has a proven track record and preferably heat geek trained. I also beefed up all the loft insulation as part of the renovation and had the gas meter removed.

The heat pumps went in first and as a comparison in Jan 24 with no solar I spent £470 on electric (2058 kWh) and in Jan 25 the cost was £139 less £20 export, so £119 for 1373 kWh. The solar and additional insulation helped reduce the cost and imported wattage.

If you are on reddit, my annual figures are there