Solar for Beginners [ask your questions here]

You're only allowed to push 25a to the grid, which nominally is 6kw if the inverter pushes the voltage up to 240v to push current against the grid voltage, though it's little more complicated than that. Any inverter or combination of inverters with an output current greater than this would need to be in off grid mode, as in wired for backup. Some inverters have built in off grid switching, some have split output whereby they give 5kw to the grid and simultaneously say 2kw to an off grid circuit, which could for example be lights, fridges, electronic appliances and so on.

To achieve 10kw for a shower plus all the other demands that might occur simultaneously such as fridge, lights, tvs, would require either a 10+kw inverter that can be choked to 5kw when in on grid mode, and I've not researched the feasibility of this, or else paired inverters, the type that can connect together to increase capacity, like the model you listed. Only one would be on grid, the other wired to either essential circuits, or idle. In backup mode both would be tied to each other and connected to the home off grid. Both could have batteries, one with solar also to combine power sources. Some of these models can also take in non synchronised generator input, and feed that into their solar/battery output. You also can consider the very costly NC7 connection which allows you to exceeds domestic limit of micro generation.

Such a system would require expert design, and there might be single inverter solutions to your quest. It might cost an awful lot more just to have the odd electric shower during the odd power cut.

If you have oil/gas HW, perhaps plumb in a normal shower mixer valve. I dumped my ensuite electric shower a decade ago, feeble flow, and useless when incoming cold water temperature was in single figures. I installed hot and cold lines from the hotpress straight to a normal mixer, via a pressure boost pump in the press. A DIY job for me, not everyone's idea of fun, but we now have high flow hot showers, and with only 0.25 kw pump consumption from the backup generator during a cut.

The off grid wouldn't support Feed in Tariff for a start, no payment for excess solar. I think there's an approval list for on grid inverters to meet the export limit. 25a input is the max, the actual power you shove in depends on the voltage at the feed in point. The actual 6kw rated inverters exceed this current, 5kw well in spec. A 5.5kw inverter is probably on the very edge. It matters not with self consumption, it's when the inverter is at full tilt and you're using nothing that you might exceed feed in current. Perhaps because you asked for back up power, the installer advised the off grid layout, but the problem with this is the inverter is in the path of all mains in, like a UPS, and if it fails, no leccie. With on grid inverter, you'll need wiring changeover relays/ switches to automatically or manually switch an on grid to an off grid inverter. That has a stiff cost burden as many installers seem shy of it.

Thank god.