Could you perhaps explain that? Are you perhaps just saying that, provided your batteries are adequately charged, they will provide about half of the power drawn by a shower (or any other large load)?
I would advise you to rethink an electric shower, to get one with a decent heat output and the cable (and pipework) lengths regards not just costs and losses with current and water flow you could go a long way to go with a storage system rather than instantaneous. The cost of a cylinder and good thermal insulation and say using for example a 3Kw immersion heater goes a long way to negating your 10Kw instantaneous shower idea. Or better still go to the main house for use of existing showering . Is the perceived gain of this luxury so important a practicality?
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