Estimated pay back time was 15 years+ using just for hot water, using for
heating was totally out of question
Will

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Don't understand this bit. What you use the power for is based on what appliances you are using when the sun's out (or it's bright). The supply from the inverter being taken in preference to that from the mains. So use power when the sun shines! In any case for solar PV the payback time is largely dependent on the FIT. As the inverters are quite expensive, it makes sense to go for as many panels as is practical.

It's also worth pointing out that the panels can be mounted in an unshaded garden, which I don't have, if the roof is unsuitable. They do need to face between SSE & SSW though.

I was limited to 1 row of panels, as the lower part of my roof gets a good deal of shade in the winter, and shading of just 1 panel in a traditional set up dramatically reduces the output. There are new systems now which, with additional wiring & electronics, overcome this, presumably by having multiple feeds into the inverter.

Graham
Penzance