I asked my plumber whether I should think about solar panels when my electric hot water heater needs replacing, and he said they’re expensive and require maintenance and his advice was to go for a gas water heater instead.

Yes but they are a long term investment in terms of recouping the money you outlay for them.

i think that over time, it is worth it.

It is a big expense, but well worth it in the long run, especially if you have quite a few so you can sell your excess power back to the grid

Yes, we do. We installed 3 years ago when we bought our first home. Worth the money, absolutely. The energy bills are a lot cheaper now.