It can be more cost effective to build a power station on your own, BUT (that’s a big but) it can also easily become a costly boondoggle. So just the usual caveats about planning well before purchasing anything.

The best diy approach I have seen is the sort built on a hand truck such as from Harbor Freight (in the US, there must be a eurozone equivalent that sells inexpensive tools). There are many variations on this plan to be found on the webz.

The key is always matching the components, the controller, the battery and panel so they work well together.

My two cents.