try and work out what your average daily Kw usage is, then go for something bigger. you also need to consider what you can fit on your roof, the angle of the roof and the direction its facing. The idea is to have an inverter that can handle a bit more than your panels max output. do you need to heavily prune your trees , Or your neigbours trees( I have 1 tree that shades my panels and my neighbour has stopped pruning it) .
These may seem like silly things but Ive seen installations where the panels face due east, even SE, panels overhanging rooflines, cableing running outside the roof, instalations that are heavily shaded. the usual comment was that the installed said it wouldnt make a difference, . finally , have a really good read of that fineprint before you sign. As with any subsidized scheme there will be some shonky operators, and they will be out there trying to find you with deals that are too good to be true