We installed a split last year. Considering our house has large windows and an open plan where it was to operate, it was noteworthy most of the sales people told us we needed about 14kw using ‘the formulas’. The largest capacity units are about 9.4 so that was two units. They did not try to sell two at once but were confident one would not do it <- Soft sell!
My installation had special issues so I was not about to buy off-the-shelf and hope the installation would be the normal cost - I wanted a confident all-up quote. 3 companies attended and one quoted from afar. The one who got my business was the only one who actually measured, applied the formula, explained why the standard formulas were not appropriate for my house, and that a single 9.4 should do everything I wanted with a high degree of confidence. An aspect I had not thought about is that 3 of the 4 ‘dumped’ the drain into the gravel while the winner routed it into a storm water downpipe. Small details can matter
It works a treat and will heat or cool the entire house if we use a single box fan in the hallway to push air toward the far rooms on bad days. On multiple 40+ days we need to over-cool the main area at night to also cool those far rooms but that is less costly than having a second unit for the far rooms and a minor inconvenience for the few days a year it happens in Melbourne.
The moral is that one needs to be wary of the formulas. Being undersold is at worst adding another unit. Being oversold is extra cost of purchase and operation forever and units that are too much over-capacity can work less well than a smaller one.
Going into a shop and buying using a formula can be a disservice or deliver a good deal, depending on how ones house matches ‘the model’ used to size units. If you have a traditional California bungalow the formulas are probably conservative; if yours is a window walled modern open plan house they will probably be optimistic. Insulation matters, the roof matters, and shading matters in addition to the obvious.