How did you start the conversations around a Tesla Powerwall, and end up with a SolarEdge Energy Bank?? Anyways...
Here's my opinion
If you buy the Battery Storage & Solar Panels plus installation at the same time, the VAT should be charged at 5%. If you buy these separately, the battery kit will be charged at 20%
Your Solar Array is 4.5kWp (not 4.5MWp) and will be capped to a maximum power output of 3.5kW (which is your inverter size).
You should still get some decent power in Summer, with the Optimizers maximising the power output of the different roof aspects. So that all looks good.
If your house uses around 7.5 kWh of energy per day, your Solar Array will easily do this in Summer, and you'll be exporting to the Grid (or your Electric Car if you have one). I would guess your Solar Array will be producing about 30kWh on it's best Summer day. So more than enough during Summer.
You can never have enough Solar Panels in my opinion, so don't downsize
The issue you'll have is in Winter. Solar will go to nothing much in November/December/January. During this time you'll be relying on your Storage Battery, Smart Meters and a Cheap Rate Tariff (like Octopus Go).
With your storage battery you need to look at two things:-
- kW (Power Capability). This is how much maximum power your battery can discharge. This is important, because anything you use above this figure means you'll have to pull the excess from solar (non existent in Winter) or Grid Power. If you're using high power during the day, beyond the capability of your battery, then it'll be Grid Power at Peak Rate.
- kWh (Energy Capacity). This is how much energy your system can generate/store/use per hour. This is also important, because it let's you know how long your stored battery power will last.
The Tesla Powerwall has 13.5 kWh of energy capacity, and 5 kW of maximum continuous power discharge. In my opinion, that's great. Because it'll handle kettles, and toasters, and hair dryers, and microwaves, and cookers, and washing machines, and tumble dryers... plus your normal background house lights, fridges/freezers etc. All up to a maximum of 5 kW before it starts hunting extra power from the Grid (at peak rates). This means you'll mostly be using cheap rate electricity stored inside it.
Seriously consider the Tesla Gateway 2 as well. This provides backup power to your home in the event of a Grid Failure. The Gateway device also allows for EV Chargers to be wired directly to it. This means you save a Consumer Board slot.
On the Tesla Gateway device you can also wire things into it on Grid Side (so power to that item fails in a powercut, ideal if it's a pool pump or EV Charger), or it's non-grid Backup side (so power is maintained in a powercut, feeding the item from your battery power). Clever kit.
Hope that helps.