Pick an inverter from the loads that will run at the same time, and from the motors that have to start. Catalog bands such as 3 kW and 5 kW are steps manufacturers offer. They are not a diagnosis of your house. Set the badges aside until the load list exists. A larger inverter does not make the modules produce more energy. The array, the weather, and the inverter's AC ceiling do that.
The overlap, and the start
The continuous rating has to cover running watts that overlap. The surge rating has to cover a motor start, for no longer than that inverter allows. How to add up inverter loads is the method, with a hypothetical list that you replace with your own nameplates. A larger badge that still cannot start the pump is the wrong badge. A smaller badge that covers the overlap and the start is enough on the load side.
Backup sizing uses a shorter list: only the circuits that transfer. Sizing backup for critical loads is that division, including a hypothetical hour count that is not a recommended system. If this inverter will never form a backup supply, do not size it as if it had to start the well. A grid-tied unit follows the utility. Its AC rating still caps the array, which is the next decision, and it does not become a motor starter because the number on the box is large.
A higher AC rating and the same modules
Solar harvest follows the modules, the irradiance, and the ceiling of the inverter. Raising that ceiling gathers more energy only in the hours when the array was already above the old rating. Those hours are clipping. Inverter clipping and the DC-to-AC ratio is the full account, including why a flat noon on a clear day can be a sizing choice rather than a breakdown. In every other hour the tilt, the shade, and the weather still set the output. Buying the next catalog size to get more solar from the same modules misses those hours.
Compare a 3 kW class and a 5 kW class only after the list is written. The larger class is inadequate when simultaneous load and surge need more than it can deliver. The larger class is surplus when a smaller unit already covers the loads and the array never sits on the smaller ceiling. Neither sentence is a recommendation of a product. Both are a reason to read nameplates instead of the step the catalog printed in bold.
Service type, then the DC window
A single-phase service and a three-phase service are a separate fork from the kilowatt badge. Single-phase versus three-phase is that fork. Matching a neighbor's inverter size does not match your service. The inverter also has to accept the array: maximum DC voltage, the MPPT window, and the string current. A comfortable AC size can still be the wrong window on a cold morning.
What a string is, and why its voltage is the sum you must compare with that window, is in what a solar string is. What the tracker does with that voltage is in what MPPT is. This page keeps the order: loads and surge first when the inverter must supply them, the array ceiling next, the catalog band last. Two quotes that name different kilowatts are comparable only after both have survived that order.
