Expandable batteries explained
Expandable units accept extra battery packs from the same maker, raising capacity without adding an inverter. Whether that is worth paying for depends on three things.
What expansion does and doesn't change
An expansion pack adds watt-hours. It does not add output: the inverter in the base unit sets the maximum watts regardless of how many packs are attached. It also does not speed up charging unless the maker says the packs charge in parallel; many charge through the base unit at the same rate, so a doubled system takes twice as long to refill.
When it's cheaper than a second unit
Compare price per kilowatt-hour. Expansion packs are usually cheaper per kWh than a complete unit because they omit the inverter and ports, but not always, and sales on complete units change the math often. Two separate units also give you two inverters, two sets of outlets and the ability to put them in different rooms. A single expanded system gives one simpler setup and one app.
A reasonable rule: if you need more hours from the same loads, expand. If you need more loads at once or power in two places, buy a second unit.
Lock-in
Packs only work with their own brand and usually only with specific base models. Buying an expandable base is a bet that the maker will still sell compatible packs in three years; the larger brands have good track records, smaller ones are a gamble.
How we treat it
Expandability earns one feature point, the same as app control. The score otherwise rates the base unit on what it ships with. Every model page lists the maximum expanded capacity, and the expandable list ranks only units that accept packs.