In their advertising, they will say a device can power up to 600W, or 1200W using their X-Boost feature. Other brands may have something similar.
So how does X-Boost achieve this magic?
What it does is this. If the Ecoflow is about to be overloaded (the device needs more watts than the Ecoflow is rated for), then the Ecoflow lowers the voltage to keep the wattage in the range. At the most extreme, it will mean the Ecoflow is delivering only 120V.
This is why they say it doesn’t work with all devices, although it will work with quite a few.
So plug a 1000W kettle into 600W Ecoflow, and it will deliver 600W at about 140V. The kettle will work, and no fuses will blow – which is why the feature is useful – but the kettle will take that much longer to boil.
In some cases it won’t matter too much, but for things like Air Fryers, Foodi, Pellet Grills and such, it might not be that good an idea. At best, it will take longer; at worst, you might never reach the temperature you require, or it may make the device’s heat control cycle unreliable, as it is based on the device’s known behaviour, at 240V, which we are changing.
So not a bad thing, and probably not a problem if the overload – and voltage drop – is marginal. But probably not to be part of the consideration of buying a new item. Buy the wattage you need; use X-Boost to only extend that need as necessary.