I've added an inverter sizing calculator to my website WattBuild. The goal is to establish what devices will be used, and to calculate continuous and surge power requirements using typical loads from our device research.
That's a good point. I'd have to think about how to design the UI in a way that doesn't add too much complexity while accommodating this kind of situation.
In practice for now its up to the user to choose their highest drawing scenario, whether that's a summer configuration or a winter one.