Record the appliances and the hours
Write down which appliances you want to use during an outage and the hours when you normally need them. Use the labels or manuals to record their ratings where available. Note which devices run at the same time and which can wait until normal supply returns.
Show the installer a busy day
Add one example of a busy evening or workday. Mention appliances that start and stop on their own or have a noticeable surge when starting. Ask the installer to explain which loads the proposed system will support together and which assumptions drive its runtime estimate.