Backup power keeps the lights on. Trading through an outage takes slightly more than that, and the gaps are usually not electrical.
Payments are the first thing to fail
A card machine on a battery is useless if the internet is down, and fibre routers and street cabinets have their own power problems that yours does not solve.
The practical arrangement is a UPS on the router as well as the terminal, and a fallback that does not depend on your line at all — a card machine on a mobile network, or a payment link customers can open on their own phones. Test it during an actual outage rather than assuming.
Taking cash is a fallback too, with its own risks, and it means having change.
Know what you can and cannot do
Decide in advance which parts of the business continue and which stop, and tell staff. A shop that can sell but cannot print a slip needs to have decided beforehand whether that is acceptable.
Where something genuinely cannot run, say so quickly rather than letting customers queue for something that will not happen.
Reschedule rather than absorb
For work that does not need customers present, moving power-hungry or system-dependent tasks around the schedule is free and effective. Do the administration when the power is off and the machine work when it is on, rather than losing both.
Where the schedule is published, planning the week around it is a small discipline with a measurable return.
Tell people, in advance and in the moment
If you are trading through outages, say so — on the door, on your listing, on your social accounts. It is a genuine advantage while competitors are closed and most businesses never mention it.
If you cannot trade, post it early. Customers forgive an outage; they do not forgive arriving to a locked door with no notice.
Look after the perishables
Keep fridges and freezers closed rather than checking them. Know how long yours actually hold temperature — measure it once — because the guess is usually wrong in one direction or the other.
Keep a temperature record. It matters for food safety, and it is the evidence for any spoilage claim.
Write it down as a routine
Half a page: what gets switched off before, what comes on, who does it, what we can still sell, what we tell customers, what happens when power returns.
The point is that it stops being a disruption managed by whoever is on shift and becomes something the business simply does. That is the difference between an outage costing you a day and costing you an hour.
General guidance on commercial practice, not technical advice.