Before buying backup power, work out what an outage actually costs you. Most businesses have never calculated it, and end up either overspending on a system far larger than they need or absorbing a cost far larger than the system would have been.
The four costs
Lost trade. Revenue you simply do not take. Highest for anyone whose customers come to them — retail, restaurants, salons — and much lower for work that can be done later.
Lost productivity. Staff paid to be there and unable to work. Real money even where no sale was lost, because the wage was paid regardless.
Spoilage and damage. Refrigerated stock, work in progress, equipment damaged by surges when power returns.
The knock-on. Deadlines missed, deliveries late, customers who went elsewhere and did not come back. Hardest to quantify and frequently the largest.
Do the arithmetic
Take an average outage length, multiply by the hourly cost of the first three, and multiply by how many you actually had last month. That is a monthly number, and it is usually larger than people expect.
Compare it against what a system costs over its life — capital, installation, batteries which are consumable and will need replacing, fuel, and servicing. Now the decision is arithmetic rather than frustration, and it will tell you what size of solution is justified.
You may not need to run everything
The most common mistake is sizing a system to run the whole premises as though nothing had happened. That is the most expensive possible answer.
Work out what actually has to stay on to keep trading: the point of sale and the internet, the fridges, particular equipment, some lighting. Frequently that is a small fraction of the total load and a fraction of the cost.
Air conditioning, geysers and heating are the loads that make systems expensive, and they are usually the ones you can do without for two hours.
Consider the cheaper options first
Not everything is solved by generation. Moving stock into fewer fridges, rescheduling power-hungry work around the schedule, a small UPS on the till and router rather than a whole-premises solution, or gas for cooking — all of these are inexpensive relative to a full installation.
For many small businesses, keeping the payment system and the internet alive is most of the benefit at a small fraction of the cost.
Tell customers what you can do
A sign or a message saying you are trading through outages is worth actual money, because your competitors are closed. Businesses that invest in backup routinely forget to tell anybody about it.
General guidance on commercial practice, not financial or technical advice. Get a qualified assessment before installing anything.