Sizing a backup system: start with what must stay on

The first question people ask about backup power is how large a system they need. It is the wrong place to start, and it is why so many installed systems either cost more than they had to or run out sooner than the owner expected.

The right first question is which circuits actually have to stay on. Almost nobody needs the whole building running through an outage. What they need is lighting, the router and the security system, the fridge, and whatever equipment the business genuinely stops without. Everything else can wait for the grid to come back, and excluding it changes the specification dramatically.

Once that list exists, two separate numbers fall out of it. The first is the load: how much power those circuits draw at the same moment, which decides the size of the inverter or generator. The second is the energy: how long they need to run, which decides battery capacity or fuel. Confusing the two is the classic error - a system can be perfectly capable of carrying your load and still exhaust itself in forty minutes, because capacity and endurance were never the same specification.

Air conditioning, water heaters and borehole pumps deserve particular attention. They draw heavily, and including any of them can double a system. That is sometimes the right decision, but it should be a decision made deliberately with the cost visible, not something that arrives silently in a quotation because the survey asked for the building rather than the essentials.

Motors also draw a surge on starting that is well above their running figure, which is why a system sized on running load alone trips the moment the pump starts. Any competent design accounts for that surge, and it is a fair question to ask a supplier whether they have.

Take the time to write the essential circuit list before anyone quotes. It is the single most useful thing you can bring to a site survey, it makes competing quotations genuinely comparable, and it tends to reveal that the system you need is smaller and less expensive than the one you were about to buy.